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Python Programming Essentials - M34 - List Comprehensions

This document discusses Python list comprehensions, which provide a concise way to generate new lists from existing lists. It covers the basic syntax of list comprehensions, using filters to apply conditions, and nesting loops to generate lists of tuples from multi-dimensional data like matrices. Examples are given to demonstrate how list comprehensions can replace traditional for loops for generating new lists in a more compact and Pythonic way.

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http://www.skillbrew.com /Skillbrew Talent brewed by the industry itself List Comprehensions Pavan Verma @YinYangPavan Founder, P3 InfoTech Solutions Pvt. Ltd. 1 Python Programming Essentials
© SkillBrew http://skillbrew.com Contents  Introduction  Syntax  filter  Nested loops 2
© SkillBrew http://skillbrew.com Introduction Lets say you are given a list and your task is to generate another list of cubes sample = [10, 20, 30, 40] cubes = [] for x in sample: cubes.append(x**3) print cubes Output: [1000, 8000, 27000, 64000] 3
© SkillBrew http://skillbrew.com Lets write a List comprehension sample = [10, 20, 30, 40] cubes = [ x**3 for x in sample_list] print cubes Output: [1000, 8000, 27000, 64000] 4 sample = [10, 20, 30, 40] cubes = [] for x in sample: cubes.append(x**3) vs
© SkillBrew http://skillbrew.com Syntax cubes = [ x**3 for x in sample_list] Expression For loop 5
© SkillBrew http://skillbrew.com Filter Lets tweak the problem a little. Generate a list of cubes but do the operation only on elements that are even. sample = [10, 21, 33, 40] cubes = [ x**3 for x in sample if x%2 == 0 ] print cubes Output: [1000, 64000] 6
© SkillBrew http://skillbrew.com Nested for loops Lets say you have a 3 3 Matrix and you have to print out all the coordinate pairs 0 1 2 0 (0,0) (0, 1) (0, 2) 1 (1,0) (1, 1) (1, 2) 2 (2,0) (2, 1) (2, 2) 7
© SkillBrew http://skillbrew.com Naive way matrix = [] for x in range(3): for y in range(3): matrix.append((x,y)) print matrix Output: [(0, 0), (0, 1), (0, 2), (1, 0), (1, 1), (1, 2), (2, 0), (2, 1), (2, 2)] 8
© SkillBrew http://skillbrew.com With List comprehensions matrix = [(x,y) for x in range(3) for y in range(3)] print matrix Output: [(0, 0), (0, 1), (0, 2), (1, 0), (1, 1), (1, 2), (2, 0), (2, 1), (2, 2)] 9
© SkillBrew http://skillbrew.com Takeaways 1. You have to write less code 2. Keeps the code clean as the syntax is precise and a one-liner 10
© SkillBrew http://skillbrew.com Resources  http://www.pythonforbeginners.com/list s/list-comprehensions-in-python/ 11
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