lecture07

lecture07 - Caltech CS 1: Fall 2010 Lecture 7 : October 18,...

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Unformatted text preview: Caltech CS 1: Fall 2010 Lecture 7 : October 18, 2010 Odds and Ends Caltech CS 1: Fall 2010 while loops break Files opening, closing, reading from files Caltech CS 1: Fall 2010 A smorgasbord of topics: More on booleans More on loops looping over files with for The range function Tuples The enumerate function Caltech CS 1: Fall 2010 A smorgasbord of topics: More on booleans More on loops looping over files with for The range function Tuples The enumerate function Sequence slices Caltech CS 1: Fall 2010 Booleans are True / False values However, Python is not strict about this Several things besides the False value are considered to be False by Python the empty string: '' the empty list: the number 0 ... and most other values can be considered to be True Caltech CS 1: Fall 2010 Python also supports three boolean operators take booleans arguments, return booleans not , and , and or not converts True False , False True >>> not True False >>> not False True Caltech CS 1: Fall 2010 >>> not 0 True >>> not 1 False >>> not '' # empty string True >>> not True Caltech CS 1: Fall 2010 and and or are operators that take two boolean arguments and return a boolean value and returns True if both arguments are True , otherwise False or returns True if either or both arguments are True , otherwise False Caltech CS 1: Fall 2010 >>> True and True True >>> True and False False >>> False or True True >>> False or False False Usually don't use and and or this way, though Caltech CS 1: Fall 2010 Generally use and and or with expressions that evaluate to booleans e.g. relational operators a = 10 b = 30 # ... later in code ... if a > 0 and b < 50 : ... # ... later in code ... if a < 0 or b > 50 : ... Caltech CS 1: Fall 2010 Recall the example problem from last time: temps = open('temps.txt', 'r') sum_nums = 0.0 while True: line = temps.readline() if line == '': break sum_nums += float(line) Caltech CS 1: Fall 2010 We had these lines of code in the example: if line == '' : break Since the empty string '' can be considered to be a False value, can write this in equivalent form: if not line : break not line means "if line is empty" or "if there is no line" Caltech CS 1: Fall 2010 With an infinite loop and not line : temps = open('temps.txt', 'r') sum_nums = 0.0 while True: line = temps.readline() if not line: break sum_nums += float(line) This is OK but still somewhat long-winded Caltech CS 1: Fall 2010 Python allows an amazing shortcut using the for statement: temps = open('temps.txt', 'r') sum_nums = 0.0 for line in temps : sum_nums += float(line) This is the preferred way to write this Caltech CS 1: Fall 2010 Previously, we had: for <name> in <list>: # block of code The thing after the in had to be a list or a string Python actually allows more than just lists or strings after the in files being one example Caltech CS 1: Fall 2010...
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This note was uploaded on 02/22/2011 for the course CS 1 taught by Professor Pinkston,d during the Fall '08 term at Caltech.

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lecture07 - Caltech CS 1: Fall 2010 Lecture 7 : October 18,...

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