Topic 4: Tuples and Lists

Transcription

1 Topic 4: Tuples and Lists 1

2 Recommended Exercises and Readings From Haskell: The craft of functional programming (3 rd Ed.) Exercises: 5.1, 5.2, 5.3, 5.15, 5.16, 5.17, 5.18, 5.19, 5.20, 5.21, 5.22, 5.23, 5.24, 5.26, and , 6.5, 6.6, 6.7, 6.29, 6.30, 6.31, and , 7.2, 7.3, 7.5, 7.6, 7.8, 7.9, 7.11, 7.13, 7.16, 7.18, 7.19, 7.20, 7.21, 7.23, 7.24, 7.25, 7.27, 7.28, 7.29, 7.32, 7.33, and 7.34 Readings: Chapters 5, 6, and 7 2

3 Tuples Tuples Combine several pieces of data into one object Pieces may have different types Similar to Struct in C/C++ Data members in a Java or C++ class Record in Pascal Tuple in Python 3

4 Tuples Examples of tuple types Examples of tuple literals Tuples with exactly two elements are referred to as pairs 4

5 Tuples as Parameters A tuple can represent a fixed length vector Write functions that compute the cross product and dot product of two vectors 5

6 Lists Lists Combine several pieces of data into one object Every piece of data must have the same type The number of elements is flexible Similar to Arrays in C / C++ / Java Lists in Python Big differences to C / C++ / Java arrays: 6

7 Lists Examples of list types Examples of list literals 7

8 List Functions head tail take drop 8

9 List Functions and Operators length : ++!! 9

10 Lists as Parameters Write a function named isemptylist that returns True if the list is empty, False otherwise Write a function named issingletonlist that returns true if the list has length 1, False otherwise 10

11 Recursion and Lists Functions that operate on lists often include recursion Perform some task involving the first element in the list Call the function recursively on the rest of the list Example: Compute the sum of all the elements in a list 11

12 Example: Run Length Encoding A (basic) compression technique Works well when there are long sequences of repeated values Used by the PCX image format Decode: Convert a list of pairs into a string First element in each pair is a character Second element in each pair is the number of times it occurs Example: [( h, 1), ( e, 1), ( l, 2), ( o, 1)] decodes to hello 12

13 Example: Run Length Encoding Encode: Convert a string into a list of pairs Example: aaabbaaaaa encodes as [( a, 3), ( b, 2), ( a, 5)] 13

14 .. Sometimes it s inconvenient to write out all of the elements in a list.. removes the need to do so in some situations Simplest case: Each element is the successor of its predecessor Different increment values are also possible 14

15 Repetition Structures What repetition structures exist in imperative and object oriented languages? 15

16 Repetition Structures How do we repeat a task in Haskell? 16

17 List Comprehensions Allow us to construct a new list from an existing list Elements in the new list are the result of a calculation performed on the elements in the existing list Elements can be removed (filtered) from the old list 17

18 List Comprehensions Examples 18

19 List Comprehensions in Python Examples 19

20 List Comprehensions with Multiple Lists List comprehensions can be performed on multiple lists data_n = [1, 2, 3] data_m = [2, 5, 2] result = [n + m n < data_n, m < data_m] putstrln (show result) What is displayed by the preceding code segment? 20

21 List Comprehensions with Multiple Lists How do we process parallel lists data_n = [1, 2, 3] data_m = [2, 5, 2] putstrln (show result) display [3, 7, 5] 21

22 Combining Lists and Tuples Elements in a list can be tuples Examples: [(1, 2, 0), (3, 0, 4), (5, 7, 6)] [("A", 65), ("a", 97)] A list of tuples can be constructed by zipping two lists together The process can be reversed with unzip 22

23 Lists and Tuples Example Example: Consider the following list of titles: Alice's Adventures in Wonderland An Accountant's Nightmare Much Ado About Nothing Pride and Prejudice The Adventures of Huckleberry Finn The Matrix The Princess Bride When titles are sorted, leading articles like A, An and The are normally ignored. Write a title sort function The titles should be ordered as An Accountant's Nightmare The Adventures of Huckleberry Finn Alice's Adventures in Wonderland The Matrix Much Ado About Nothing Pride and Prejudice The Princess Bride Data.List contains a sort function that sorts strings into ASCII order 23

24 Lists and Tuples Example 24

25 Summary Lists and tuples make it practical to work with larger amounts of data Tuples: Similar to a C struct or data members in a class Lists: An ordered collection of values that all have the same type List and tuple elements are accessed via Pattern matching Functions Elements in a list can be processed using Recursion List Comprehensions 25