Slides from the training on the topic "Python Programming Essentials"

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Talent brewed by the industry itself
List Comprehensions
Pavan Verma
@YinYangPavan
Founder, P3 InfoTech Solutions Pvt. Ltd.
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Python Programming Essentials

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Contents
 Introduction
 Syntax
 filter
 Nested loops
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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]
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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]
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sample = [10, 20, 30, 40]
cubes = []
for x in sample:
cubes.append(x**3)
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Syntax
cubes = [ x**3 for x in sample_list]
Expression For loop
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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]
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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)
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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)]
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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)]
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Takeaways
1. You have to write less code
2. Keeps the code clean as the syntax is precise and
a one-liner
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Resources
 http://www.pythonforbeginners.com/list
s/list-comprehensions-in-python/
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