Lecture 13: Nested Lists, Tuples, and Dictionaries (Sections - PowerPoint PPT Presentation

http://www.cs.cornell.edu/courses/cs1110/2019sp Lecture 13: Nested Lists, Tuples, and Dictionaries (Sections 11.1-11.5, 12.1-12) CS 1110 Introduction to Computing Using Python [E. Andersen, A. Bracy, D. Gries, L. Lee, S. Marschner, C. Van Loan, W. White]

Nested Lists • Lists can hold any objects • Lists are objects • Therefore lists can hold other lists! x[2] b = [3, 1] x[2][2] x[1] c = [1, 4, b] x = [1, [2, 1], [1, 4, [3, 1]], 5] a = [2, 1] x = [1, a, c, 5] x[2][0] x[2][2][0] x[0] x[1][1] 2

Two Dimensional Lists Table of Data Images 0 1 2 3 0 1 2 3 4 5 6 7 8 9 101112 0 Each row, col 0 5 4 7 3 1 2 has a value 3 Each row, col has 1 4 8 9 7 4 5 an RGB value 5 1 2 3 6 2 7 8 4 1 2 9 3 9 10 11 6 7 8 0 4 12 Store them as lists of lists (" row-major order ") d = [[5,4,7,3],[4,8,9,7],[5,1,2,3],[4,1,2,9],[6,7,8,0]] 3

Overview of Two-Dimensional Lists 0 1 2 3 5 4 7 3 0 1 4 8 9 7 5 1 2 3 2 4 1 2 9 3 >>> d = [[5,4,7,3],[4,8,9,7],[5,1,2,3],[4,1,2,9]] Access value at row 3, col 2 >>> d[3][2] 2 Assign value at row 3, col 2 >>> d[3][2] = 8 Number of rows of d >>> len(d) 4 Number of cols in row 2 of d >>> len(d[2]) 4 >>> d [[5, 4, 7, 3], [4, 8, 9, 7],[5, 1, 2, 3], [4, 1, 8, 9]]

How Multidimensional Lists are Stored b = [[9, 6, 4], [5, 7, 7]] 9 6 4 5 7 7 Heap Space id2 id3 id1 Global 0 5 0 9 Space 0 id2 1 7 1 6 1 id3 id1 b 2 2 7 4 • b holds id of a one-dimensional list § Has len(b) elements • b[i] holds id of a one-dimensional list § Has len(b[i]) elements 5

Ragged Lists: Rows w/ Different Length • b = [[17,13,19],[28,95]] Heap Space Global Space id2 id1 id3 id1 0 17 b 0 id2 0 28 1 13 1 id3 1 95 19 2 6

Slices and Multidimensional Lists • Only “top-level” list is copied. • Contents of the list are not altered id2 9 b = [[9, 6], [4, 5], [7, 7]] 0 6 1 Heap Space Global Space id1 id1 b id3 0 id2 0 4 id5 x 1 id3 1 5 2 id4 x = b[:2] id4 id5 7 0 id2 0 7 1 id3 7 1

Slices & Multidimensional Lists (Q1) • What is now in x ? • Create a nested list >>> b = [[9,6],[4,5],[7,7]] • Get a slice A: [[9,6,10]] >>> x = b[:2] B: [[9,6],[4,5,10]] • Append to a row of x C: [[9,6],[4,5,10],[7,7]] D: [[9,6],[4,10],[7,7]] >>> x[1].append(10) E: I don’t know 8

Slices & Multidimensional Lists (A1) • What is now in x ? • Create a nested list >>> b = [[9,6],[4,5],[7,7]] • Get a slice A: [[9,6,10]] >>> x = b[:2] B: [[9,6],[4,5,10]] • Append to a row of x C: [[9,6],[4,5,10],[7,7]] D: [[9,6],[4,10],[7,7]] >>> x[1].append(10) E: I don’t know 9

Slices & Multidimensional Lists (Q2) • What is now in b ? • Create a nested list >>> b = [[9,6],[4,5],[7,7]] • Get a slice A: [[9,6],[4,5],[7,7]] >>> x = b[:2] B: [[9,6],[4,5,10]] • Append to a row of x C: [[9,6],[4,5,10],[7,7]] D: [[9,6],[4,10],[7,7]] >>> x[1].append(10) E: I don’t know • x now has nested list [[9, 6], [4, 5, 10]] 10

Slices & Multidimensional Lists (A2) • What is now in b ? • Create a nested list >>> b = [[9,6],[4,5],[7,7]] • Get a slice A: [[9,6],[4,5],[7,7]] >>> x = b[:2] B: [[9,6],[4,5,10]] • Append to a row of x C: [[9,6],[4,5,10],[7,7]] D: [[9,6],[4,10],[7,7]] >>> x[1].append(10) E: I don’t know • x now has nested list [[9, 6], [4, 5, 10]] 11

Data Wrangling: Transpose Idea 1 2 1 3 5 7 3 4 2 4 6 8 5 6 7 8 4 lists: 2 elements in each 2 lists: 4 elements in each How to transpose? 1 st element of each list gets appended to 1 st list • 2 nd element of each list gets appended to 2 nd list • 12

Data Wrangling: Transpose Code def transpose(table): """Returns: copy of table with rows and columns swapped 1 2 Precondition: table is a (non-ragged) 2d List""" 3 4 n_rows = len(table) n_cols = len(table[0]) # All rows have same no. cols 5 6 new_table = [] # Result accumulator for c in range(n_cols): row = [] # Single row accumulator for r in range(n_rows): row.append(table[r][c]) # Build up new row 1 3 5 new_table.append(row) # Add new row to new table 2 4 6 return new_table d = [[1,2],[3,4],[5,6]] d_v2 = transpose(d) 13

Tuples strings: tuples*: lists: immutable sequences immutable sequences mutable sequences of characters of any objects of any objects * “tuple” generalizes “pair,” “triple,” “quadruple,” … • Tuples fall between strings and lists § write them with just commas: 42, 4.0, ‘x’ § often enclosed in parentheses: ( 42, 4.0, ‘x’) Use tuples for: Use lists for: • short sequences • long sequences • heterogeneous sequences • homogeneous sequences • fixed length sequences • variable length sequences 14

Returning multiple values • Can use lists/tuples to return multiple values INCHES_PER_FOOT = 12 def to_feet_and_inches(height_in_inches): feet = height_in_inches // INCHES_PER_FOOT inches = height_in_inches % INCHES_PER_FOOT return (feet, inches) all_inches = 68 (ft,ins) = to_feet_and_inches(all_inches) print(You are “+str(ft)+” feet, “+str(ins)+” inches.”) 15

Dictionaries (Type dict ) Description Python Syntax • Create with format: • List of key-value pairs {key1:value1, key2:value2, …} § Keys are unique • Keys must be immutable § Values need not be § ints, floats, bools, strings • Example: net-ids § Not lists or custom objects § net-ids are unique (a key) • Values can be anything § names need not be (values) • Example: § js1 is John Smith (class ’13) d = {'ec1':'Ezra Cornell', § js2 is John Smith (class ’16) 'ec2':'Ezra Cornell', 'tm55':'Toni Morrison'} 16

Using Dictionaries (Type dict ) >>> d = {'ec1':'Ezra', 'ec2':'Ezra', 'tm55':'Toni'} >>> d['ec1'] 'Ezra' Global Space >>> d[0] Traceback (most recent call last): d id8 File "<stdin>", line 1, in <module> KeyError: 0 Heap Space >>> d[:1] Traceback (most recent call last): id8 File "<stdin>", line 1, in <module> dict TypeError: unhashable type: 'slice' 'Ezra' 'ec1' >>> 'Ezra' 'ec2' • Can access elements like a list 'Toni' 'tm55' • Must use the key, not an index 17 • Cannot slice ranges

Using Dictionaries (Type dict ) d = {'ec1':'Ezra','ec2':'Ezra', • Dictionaries are mutable 'tm55':'Toni'} § Can reassign values § d['ec1'] = 'Ellis' d id8 id8 dict 'Ezra' 'ec1' 'Ezra' 'ec2' 'Toni' 'tm55' 18

Using Dictionaries (Type dict ) d = {'ec1':'Ezra','ec2':'Ezra', • Dictionaries are mutable 'tm55':'Toni'} § Can reassign values § d['ec1'] = 'Ellis' d id8 id8 dict 'ec1' û 'Ezra' 'Ellis' 'Ezra' 'ec2' 'Toni' 'tm55' 19

Using Dictionaries (Type dict ) d = {'ec1':'Ezra','ec2':'Ezra', • Dictionaries are mutable 'tm55':'Toni'} § Can reassign values § d['ec1'] = 'Ellis' d id8 § Can add new keys id8 dict § d['psb26'] = 'Pearl' 'ec1' û 'Ezra' 'Ellis' 'Ezra' 'ec2' 'Toni' 'tm55' 20

Using Dictionaries (Type dict ) d = {'ec1':'Ezra','ec2':'Ezra', • Dictionaries are mutable 'tm55':'Toni','psb26':'Pearl'} § Can reassign values § d['ec1'] = 'Ellis' d id8 § Can add new keys id8 dict § d['psb26'] = 'Pearl' 'ec1' û 'Ezra' 'Ellis' 'Ezra' 'ec2' 'Toni' 'tm55' 'Pearl' 'psb26' 21

Using Dictionaries (Type dict ) d = {'ec1':'Ezra','ec2':'Ezra', • Dictionaries are mutable 'tm55':'Toni','psb26':'Pearl'} § Can reassign values § d['ec1'] = 'Ellis' d id8 § Can add new keys id8 dict § d['psb26'] = 'Pearl' 'ec1' û 'Ezra' 'Ellis' § Can delete keys 'Ezra' 'ec2' § del d['tm55'] 'Toni' 'tm55' 'Pearl' 'psb26' 22

Using Dictionaries (Type dict ) d = {'ec1':'Ezra','ec2':'Ezra', • Dictionaries are mutable 'psb26':'Pearl'} § Can reassign values § d['ec1'] = 'Ellis' d id8 § Can add new keys id8 dict § d['psb26'] = 'Pearl' 'ec1' û 'Ezra' 'Ellis' § Can delete keys 'Ezra' 'ec2' § del d['tm55'] û û 'Erik' 'tm55' 'Pearl' 'psb26' Deleting key deletes both 23