Closed Lab Solution Review Closed Lab 01 CS 224 Introduction to - PowerPoint PPT Presentation

2/10/20 Closed Lab Solution • Review Closed Lab 01 CS 224 Introduction to Python Spring 2020 Class #07: Iteration 1

2/10/20 Preliminaries: Random Preliminaries: Additional List Operations • random provides a number of helpful methods • List membership: in • random() returns a float in [0, 1) • returns Boolean • randint(x, y) returns an int in [x, y] • e in L • shuffle(L) permutes L in place • List of consecutive integers: range • Multiple ways to import • returns a list • import random • range(10) [0, 1, …, 9] • random.random, random.randint, random.shuffle • range(100, 200) [100, 101, …, 199] • import random as rd • Assignment: = • rd.random, rd.randint, rd.shuffle • creates an alias • from random import random, randint, shuffle • random, randint, shuffle 2

2/10/20 Iteration Iteration • while loops • for loops with range and len i = 0 for i in range(len(L)): • range(len(L)) returns list with an • similar to other languages with minor while i < 10: instructions syntactic differences int value, beginning with 0, for each instructions element in a list L i += 1 • iterator • for loops for e in L: for i in range(x): • operate on each element of list • primarily list based instructions instructions • e takes value of each element in list in turn 3

2/10/20 A big BUT concerning previous note Important note about iterators • Consider code this code fragment • Let L be a list of lists for e in L: • Consider code this code fragment for e in L: e = 2 * e • e is really a reference to each e[0] = 2 * e[0] element in list • e is a reference to each element • e = 2 * e reassigns the in list reference but doesn’t affect the • e[0] = 2 * e[0] does value stored in the list not reassign the reference • Thus L is unchanged • Thus L is updated 4