understanding the creation of the array of lists in python

Question

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.

list comprehension is explained on several SO topics. here is one. stackoverflow.com/questions/20639180/… — Anil_M
– Anil_M, Commented Mar 29, 2017 at 3:20

delta · Accepted Answer · 2017-03-29 03:18:10Z

1

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'}

answered Mar 29, 2017 at 3:18

delta

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Athina · Accepted Answer · 2017-03-29 03:21:40Z

1

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.

answered Mar 29, 2017 at 3:21

Athina

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Community · Accepted Answer · 2017-05-23 10:30:56Z

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.

edited May 23, 2017 at 10:30

CommunityBot

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answered Mar 29, 2017 at 3:18

Treefish Zhang

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