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I was reading about how to declare a matrices in python and I find this question How to define two-dimensional array in python

It's work with me, but I understanding what was done here in part, what I don't understanding is the parameter before the "for" in both loops...So I going to my terminal and testing parts of this for one by one, so when I type:

0 for x in range(w):

I receive:

File "< stdin >", line 1

So I try:

[0 for x in range(w)] for y in range(h):

receive:

File "< stdin >", line 1 [0 for x in range(w)] for y in range(h):

So I try:

[0 for x in range(w)]

and

[[0 for x in range(w)] for y in range(h)]

and it's work...

So why the loop works when I put the brackets and not work without the brackets?

Thanks in advance.

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Douglas da Dias Silva
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3 Answers3

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It is list comprehension in python. And similar, there is also set comprehension, dict comprehension. It uses the elements in for loop to construct the list. Such as

>>> [2*i for i in range(10)]
[0, 2, 4, 6, 8, 10, 12, 14, 16, 18]
>>> [i for i in range(10) if i % 2 == 1]
[1, 3, 5, 7, 9]
>>> [0 for _ in range(10)]
[0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
>>> [[i for i in range(j)] for j in range(5)]
[[], [0], [0, 1], [0, 1, 2], [0, 1, 2, 3]]
>>> {i : chr(65+i) for i in range(5)}
{0: 'A', 1: 'B', 2: 'C', 3: 'D', 4: 'E'}
  1. https://www.python.org/dev/peps/pep-0202/
  2. https://www.python.org/dev/peps/pep-0274/
delta
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You need the brackets so that the python interpreter processes the operations within the brackets in the correct order. Otherwise, the statement is not being interpreted correctly.

Athina
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0

You are looking at a list comprehension(please look over 5.1.4. Nested List Comprehensions of the link). The same task may be achieved using generator expression which does not involve brackets, but a generator does not create a list.

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Treefish Zhang
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