Python List built-in methods, accessing elements, selection, slicing, recursive functions, list comprehension

Transcription

1 ITP Programming Languages Chapter 2 Building Abstractions with Data 2.1 Introduction 2.2 Data Abstraction 2.3 Sequences List, Tuple, String, Linked List Major references: 1. Structure and Interpretation of Computer Programs(SICP) by Abelson and Sussman, MIT Composing Programs by John DeNero, Google 2. MITx OPENCOURSEWARE(OCW) 6.00 SC Lecture 3. UC Berkley CS61A Lecture 4. Python, Wikipedia and many on-line resources. Prof. Youngsup Kim, idebtor@handong.edu, 2014 Programming Languages, CSEE Dept., Handong Global University

2 ITP Programming Languages Chapter 2 Building Abstractions with Data 2.1 Introduction 2.2 Data Abstraction 2.3 Sequences List, Tuple, String, Linked List Python List built-in methods, accessing elements, selection, slicing, recursive functions, list comprehension Major references: 1. Structure and Interpretation of Computer Programs(SICP) by Abelson and Sussman, MIT Composing Programs by John DeNero, Google 2. MITx OPENCOURSEWARE(OCW) 6.00 SC Lecture 3. UC Berkley CS61A Lecture 4. Python, Wikipedia and many on-line resources. Prof. Youngsup Kim, idebtor@handong.edu, 2014 Programming Languages, CSEE Dept., Handong Global University

3 2.3 The Sequence Abstraction Christ 3

4 2.3 The Sequence Abstraction Christ in whom are hidden all the treasures of wisdom and knowledge Col2:3 4

5 2.3 The Sequence Abstraction Christ in whom are hidden all the treasures of wisdom and knowledge Col2:3 Taste and see that the LORD is good; Ps34:8 5

6 2.3 The Sequence Abstraction Christ in whom are hidden all the treasures of wisdom and knowledge Col2:3 Taste and see that the LORD is good; Ps34:8 The fear of the LORD is the beginning of knowledge, but fools despise wisdom and discipline. Pro1:7 6

7 2.3 The Sequence Abstraction red, orange, yellow, green, blue, indigo, violet. 7

8 2.3 The Sequence Abstraction red, orange, yellow, green, blue, indigo, violet. There isn't just one sequence type (in Python or in general) 8

9 2.3 The Sequence Abstraction red, orange, yellow, green, blue, indigo, violet. There isn't just one sequence type (in Python or in general) This abstraction is a collection of behaviors: 9

10 2.3 The Sequence Abstraction red, orange, yellow, green, blue, indigo, violet. There isn't just one sequence type (in Python or in general) This abstraction is a collection of behaviors: Length. A sequence has a finite length. Element selection. A sequence has an element corresponding to any non negative integer index less than its length, starting at 0 for the first element. 10

11 2.3 The Sequence Abstraction red, orange, yellow, green, blue, indigo, violet. 0, 1, 2, 3, 4, 5, 6. There isn't just one sequence type (in Python or in general) This abstraction is a collection of behaviors: Length. A sequence has a finite length. Element selection. A sequence has an element corresponding to any non negative integer index less than its length, starting at 0 for the first element. 11

12 2.3 The Sequence Abstraction red, orange, yellow, green, blue, indigo, violet. 0, 1, 2, 3, 4, 5, 6. There isn't just one sequence type (in Python or in general) This abstraction is a collection of behaviors: Length. A sequence has a finite length. Element selection. A sequence has an element corresponding to any non negative integer index less than its length, starting at 0 for the first element. The sequence abstraction is shared among several types, including tuples. 12

13 2.3 Python Sequence Abstraction Built-in sequence types provide the following behavior. Type specific constructor tuple >>> a = (1, 2, 3) >>> b = tuple([4, 5, 6, 7]) length tuple >>> len(a), len(b) (3, 4) list : variable length, mutable object stay tuned for more Element selection Slicing Membership >>> a[1], b[ 1] (2, 7) >>> a[1:3], b[1:1], a[:2], b[1:] ((2, 3), (), (1, 2), (5, 6, 7)) >>> 2 in a, 4 in a, 4 not in b (True, False, False) 13

14 2.3 Python Sequence Abstraction Built-in sequence types provide the following behavior. Type specific constructor tuple >>> a = (1, 2, 3) >>> b = tuple([4, 5, 6, 7]) length tuple >>> len(a), len(b) (3, 4) list : variable length, mutable object stay tuned for more Element selection >>> a[1], b[ 1] (2, 7) Counting from the end; -1 is the last element Slicing Membership >>> a[1:3], b[1:1], a[:2], b[1:] ((2, 3), (), (1, 2), (5, 6, 7)) >>> 2 in a, 4 in a, 4 not in b (True, False, False) 14

15 2.3 List as a Type in Python Tuple: fixed-length, immutable sequence of Python objects List: variable-length, and mutable >>> a = [3, 1, 2] >>> a [3, 1, 2] >>> len(a) 3 >>> a[1] 1 >>> c, d = a, a[:] >>> a, c, d ([3, 1, 2], [3, 1, 2], [3, 1, 2]) >>> c[0] = 4 >>> a, c, d ([4, 1, 2], [4, 1, 2], [3, 1, 2]) >>> d[0] = 5 >>> a, c, d ([4, 1, 2], [4, 1, 2], [5, 1, 2]) >>> a[1:2] = [7, 8, 9] >>> a, c, d ([4, 7, 8, 9, 2], [4, 7, 8, 9, 2], [5, 1, 2]) Create a list using square brackets: Lists are sequences. Bind another name to a list or a slice of a list. Modify contents of a list. wut()? 15

16 2.3 Python Sequence Iteration Python has a special statement for iterating over the elements in a sequence def count(s, value): total = 0 Name elem bound in the first frame of the current environment for elem in s: if elem == value: total += 1 return total 16

17 2.3 Python Sequence Iteration For Statement Execution Procedure for <name> in <expression>: <suite> 17

18 2.3 Python Sequence Iteration For Statement Execution Procedure for <name> in <expression>: <suite> 1. Evaluate the header <expression>, which must yield an iterable value. 18

19 2.3 Python Sequence Iteration For Statement Execution Procedure: for <name> in <expression>: <suite> 1. Evaluate the header <expression>, which must yield an iterable value. 2. For each element in that sequence, in order: Bind <name> to that element in the first frame of the current environment. Execute the <suite>. 19

20 2.3 List Comprehensions see One large expression that combines mapping and filtering to produce an iterable. [<map exp> for <name> in <iter exp> if <filter exp>] Evaluates to an iterable. <iter exp> is evaluated when the generator expression is evaluated. Remaining expressions are evaluated when elements are accessed. No filter version: [ <map exp> for <name> in <iter exp> ] Stay tuned: Precise evaluation rule introduced in Chapter 4. [Demo] 20