Find index in a case of 2D array

Question

I want to find 2D array's index and make it array. for example:

data_pre=[[1,1,1,0,0,0],[1,0,1,0,0,0],[1,0,0,0,1,0],[1,0,0,0,0,0]]

i wanna find index that have one and wanna make it like this

b=[[0,1,2],[0,2],[0,4],[0]]

Code:

result = []
for i in range(len(data_pre)):
    arr=data_pre[i]
    currentArrResult=[]
    for j in range(len(arr)):
        if arr[j]==1:
            currentArrResult.append(j)
            result.append(currentArrResult)

I tried like that but output is wrong.

[[0, 1, 2], [0, 1, 2], [0, 1, 2], [0, 2], [0, 2], [0, 4], [0, 4], [0]]

I don't know which part is wrong...

You should reduce indentation of the last line so that it is at the same level as the innermost for loop. Right now, you are executing `result.append(currentArrResult)` for each `i` and `j`. I suppose that you mean to execute it once for each `i`. — hilberts_drinking_problem, Jan 01 '22 at 06:49
By the way, you can express this more compactly as a list comprehension: `[[i for i, b in enumerate(row) if b] for row in data_pre]`. — hilberts_drinking_problem, Jan 01 '22 at 06:50
This pattern: `for i in range(len(data_pre))` is unnecessarily roundabout. We can directly iterate over `data_pre` with `for arr in data_pre`. If the index is needed, we can use `enumerate`: `for i, arr in enumerate(data_pre)`. — Chris, Jan 01 '22 at 06:55
Using `numpy`: [`np.where(np.any(x==15, axis=1)) for x in data_pre]`. See https://stackoverflow.com/questions/16094563/numpy-get-index-where-value-is-true — Larry the Llama, Jan 01 '22 at 07:04

score 0 · Accepted Answer · answered Jan 01 '22 at 08:38

you should not collect output inside the inner loop. that may get a result like this :

[[0],[0, 1],[0, 1, 2],
[0],[0, 2],
[0],[0, 4],
[0]]

you can check that by printing currentArrResult after append finish.
but you got different outcome because of data reference.

you should collect result after inner loop finish its work.
like:

result = []
for i in range(len(data_pre)):
    arr=data_pre[i]
    currentArrResult=[]
    for j in range(len(arr)):
        if arr[j]==1:
            currentArrResult.append(j)
           #print(currentArrResult)
    result.append(currentArrResult)

Find index in a case of 2D array

1 Answers1