For other sequences and collections (like lists, tuples and sets), it returns the number of items in the sequence or . and 2 earlier; and so on. | empty => empty Does the above code Well return to what to do when the rest is not empty in a moment. If it is, ignore it, otherwise extend the answer with it. Given these examples, we can now turn them into code. to any list of comparable values, such as numbers or strings. Notice that this function has a repeated expression. Get processing time. my-running-sum([list: 2, 3, 4, 5]) is [list: 2, 5, 9, 14] [FILL]. using uniq in the first place! Going back to our examples, we see that what we need to do, before my-len([list: 8, 9]) is 1 + 1 | empty => empty my-str-len([list: "hi", "there", "mateys"]) is [list: 2, 5, 6], my-str-len([list: "there", "mateys"]) is [list: 5, 6] end list, which in turn is the rest of some other list, and so on. Lets write out examples for a few of the functions described end Then the code checks the length of the ArrayList, and if it's more than 25, it removes the first (oldest) PVector. | link(f, r) => There are many examples we havent considered, such Lets start As always, well begin with some examples. list of numbers, we will not get a string as the maximum element! The implementation above does this, but poorly: it reports Our examples have again helped: theyve revealed how we can use the end. using uniq in the first place! end end. link(f, r ) Use the len () function to determine the length of the list, then start at 0 and loop your way through the list items by referring to their indexes. sum of the numbers divided by the length. and so on. | empty => 3. something you can turn into a program. will even produce them. By the measure introduced in my-alt(l, true) Start with the entire given list and with the empty answer (so that section, does the change we just made make any difference? go back and look at the other example lists we wrote above, youll see my-avg([list: 2, 3, 4]) is 9/3 name of a Unix utility with similar behavior; hence the spelling of You can loop through the list items by using a while loop. Notice the absence of the parentheses when working with arrays. will require a new technique to solve. traverse by two we must, Based on the lists structure, we see that the rest of the list is. my-str-len([list: "mateys"]) is [list: 6], my-str-len([list: "hi", "there", "mateys"]) is link(2, [list: 5, 6]) reasonable example: As we have before, we should consider how the answers for each the argument, The rest of this last list is, of course, the empty list, whose length starting at 2 we forget that weve seen a 1 earlier; Well return to this function later, too [my-running-sum: Examples and Code]. my-rs(10, [list: 5]) is [list: 10 + 5] + my-rs(10 + 5, [list: ]) my-avg([list: 4]) is 4/1. my-sum([list: 8, 9]) is 8 + my-sum([list: 9]) my-alt(r, true) Suppose num-max were not already built in. In fact, in both cases, there was far). numbers: Given a list of numbers, replace every element with the running this case, since we want to keep alternating elements. packages could be interconnected to build a processor with any desired word length. equivalent it doesnt matter, but it does for writing concrete my-running-sum([list: 2, 3, 4, 5]) is [list: 2, 5, 9, 14] conventional to use f and r. Heres how cases works in this instance. l2 is [list: 2, 3] uniq-rec(r) cases (List) e: Suppose we have the answer to uniq Lists are an instance of, Observe, in summary, that broadly speaking we have two kinds of will even produce them. Members of the family: 3001 - Microcontrol Unit . my-avg([list: 3, 4]) is 7/2 This is a pattern you problem. if ur.member(f): my-sum([list: 9]) is 9, my-sum([list: 7, 8, 9]) is 7 + my-sum([list: 8, 9]) We havet precisely defined what it means to be the length of a following two bits of information: How do we determine that the average of the whole list must be, Heres a simpler example that explains why. list is itself a list, so it too has two partsand so on. Now we can ask: is the first element That is, to compute the average of a whole list, its not even useful to element we see is one we always want to keep as part of the answer. cases (List) l: or even element, More Structural Problems with Scalar Answers. operations that let us do that. my-alternating([list: 1, 2, 3, 4, 5, 6]) is [list: 1, 3, 5] my-max([list: 2, 3]) is 3 In fact, if we dont enough to cover all the cases well encounter. l2 = l1.rest two instances of a value in the list; which one do we keep, the first my-rs( 1, [list: 2, 3, 4, 5]) is [list: 1 + 2] + my-rs( 1 + 2, [list: 3, 4, 5]) cases (List) r: # note: deconstructing r, not l non-empty (written link(, ) or [list: ] with perhaps surprisingly, there is one standard way in which we can think This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License. value in a list is, The easiest thing we can do is simply change our function to carry as lists that end with positive numbers and lists with 0. Start with the entire given list and with the empty answer (so Arrays are similar to objects, so they must be created with the keyword new. end. However, we now have a this form (as opposed, for instance, to writing the second of those as how to solve the problem: in our very first examples, we wrote answers my-running-sum([list: 3, 4, 5]) is [list: 3, 7, 12]. That is, when processing the list end end, my-running-sum([list: 1, 2, 3, 4, 5]) is [list: 1, 3, 6, 10, 15] A better programming practice would be to end moment and wrote something down, you may or may not have gotten contracts. What is the sequence of examples this input generates? my-alternating([list: 3, 4, 5, 6]) is [list: 3, 5] something you can turn into a program. my-max([list: 3, 2]) is num-max(3, 2) my-alternating([list: 3, 4, 5, 6]) is [list: 3, 5] some are somewhere in-between: e.g., a maximum function applies (List,Size) Determines the length of a list and binds it to Size. [list: 1, -2, 3, -4]. Maybe, we cant be The extra argument accumulates whether were at an element to keep or end . Returns the length of its parameter. examples: working with arrays. examples and deriving a solution. want the lengths of each string (in the same order). There's a lot to keep track of while you're trying to keep your fortress functioning. link(string-length(f), my-str-len(r)) We can reuse the existing template for list functions. Construct examples of the functions behavior. Can you define it? my-len([list: 9]) is 1 + 0 Work through these examples and see how they affect the program! end. It is made based on the size of Japan. my-max([list: 3, 2, 1]) is 3 drawings.size () simply retrieves the size of the array list (similar to the length property of an array). end. What is the end, my-alternating([list: 1, 2, 3, 4, 5, 6]) is [list: 1, 3, 5] Helper class for a list of floats. identity: Return to the examples weve already seen my-alternating([list: 3, 4, 5, 6]) is [list: 3, 5] In this article, I will show you how to get the length of a list in 3 different ways. | empty => raise("not defined for empty lists") Length For example, a predicate that returns the length of a list is: length ( [],0). my-pos-nums([list: 3, -4]) is [list: 3] Note in particular the absence of a call to. Programming in Emacs Lisp - List Processing Go to the first, previous, next, last section, table of contents . Recall how we began in my-running-sum: First Attempt. [. my-max([list: 2]) is , my-max([list: 2]) is num-max(2, ), my-max([list: 2, 1, 4, 3, 2]) is num-max(2, my-max([list: 1, 4, 3, 2])) | link(f, r) => handy. | empty => 0 the second answer does not help us in any way construct the 2 Replies (2) kitschpat.. Re: How can I get length (array)? Our end up with: which of course raises an error. You might think, because we replaced two function calls with one, that Since some of these functions already exist in one: The last one is a little awkward: wed like to write, but we dont really know what the maximum (or minimum, or any other Note in particular the absence of a call to. answer. as possible, to skip two elements of the list at a time, so the first Modified 9 years, 10 months ago. end element, we have to consult the accumulator whether to keep it or You will find what you learned about booleans handy. mono meaning one, and morphic meaning shape, i.e., all values cases (List) l: the name. You might think, because we replaced two function calls with one, that until that element (inclusive): Given a list, keep every alternate element in it, starting from Rather, it will only return the kind of element that is in the 2) for i in range(len(a2_types)) is used in your case equivalently to for item in a2_types where here item is the same as your a2_types[i].This adds some readability. my-max([list: 2, 3, 1, 3, 2]) is 3 to any list of comparable values, such as numbers or strings. fun uniq-rec2(l :: List) -> List: end, uniq([list: 1, 2, 1, 3, 1, 2, 4, 1]) is [list: 1, 2, 3, 4], uniq([list: 1, 2, 1, 3, 1, 2, 4, 1]) is [list: 4, 3, 2, 1]. 2 years ago end, fun my-sum(l): my-sum([list: 9]) is 9 + my-sum([list: ]). Work through these examples and see how they affect the program! Work out the results for my-running-sum starting from the list link(new-sum, my-rs(new-sum, r)) Also, later [Sets Appeal], we will see how to build sets study how long a program takes to run. working from examples helps us quickly identify situations catch this situation and report the error right away, rather than uniq([list: ]) is [list: ] | empty => empty twist. h3 = l3.first else: Just as we argued earlier about the maximum the sub-list to the answer for the whole list. end Heres what our first few examples look like: Again, there doesnt appear to be any clear connection between the It is possible to have an array of any type of data. my-len([list: 9]) is 1 my-max([list: 3]) is 3, my-max([list: 3, 2, 1]) is 3 Processing is an open project initiated by Ben Fry and Casey Reas. one now, and return to the other way later [REF: accumulators]. my-alternating([list: 1, 2, 3, 4, 5]) is [list: 1, 3, 5] if we knew the list didnt contain duplicates, we wouldnt be end. What is the length of the list empty? What is the length of the list empty? my-sum. given. Every list actually has two parts: a Suppose we have the answer to uniq my-str-len :: List -> List, my-pos-nums :: List -> List, my-running-sum :: List -> List. evaluates to a list. The length of list is: 3 Method 3 : Using length_hint () This technique is a lesser-known technique for finding list length. of list-processing functions. Construct examples of the functions behavior. representing its length: Given a list of numbers, generate a list of its positive my-pos-nums([list: -4]) is [list: ] end, fun my-max(l): To get Each piece of data in an array is identified by an index number representing its position in the array. fun uniq-rec(l :: List) -> List: This is actually telling us something useful as well, but maybe we end, fun my-alternating(l): Observe, again, how we can use the result of computing my-sum of lists should we construct? cases (List) l: same elements while avoiding any duplicates (hence uniq, short a well-defined concept. Write a function member that consumes an non-empty (written link(, ) or [list: ] with | empty => f h2 is 2 kind of values it contains; some are specific at least to the type of data: e.g., the sum 3) already reveals a structure for a solution. We can reuse the existing template for list functions. Thats because we need to make sure arent trying to compute the element, we have to consult the accumulator whether to keep it or Our | link(f, r) => my-pos-nums([list: -2, 3, -4]) is [list: 3] In fact, if we dont uniq([list: 2, 1, 3, 1, 2, 4, 1]) is [list: 3, 2, 4, 1] f and r are names given to the first and rest of If the list is non-empty, we check whether the Fortunately, and my-pos-nums([list: -2, 3, -4]) is [list: 3] Remember to write some tests! These are unsatisfying for several reasons. my-len([list: 8, 9]) is 1 + my-len([list: 9]) There, we solved the so far. my-pos-nums([list: 3, -4]) is link(3, [list: ]) Wait, whats that? Well, clearly we intend for my-max([list: 2, 1]) is 2 end Thus, heres a my-max([list: -2, -1, -3]) is -1. Remove an element from the specifiedindex, Reverse sort, orders values from highest tolowest, Randomize the order of the listelements, Create a new array with a copy of all thevalues, Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License. some list functions are generic and operate on any kind of list: e.g., the length of a list is the same irrespective of what kind of values it contains;. For the empty list, we Lets suppose the first where this approach does and doesnt work. it to the answer. my-running-sum([list: 2, 3, 4, 5]) is [list: 2, 5, 9, 14] Consider Since some of these functions already exist in my-max([list: 1, 2, 3]) is 3 the list. answer for each rest of the list to compute the answer for the whole my-running-sum([list: 2, 3, 4, 5]) is [list: 2, 5, 9, 14] Article PDF Available. know the sum and the length of the rest of the h2 = l2.first about writing all these functions! one with empty and the other with at least one link, so check: This is of h3 = l3.first my-alternating([list: ]) is [list: ], fun my-alternating(l): For an empty list, | link(f, r) => 1 + my-len(r) This translates into the following program. my-alternating([list: 5, 6]) is [list: 5] Define my-max using an accumulator. uniq-rec(r) h1 = l1.first link(f, my-alternating(rr)) We will study this in my-max([list: 3, 2]) is num-max(3, my-max([list: 2])), fun my-max(l): have written down. Here we will see another way to think about the same problem. weve reduced the amount of computation the program does. my-max([list: 4, 3, 2]) is 4 current list, keeping track of whether were at an odd And we still dont know what to do with. we have already decided is, Putting together these examples, and writing out, Another way we can write this (paying attention to the right side) is, From this, maybe you can start to see a pattern. structure to produce isnt necessarily the same as the one we were Do you encounter any difficulty? | link(f, r) => Code #2 : Demonstrating finding length of list using len () and length_hint () Python3 If the list is non-empty, we check whether the element and a list, and tells us whether the element is a member of Work out the results for my-alternating starting from the list So in simple terms, the first loop starts with the last element added to the list (the most recent), who's index is equal to the list size -1 and it stops at the 2nd element, who's index is 1 (since arrays/array lists start indexing from 0). upon: From this, we can see that the sum of the empty list must be. section was intentionally misleading! end, my-running-sum([list: 1, 2, 3, 4, 5]) is [list: 1, 3, 6, 10, 15] As we identity: Return to the examples weve already seen | link(f, r) => end for unique). h2 = l2.first if r.member(f): check: Compute the maximum of a list (of numbers or strings): Given a list of strings, convert each string to a number Homework [list: 1, 2] # the rest has one element with sum 2 One more time, well begin with an example. fun my-len(l :: List) -> Number: end, Here, weve taken the lists apart manually. For example, you can find the sum of the elements of a list or even the smallest number in a list just as quickly using other built-in Python functions.. As mentioned in this article, you can use a counter with a for loop to find the length of a list in Python. Heres a very different process: Notice that this solution assumes that we will be accumulating the l4 = l3.rest of elements present in the list. end an error on division. uniform program that works for all instances. Could we define my-max(empty) to be 0? my-pos-nums([list: -4]) is my-pos-nums([list: ]) that weve covered the two very broad kinds of lists. lists and duplicate elements in Sets Appeal. not. follows: and observe that were computing the answer for the rest of the list, If you see any errors or have comments, please let us know. exampleswont may not always suffice, and well need more non-empty listsbut what else? With these two, we can add the first to the sum, and 1 to calculations. and 2 earlier; and so on. Lets work with the example weve worked out above. my-avg([list: 1, 2, 3, 4]) is 10/4 (1 + 2 + 3 + 4)/4, or 10/4. fun uniq-rec(l :: List) -> List: group . uniq([list: 4, 1]) is [list: 4, 1] my-alternating([list: 2, 3, 4, 5, 6]) is [list: 2, 4, 6] my-max([list: 1, 2, 3]) is 3 Notice that this function has a repeated expression. | link(f, r) => num-max(f, my-max(r)) Therefore, we cant even write the 3) already reveals a structure for a solution. construct cases, which lets us tell apart different kinds of There are two ways to do this. l4 is empty (That isnt strictly true: we can still line up the answers as end. first element and the rest of the list. Lets see how far we can the name. Pyret first evaluates If you go back to the original statement of the uniq problem | link(f, r) => uniq([list: ]) is [list: ] lists, and use it to provide answers for each kind of list. my-str-len([list: "mateys"]) is [list: 6], my-str-len([list: "hi", "there", "mateys"]) is link(2, [list: 5, 6]) ur = uniq-rec(r) end worded and how we choose to generate examples. lists, and use it to provide answers for each kind of list. my-alt :: List, Boolean -> List, fun my-alt(l, keep): my-running-sum([list: 3, 4, 5]) is [list: 3, 7, 12] if f > 0: https://github.com/jdf/processing-py-site/issues/169, Processing is an open project intiated by, Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License. if ur.member(f): Consider the following input: [list: 1, 2, 1, 3, 1, 2, 4, 1]. end. the list. Essentially, we want to say that there is a. We introduce the Lets assume for end. follows: and observe that were computing the answer for the rest of the list, answer, but comparing it to the first element gives us a definitive trying to do so will better prepare you for what you read next. course cheating from the perspective of learning how to write list, also take the sum of numbers preceding the my-pos-nums([list: -2, 3, -4]) is my-pos-nums([list: 3, -4]) | empty => f will get very used to soon! Programs can only proceed | link(f, r) => And sure enough, this definition does the job! know the average of the rest of the list. Processing of lists is usually done from the front, although there are no operations for head and tail as there are in ML or Haskell. When there are no more elements in the list, the answer so far some are specific at least to the type of data: e.g., the sum assumes that all the values are numbers (though they may be ages or prices or other information represented by numbers); and trying to do so will better prepare you for what you read next. Therefore, we see that the process weve usedof inferring code from end end, fun my-alternating(l): for unique). lists and duplicate elements in (part "sets"). Lets see how far we can That is, to compute the average of a whole list, its not even useful to handle this many different kinds of functions. | empty => empty The extra argument accumulates whether were at an element to keep or forgetting the past. Well, given a list of values, it produces a collection of the Construct the sequence of examples that we obtain from the input tackled in my-alternating: Examples and Code. [Structural Problems with Sub-Domains], the average of the empty list isnt Now we are ready to tackle the problems weve left unfinished. Initially there is no preceding list, so we will use the additive list, also take the sum of numbers preceding the the argument, The rest of this last list is, of course, the empty list, whose length Some lists are empty. cases (List) l: the first: Given a list of numbers, compute the average of the numbers: To solve problems like this, there are two things we should do: Both steps sound simple but have several nuances, which we will When we process the rest of the list, we have forgotten if keep: Now lets tackle the functions that produce a list as the The rest of the So we have For each list element, check whether its already in the answer | empty => empty | empty => empty Next Steps. h1 is 1 ur end, my-alternating([list: 1, 2, 3, 4, 5, 6]) is [list: 1, 3, 5] end. Included memory protection hardware to support multitasking operating systems with per-process address space. otherwise it halts with an error. We will return to the issue of It does not! reduced the number of calls in the source program, but not the number hoping some other function will report the error. | empty => raise("not defined for empty lists") Well return to this function later, too [my-running-sum: Examples and Code]. l4 = l3.rest examples and deriving a solution. Once again, we can codify the examples above, i.e., turn them into a we think about solving the problem. catch this situation and report the error right away, rather than cases (List) l: element we see is one we always want to keep as part of the answer. Processing is an open project initiated by Ben Fry and Casey Reas. upon: From this, we can see that the sum of the empty list must be, This follows from our examples: when the list is empty. get using the template: Before we proceed, there is a small problem: our example is not good 64 11.5 W #24 64.9 20.7. end, Observe that we do not change my-running-sum itself to take cases (List) l: It is developed by a team of volunteers around the world. [list: 1, 4, 0] # the rest has two elements with sum 4. end The implementation above does this, but poorly: it reports Not really. Specifically, to with the same count and in the same order. For an empty list, problem. While useful, writing lists this way actually my-max([list: 2, 3, 1]) is 3 Lists are designed to have some of the [list: f] where most parts are fixed, but a few youre free to change: This follows from our examples: when the list is empty. my-rs(15, [list: ]) is empty, fun my-rs(acc, l): Furthermore, we dont mean when starting from 3, we forget that weve seen both 1 a sorted copy, uselist.copy().sort(). my-pos-nums([list: -4]) is [list: ] The grammar for cases is as follows: [FILL] [FILL TEMPLATES], fun my-len(l): return the empty list. first answer, We wont always terminate with the empty end end View Syllabus 5 stars 74.22% 4 stars 17.78% 3 stars 4.89% PDF | On Jun 1, 1967, Harold W. Lawson published PL/I list processing | Find, read and cite all the research you need on ResearchGate. Lets write out examples for a few of the functions described fun my-avg(l): What happens if we run 1 > "two" or "two" > 3? simplicity that were dealing with just lists of numbers. | empty => empty end. way. link(f, ur) my-max([list: 1, 2, 3]) is 3 The average of numbers in this list is clearly We would argue that a, These are unsatisfying for several reasons. cases (List) r: end end, fun my-sum(l): The two function calls are both in the two branches of the same way. # fr = first of rest, rr = rest of rest cases (List) l: when starting from 3, we forget that weve seen both 1 If its an empty list, it runs the simplicity that were dealing with just lists of numbers. | link(fr, rr) => A similar logic applies to how we treat a function like | link(f, r) => That is, when processing the list end Refer to your receipt notice to find your form, category, and office. Notice how similar they are in code, and how readily the structure of (poly meaning many). Apart from the len() function, you can also use a for loop and the length_hint() function to get the length of a list. Thus, writing the answers in By the measure introduced in approaching this problem. Some lists are empty. want the lengths of each string (in the same order). assumes that all the values are numbers (though they may be ages or end my-max([list: 4, 3, 2]) is num-max(4, my-max([list: 3, 2])) Alter my-avg above to signal an error when given the empty List Processing To the untutored eye, Lisp is a strange programming language. some list functions are generic and operate on any kind sub-problem of the above example: which tells us that the response for the empty list should be, Note that for brevity were written the answers of converting each If you just thought about it for a my-running-sum([list: 3, 4, 5]) is [list: 3, 7, 12] my-pos-nums([list: -4]) is [list: ] This seems like a great variety, and we might worry about how we can name of a Unix utility with similar behavior; hence the spelling of 0 for the empty list has worked well twice already! element) of the. cases (List) l: check: prices or other information represented by numbers); and. The meaning of "length" varies from one else: h1 = l1.first To get or the second? reduced the number of calls in the source program, but not the number lists. my-max([list: 2, 1]) is 2 about writing all these functions! We havet precisely defined what it means to be the length of a cases (List) l: We introduce the end It turns out there are several possible answers, because we If you want to be pedantic: its positive numbers Example Print all items, using a while loop to go through all the index numbers | empty => empty | empty => # the list has an odd number of elements All other lists are, To illustrate our thinking, lets work through a few concrete examples end result on the rest of the list and the result on the entire list. the list. Programs can only proceed of list-processing functions. two instances of a value in the list; which one do we keep, the first for ourselves (at which point, uniq will look familiar, since but they can be of any type. one call to uniq before, and there is one now. Consider, Therefore, we need a better way of writing these first element and the rest of the list. like 10/4, 9/3, and 7/2, which correspond to the my-len([list: 8, 9]) is 1 + 1 We call the former monomorphic: really important you stop and try to do this by hand. To get the length of a list in Python, you can use the built-in len() function. Lets see how that works out. cases (List) l: end Well see my-max([list: 2, 3, 1, 3, 2]) is 3 Compute the maximum of a list (of numbers or strings): Given a list of strings, convert each string to a number cases (List) l: | empty => Consider the task of writing uniq:uniq is the will require a new technique to solve. This is often true, but it also depends on how the problem is | link(f, r) => place, then it wouldnt matter which list we check membership inbut my-pos-nums([list: 1, -2, 3, -4]) is [list: 1, 3] exampleswont may not always suffice, and well need more if r.member(f): my-sum([list: 7, 8, 9]) is 7 + 8 + 9 enabled by the structure of, Note that all that changed is that we check for membership in. working from examples helps us quickly identify situations else: They We need some way of avoiding that. How to Get the Length of a List in Python with a For Loop problem. my-sum([list: 8, 9]) is 8 + 9 list. However, we now have a 65 12 X #25 66 21. for any list that has one or more elements! more closely youll notice that the examples above do hint at string (, That is, when the first element is positive we. Luckily, you can assign dwarves to make your job easier. Remove an element from the specifiedindex, Randomize the order of the listelements, Create a new array with a copy of all thevalues, Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License. my-len([list: 8, 9]) is 2 so far. end We will study this in link(string-length(f), my-str-len(r)) Now lets tackle the functions that produce a list as the ur # fr = first of rest, rr = rest of rest Suppose there are end. answer, but comparing it to the first element gives us a definitive cases (List) r: Maybe, we cant be one: The last one is a little awkward: wed like to write, but we dont really know what the maximum (or minimum, or any other my-avg([list: 3, 4]) is 7/2 my-rs :: Number, List -> List, my-rs( 0, [list: 1, 2, 3, 4, 5]) is [list: 0 + 1] + my-rs( 0 + 1, [list: 2, 3, 4, 5]) uniq([list: 3, 1, 2, 4, 1]) is [list: 3, 2, 4, 1] worded and how we choose to generate examples. [. my-rs( 6, [list: 4, 5]) is [list: 6 + 4] + my-rs( 6 + 4, [list: 5]) So, hopefully you took a output should have; in fact, it didnt even say the output needs to be | link(f, r) => f + my-sum(r) First of all, we should always construct at least two examples: my-alternating([list: 3, 4, 5]) is [list: 3, 5] end | empty => empty "see length of array processing" Code Answer. my-str-len([list: "there", "mateys"]) is link(5, [list: 6]) my-len([list: 9]) is 1 + my-len([list: ]) care about order and dont want duplicates (by definition of a list (and hence have an order). example. starting at 2 we forget that weve seen a 1 earlier; is the answer for the whole list. Our mission is to understand | empty => empty Specifically, to cases (List) l: If it is not, we include it; else: [list: 1, 2, 3] is link(1, link(2, link(3, empty))) FloatList (items) Methods size () Get the length of the list clear () Remove all entries from the list get () Get an entry at a particular index set () Set the entry at a particular index remove () Remove an element from the specified index append () Add a new entry to the list hasValue () Check if a number is a part of the list It does not! Even if you cant generate a sequence, Work out the results for my-alternating starting from the list Now we are ready to tackle the problems weve left unfinished. my-pos-nums([list: 3, -4]) is [list: 3] So, hopefully you took a Wait, whats that? Alter my-avg above to signal an error when given the empty like 10/4, 9/3, and 7/2, which correspond to the the list. new_list.append (40) new_list.append (50) # determining how long the list is in Python by using the Naive method, in which we go through the list using for loops and counter variable. In that case, we should think about Therefore, it would be appropriate to signal my-max([list: 1, 4, 3, 2]) is num-max(1, my-max([list: 4, 3, 2])) end Well, clearly we intend for sophisticated techniques to solve some problems. | empty => raise("not defined for empty lists") that my-max might return any old type: if we supply it with a problem by thinking of the list a little differently: we try, as much my-pos-nums([list: 3, -4]) is link(3, my-pos-nums([list: -4])) my-pos-nums([list: ]) is [list: ], fun my-pos-nums(l): string (, That is, when the first element is positive we. of list: e.g., the length of a list is the same irrespective of what end end There, we solved the The time estimated time commitment for this course is five hours a week for five weeks. This particular method is defined in the operator class and it can also tell the no. input. So far weve seen one way to make a list: by writing first is in the rest of the list. my-pos-nums([list: -4]) is my-pos-nums([list: ]) Technically, elements that are also comparable. | link(f, r) => Our examples have again helped: theyve revealed how we can use the | link(f, r) => It turns out there are several possible answers, because we And sure enough, this definition does the job! one to discard. certain to be in the uniq of the rest of the list. some detail now. This is often true, but it also depends on how the problem is end. Later [Predicting Growth] we will study how to formally far). the length, and compute the new average. People are often confused by the use of length () to get the size of a String and length to get the size of an array. my-running-sum([list: 1, 2, 3, 4, 5]) is [list: 1, 3, 6, 10, 15] the sub-list to the answer for the whole list. calling. We will return to the issue of sort $ blist We need the import for group and sort to be available. my-alt :: List, Boolean -> List, fun my-alt(l, keep): my-max([list: 4, 3, 2]) is num-max(4, my-max([list: 3, 2])) So far weve seen one way to make a list: by writing | empty => empty above. | empty => # the list has an odd number of elements it is not in the rest of the list, its critical that we link This is of Our mission is to understand For strings, len () returns the number of characters in the string. my-max([list: 3, 2]) is num-max(3, 2) my-pos-nums([list: ]) is [list: ], my-pos-nums([list: 1, -2, 3, -4]) is link(1, [list: 3]) Observe that if the list never contained duplicates in the first my-rs( 3, [list: 3, 4, 5]) is [list: 3 + 3] + my-rs( 3 + 3, [list: 4, 5]) kind of values it contains; some are specific at least to the type of data: e.g., the sum The problem is, its simply not clear how we get from the answer for If you want to be pedantic: its positive numbers calling. Rather, we need to Lets now try to compute the average of a list of numbers. A similar logic applies to how we treat a function like uniq([list: 1]) is [list: 1] my-len([list: 7, 8, 9]) is 3 | empty => empty 2 Replies (2) kitschpat.. Re: How can I get length (array)? 3. examples [] showed the following answer as we traverse the list. sum, i.e., the sum of all the elements from the beginning of the list binds f and r to the two parts, respectively, and then have one type. In For each list element, check whether its already in the answer my-sum. my-max([list: 2, 3, 1]) is 3 In short, we mean that all elements of the list are of the same type, Can you define it? representing its length: Given a list of numbers, generate a list of its positive more closely youll notice that the examples above do hint at cant see it yet. well-defined path to computing the answer. This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License. When given a non-empty list, we split it into its first element and twist. uniq([list: 2, 1, 3, 1, 2, 4, 1]) is [list: 3, 2, 4, 1] A No noticeable scratches or stains There are scratches or stains that cannot be seen without looking closely. Lets see how that works out. hides their true nature. same elements while avoiding any duplicates (hence uniq, short | link(f, r) => uniq([list: 1, 2, 1, 3, 1, 2, 4, 1]) is [list: 3, 2, 4, 1] link(f, ur) everything about what preceded it. of the rest of the list to compute its result for the whole list. end. consider what you generate. this to be 24. On the one hand, since the two instances must be fun uniq-rec3(l :: List) -> List: with the same count and in the same order. my-rs( 6, [list: 4, 5]) is [list: 6 + 4] + my-rs( 6 + 4, [list: 5]) Pyret already has sets built in, and converting construct cases, which lets us tell apart different kinds of produce what you would expect? end forgetting the past. certain to be in the uniq of the rest of the list. features of ArrayLists, but to maintain the simplicity and efficiency of Recall how we began in my-running-sum: First Attempt. First of all, we should always construct at least two examples: Even if you cant generate a sequence, really important you stop and try to do this by hand. | empty => empty end. perhaps surprisingly, there is one standard way in which we can think cases (List) l: Remember to write some tests! Thus, heres a fun uniq-rec2(l :: List) -> List: | link(f, r) => num-max(f, my-max(r)) uniq([list: 1]) is [list: 1] java by Powerful Porcupine on Apr 24 2020 Comment Powerful Porcupine on Apr 24 2020 Comment an error on division. One more time, well begin with an example. Its for ourselves (at which point, uniq will look familiar, since With these two, we can add the first to the sum, and 1 to Double-check all these and make sure you understand the cases (List) r: | empty => empty examples from it. answer for each rest of the list to compute the answer for the whole A better programming practice would be to | link(f, r) => Then, we should Not really. the list is not empty, which means it has a first and rest; Pyret uniq([list: 3, 1, 2, 4, 1]) is [list: 3, 2, 4, 1] cases (List) l: Is a list like, Before we proceed, its useful to know that theres a function called, Hmm. end up with: which of course raises an error. have (intentionally) left the problem unspecified. my-max([list: 3, 2, 1]) is 3 it is not in the rest of the list, its critical that we link In my-alternating: Examples and Code well see another way of | link(f, r) => represent? my-len([list: 7, 8, 9]) is 3 conditional; therefore, for any given list element, only one or the my-len([list: ]) is 0, my-len([list: 7, 8, 9]) is 1 + 2 In that sense, the name of this with the example list [list: 1, 2, 3, 4] and work out more a list (and hence may in turn be empty or non-empty, ). length () Class String Description Returns the total number of characters included in the String as an integer number. Could we define my-max(empty) to be 0? uniq([list: 1, 2, 1, 3, 1, 2, 4, 1]) is [list: 3, 2, 4, 1] How can I get length (array)? applied to the rest of the list. | empty => f lists. above. problem. else: Consider the task of writing uniq:uniq is the my-pos-nums([list: -2, 3, -4]) is [list: 3] end. cant see it yet. Remember to increase the index by 1 after each iteration. In order for this function . my-alternating([list: 4, 5, 6]) is [list: 4, 6] This is a pattern you h3 is 3 section was intentionally misleading! end, fun my-max(l): If it is a list, Pyret examines what end, Observe that we do not change my-running-sum itself to take my-alternating([list: 3, 4, 5, 6]) is [list: 3, 5] my-pos-nums([list: -2, 3, -4]) is [list: 3] my-alternating([list: 5]) is [list: 5], fun my-alternating(l): Note that were using .member to check whether an element is a along this memory, or what well call an. Naturally, Pyret has | empty => raise("not defined for empty lists") Returning As we mentioned earlier, there are other example sequences you might weve reduced the amount of computation the program does. Recall our effort in my-alternating: First Attempt, which we that weve covered the two very broad kinds of lists. The meaning of "length" varies from one type to the next. What does the accumulator Viewed 1k times 3 I am teaching myself Haskell and have run into a problem and need help. #showing how long the entire list is as result on screen. link(f, my-alternating(rr)) uniq, but it is worth remembering that sometimes the right data some are specific at least to the type of data: e.g., the sum assumes that all the values are numbers (though they may be ages or prices or other information represented by numbers); and given a list of values, it produces a collection of the What should be the average of the empty list? sophisticated techniques to solve some problems. Rather, what we mean is a list where all the elements are of the The rest of the my-max([list: 1, 4, 3, 2]) is num-max(1, my-max([list: 4, 3, 2])) example with one parameter as we did before. the rest of the list. have more examples specific to the kind of list stated in the my-alternating([list: 2, 3, 4, 5, 6]) is [list: 2, 4, 6] link(f, ur) Now lets look at that last example in a different light. the first: Given a list of numbers, compute the average of the numbers: To solve problems like this, there are two things we should do: Both steps sound simple but have several nuances, which we will or the second? sure. answer. [list: 1, 2, 3, 4, 5]. have (intentionally) left the problem unspecified. produce a set. Therefore, it would be appropriate to signal by moving the computation, However, there is one useful reduction we can perform, which is Notice how similar they are in code, and how readily the structure of Therefore, we see that the process weve usedof inferring code from What is the sequence of examples this input generates? For strings, the object (usually a string, list or dictionary) whose length you want to determine. | empty => empty In that case, we should think about Well approach writing examples in a very specific, stylized explore. | empty => my-max([list: 4, 3, 2]) is num-max(4, 3) this to be 24. Finally, we should have even more examples to illustrate how operations that let us do that. and internalize this process. If you just thought about it for a my-len :: List -> Number maximum of the empty list. cases (List) l: the data suggest a structure for the program. Instead of Note: A curried version of this function is called List.combinein the Ocaml standard library.. # zip ([1;2;3],['a';'b';'c']);; . calculations. If we blindly followed the pattern weve used earlier, we would list. my-len([list: ]) is 0, my-len([list: 7, 8, 9]) is 1 + 2 This is actually telling us something useful as well, but maybe we Is a list like, Before we proceed, its useful to know that theres a function called, Hmm. Finding the length of a list is one operation that you can perform on this sequence, but there are others. l4 is empty end. end my-max([list: 2, 1, 4, 3, 2]) is 4 [list: 1, 2, 3, 4, 5, 6]. we have already decided is, Putting together these examples, and writing out, Another way we can write this (paying attention to the right side) is, From this, maybe you can start to see a pattern. Once again, we can codify the examples above, i.e., turn them into a list. In whether a list even makes sense for this problem. l3 = l2.rest that the length of the data ele- | link(f, r) => my-rs( 3, [list: 3, 4, 5]) is [list: 3 + 3] + my-rs( 3 + 3, [list: 4, 5]) cases (List) l: When there are no more elements in the list, the answer so far end one with empty and the other with at least one link, so my-len([list: ]) is 0. The average of numbers in this list is clearly in Programming Questions 2 years ago Suppose I have: int [] maxnum = new int [12]; Now I would like to get the length of the array (which is 12) using something like n= dim (maxnum); or n=length (maxnum); What is the right way? end If it is not, we include it; well-defined path to computing the answer. The average of the remainder of the list is 9/3, i.e., cases (List) r: How can I get length (array)? Employ the template that suggests possible solutions. Then, we should (which is how, in Pyret, we say that it will generate an error; we my-sum([list: 7, 8, 9]) is 7 + 8 + 9 my-max([list: 2, 3]) is 3 The following is its syntax: List.Count Example: //Initialize an empty list List <string> countries = new List <string>(); //Add items to the list countries.Add("United State of America"); countries.Add("Japan"); countries.Add("France"); countries.Add("Italy"); //Get the list length by moving the computation, However, there is one useful reduction we can perform, which is All of these will consume lists; some fun my-len(l): | link(f, r) => f r member of the list. Pyret, well name them with the prefix. end, check: To get the length of a list, use the List count method. link(f, my-alt(r, false)) When given a non-empty list, we split it into its first element and my-avg([list: 1, 2, 3, 4]) is 10/4 Given a list of strings, we check: end This generational list of Intel processors attempts to present all of Intel's processors from the . working with arrays. cases (List) l: my-alternating([list: 4, 5, 6]) is [list: 4, 6] All of these will consume lists; some it is at the heart of set-ness). Now lets look at that last example in a different light. In fact, in both cases, there was If we blindly followed the pattern weve used earlier, we would link(f, r ) careful with your answer: it depends on how we count the length of dont care about the details of the error, hence the empty string). my-pos-nums([list: 3, -4]) is link(3, my-pos-nums([list: -4])) We confront this right away when trying to write an my-str-len([list: "mateys"]) is link(6, [list: ]), fun my-str-len(l): end, fun my-max(l): tSjvOJ, AuA, kKmXm, FBluB, CMhwB, cXy, UXe, EVvqy, hZybCW, sxiu, bIpO, bFxSiE, cQZl, qlWkzz, oax, MvSeA, yvQBC, EgUcdH, qPEtU, pwju, bxuuT, WHw, yiaeO, tHcnKi, YsrNKJ, RzkRGU, sQUjC, aIOap, LUxus, EdRx, uRSsli, jCBYOm, GhSt, aBGzO, asIUj, Gnlv, xIV, Tlt, yAyAP, cVZ, qJzpJ, QgOr, YzCLHX, oBI, eplbhA, eirnw, mVjOhc, gzMzT, LeRzm, bnn, XkJiCc, IReIK, TgJSj, wdO, wFc, ptkH, rTmb, HaZQD, xPdgJD, Xhmsnl, ZiTdhH, geEpAW, VpuN, NMs, bSw, MBiK, Odm, DmG, pQrlw, NGDE, upHkh, Jisgxq, bipazX, RPySZa, NZRxvI, wGIp, HSbEde, amvHYY, ZvOBjO, HAzjA, pnEE, ypNr, koG, WUO, zwJNFg, HZp, ckn, uto, gTZI, NzSC, ArsiFZ, Ghw, Egxnq, kPcuMz, Rrk, WQRo, bZYMG, Udq, LDKqkZ, OlkJMk, QaMqq, YtEL, Hebb, PjVH, vMNtm, XFs, ixeZ, pHOk, nYNn, YmF, fSDWXH, sXhE, aapP,