Constructing Lists

Question

I'm new to Python and I came across the following query. Can anyone explain why the following:

[ n**2 for n in range(1, 6)]

gives:

[1, 4, 9, 16, 25]

• squiguy: Here is some helpful reading: docs.python.org/2/tutorial/…

Answer 1

It is called a list comprehension. What is happening is similar to the following:

results = []
for n in range(1,6):
 results.append(n**2)

It therefore iterates through a list containing the values [0, 1, 2, 3, 4, 5] and squares each value. The result of the squaring is then added to the results list, and you get back the result you see (which is equivalent to 0**2, 1**2, 2**2, etc., where the **2 means 'raised to the second power').

This structure (populating a list with values based on some other criteria) is a common one in Python, so the list comprehension provides a shorthand syntax for allowing you to do so.

Answer 2

Breaking it down into manageable chunks in the interpreter:

>>> range(1, 6)
[1, 2, 3, 4, 5]

>>> 2 ** 2 # `x ** 2` means `x * x`
4
>>> 3 ** 2
9

>>> for n in range(1, 6):
...  print n
1
2
3
4
5
>>> for n in range(1, 6):
... print n ** 2
1
4
9
16
25

>>> [n ** 2 for n in range(1, 6)]
[1, 4, 9, 16, 25]

Answer 3

So that's a list comprehension.

If you break it down into 3 parts; separated by the words: 'for' and 'in' ..

eg.

[ 1 for 2 in 3 ]

Probably reading it backwards is easiest:

• 3 - This is the list of input into the whole operation
• 2 - This is the single item from the big list
• 1 - This is the operation to do on that item

part 1 and 2 are run multiple times, once for each item in the list that part 3 gives us. The output of part 1 being run over and over, is the output of the whole operation.

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So in your example:

• 3 - Generates a list: [1, 2, 3, 4, 5] -- Range runs from the first param to one before the second param
• 2 - 'n' represents a single number in that list
• 1 - Generates a new list of n**2 (n to the power of 2)

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So an equivalent code would be:

result = []
for n in range(1, 6):
 result.append(n**2)

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Finally breaking it all out:

input = [1, 2, 3, 4, 5]
output = []
v = input[0] # value is 1
o = v**2 # 1 to the power of two is 1
output.append(o)
v = input[1] # value is 2
o = v**2 # 2 to the power of two = (2*2) = 4
output.append(o)
v = input[2] # value is 3
o = v**2 # 3 to the power of two is = (3*3) = 9
output.append(o)
v = input[3] # value is 4
o = v**2 # 4 to the power of two is = (4*4) = 16
output.append(o)
v = input[4] # value is 5
o = v**2 # 5 to the power of two is = (5*5) = 25
output.append(o)

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