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python3:list_comprehensions

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python3:list_comprehensions [2019/05/07 13:57]
jguerin Added several list-comprehension examples. 		python3:list_comprehensions [2019/05/07 16:19] (current)
jguerin Moved the multiple loop example, switched x*2 to abs(x) in list example.
Line 20: Line 20:
 [0, 1, 4, 9, 16, 25, 36, 49, 64, 81] [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
 </code> </code>
 +
 +
 +
 +===== Example Functionality =====
  
 ==== Multiple Loops ==== ==== Multiple Loops ====
-Note that this basic syntax is easily extended to multiple loops and variables: 
 <code python> <code python>
 >>> [(x**2, y**2) for x in range(3) for y in range(3)] >>> [(x**2, y**2) for x in range(3) for y in range(3)]
Line 28: Line 31:
 </code> </code>
  
-===== Examples ===== +====Conditionals====
-Conditionals:+
 <code python> <code python>
 >>> [(x, y) for x in [1,2,3] for y in [3,1,4] if x != y] >>> [(x, y) for x in [1,2,3] for y in [3,1,4] if x != y]
Line 35: Line 37:
 </code> </code>
  
-Repetitions:+====Repetitions====
 <code python> <code python>
 >>> [i for i in range(1, 5) for j in range(i)] >>> [i for i in range(1, 5) for j in range(i)]
Line 41: Line 43:
 </code> </code>
  
-Processing non-range() iterables:+====Processing non-range() iterables====
 <code python> <code python>
 >>> vec = [-4, -2, 0, 2, 4] >>> vec = [-4, -2, 0, 2, 4]
->>> [x*2 for x in vec] +>>> [abs(x) for x in vec] 
-[-8, -4, 0, 4, 8]+[4, 2, 0, 2, 4]
 </code> </code>
  
-Flattening a double list:+ 
 +====Creating a double list==== 
 +<code python> 
 +>>> n = 3 
 +>>> [[i for i in range(j*n, j*n+n)] for j in range(n)] 
 +[[0, 1, 2], [3, 4, 5], [6, 7, 8]] 
 +</code> 
 + 
 +====Flattening a double list====
 <code python> <code python>
 >>> vec = [[1,2,3], [4,5,6], [7,8,9]] >>> vec = [[1,2,3], [4,5,6], [7,8,9]]
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 </code> </code>
  
 +==== Benchmarks ====
 +List comprehensions are typically faster than their loop-based counterparts, often without fine-tuning. The below tests are taken directly from the above examples.
 +
 +<file python square_comprehension.py>
 +n = int(10000)
 +squares = [([(x**2, y**2) for x in range(n) for y in range(n)])]
 +</file>
 +
 +<file python square_loop.py>
 +n = int(argv[1])
 +
 +squares = []
 +
 +for i in range(n):
 +    for j in range(n):
 +        squares.append((i**2, j**2))
 +</file>
 +
 +^ %%n%%          ^ square_comprehension.py((Python 3.5.2))          ^ square_loop.py          ^
 +^ 500   | .1864s    | .2152s        |
 +^ 1000  | .7104s    | .8147s        |
 +^ 1500  | 1.572s    | 1.8304s       |
 +^ 2000  | 2.7992    | 3.28s         |
 +
 +Note that both of the above examples require storage of (potentially) millions of tuples. Many problems will not necessarily require //storage// of this many values in memory, but instead iterating over them. In such an instances iteration using for loops or [[python3:generators|Generators]] may be advisable.
python3/list_comprehensions.1557255444.txt.gz · Last modified: 2019/05/07 13:57 by jguerin

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