Python create empty 2-dimensional array

Question

I am trying to generate an empty 2-dimensional array by using to for-loops. I have found one method that works, and it looks like this:

rows = 5
cols = 5
grid1 = []
grid1 = [[0 for i in range(cols)] for j in range(rows)] 
print(grid1)

Output:

[[0, 0, 0, 0, 0], [0, 0, 0, 0, 0], [0, 0, 0, 0, 0], [0, 0, 0, 0, 0], [0, 0, 0, 0, 0]]

However, when I try to write the for loops in "normal" syntax it throws an error. Why can't I write it in normal syntax?

rows = 5
cols = 5

grid2 = []
for i in range(rows):
    for j in range(cols):
        grid2[i][j] = 0
print(grid2)

Output:

Exception has occurred: IndexError
list index out of range
  File "C:\Users\Bruker\Downloads\test.py", line 8, in <module>
    grid2[i][j] = 0

You can't assign to an index in the list beyond the current length of the list. The equivalent way to write the list comprehension would be to use append. — dspencer
– dspencer, Commented Apr 3, 2020 at 8:37

dspencer · Accepted Answer · 2020-04-03 08:45:39Z

2

You get the IndexError because you can't assign to an index in the list beyond the current length of the list. Since grid2 is initialized to an empty list, any attempt to index it will raise this error.

One correct way to write your nested list comprehension using for loops would be to construct the inner list first for each row, then append this to grid2:

grid2 = []
for i in range(rows):
    inner = []
    for j in range(cols):
        inner.append(0)
    grid2.append(inner)

edited Apr 3, 2020 at 8:45

answered Apr 3, 2020 at 8:39

dspencer

4,5014 gold badges25 silver badges46 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

yuri · Accepted Answer · 2020-04-03 08:45:28Z

2

you should initialize grid2=np.empty([rows, cols]) instead please refer to https://docs.scipy.org/doc/numpy/reference/generated/numpy.empty.html#numpy.empty for more details

answered Apr 3, 2020 at 8:45

yuri

1191 silver badge9 bronze badges

2 Comments

dspencer Over a year ago

If using numpy, why not np.zeros? This seems more aligned with what OP wants to do.

yuri Over a year ago

since you want to fill your array with zeros then it makes more sense to initialize it with zeros (np.zeros), otherwise empty declaration is faster if you later want to fill it with non zeros values

azro · Accepted Answer · 2020-04-03 08:39:18Z

0

In the first case each cell exists, so you can access them by index, but in the second case they do not still exist, the other way is

grid2 = []
for i in range(rows):
    tmp = []
    for j in range(cols):
        tmp.append(0)
    grid2.append(tmp)

And in fact if you reduce this code once you have the following, and reducing again with another list comprehension you have your first code

grid2 = []
for i in range(rows):
    grid2.append([0 for j in range(cols)])

answered Apr 3, 2020 at 8:39

azro

54.2k9 gold badges38 silver badges75 bronze badges

Comments

kederrac · Accepted Answer · 2020-04-03 08:42:08Z

0

you are trying to access the second "dimension" or an inner loop without creating here, you may use:

rows = 5
cols = 5

grid2 = []
for _ in range(rows):
    grid2.append([])
    for _ in range(cols):
        grid2[-1].append(0)
print(grid2)

or you may use:

grid2 = []
for _ in range(rows):
    grid2.append([0] * cols)

answered Apr 3, 2020 at 8:42

kederrac

17.4k6 gold badges36 silver badges58 bronze badges

Comments

Hemant Jaiman · Accepted Answer · 2022-01-27 14:27:07Z

0

Try this:

rows = 5
cols = 5

arr = [[0 for x in range(cols)] for x in range(rows)]

Output:

[[0, 0, 0, 0, 0],
 [0, 0, 0, 0, 0],
 [0, 0, 0, 0, 0],
 [0, 0, 0, 0, 0],
 [0, 0, 0, 0, 0]]

answered Jan 27, 2022 at 14:27

Hemant Jaiman

1,1817 silver badges15 bronze badges

Comments

Ben Woo · Accepted Answer · 2020-04-03 08:42:27Z

-1

You need to use .append() fucntion to add elements to list.

grid2 = []
for i in range(rows):
    grid2.append([])
    for j in range(cols):
        grid2[i].append(0)
print(grid2)

more efficiently:

grid2 = [[0]*cols] * rows

answered Apr 3, 2020 at 8:42

Ben Woo

701 silver badge5 bronze badges

1 Comment

xvan May 13 at 4:59

grid2 = [[0]*2] * 3 ; grid2[2][1]=3 -> [[0, 3], [0, 3], [0, 3]]

Collectives™ on Stack Overflow

Python create empty 2-dimensional array

6 Answers 6

Comments

2 Comments

Comments

Comments

Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

6 Answers 6

Comments

2 Comments

Comments

Comments

Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related