What I want to do is to subtract 1d array from other 2d array’s rows. I found, in some case, result was wrong and confused me. Below code worked good as I wanted.
lst1 = [[10,11,12,13,14], [20,21,22,23,24], [30,31,32,33,34]] lstave = [10,20,30,40,50] # lstave = [0.1,0.2,0.3,0.4,0.5] # <- This is also no problem. # transform them to nparray. arr1 = np.array(lst1).reshape(3,5) arrave = np.array(lstave) for i in range(len(lst1)): print(arr1[i] - arrave) # print(np.subtract(arr1[i], arrave)) # <- This is also work
Then I got following as expected.
[ 0 -9 -18 -27 -36] [ 10 1 -8 -17 -26] [ 20 11 2 -7 -16]
However, when I change the array “lstave” to [1, 2, 3, 4, 5] and subtract,
lst1 = [[10,11,12,13,14], [20,21,22,23,24], [30,31,32,33,34]] lstave = [1,2,3,4,5] arr1 = np.array(lst1).reshape(3,5) arrave = np.array(lstave) for i in range(len(lst1)): print(arr1[i] - arrave)
I got following.
[9 9 9 9 9] [19 19 19 19 19] [29 29 29 29 29]
I cannot understand why the only 1st element subtraction performed 5 times. I feel that broadcast is working wrong way, but I don’t know how to fix this. Can someone help me out? Thank you,
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Answer
It does actually work, just:
10 - 1
Gives:
9
But:
11 - 2
Also gives:
9
This is more of a math question.
[10 - 1, 11 - 2, 12 - 3, 13 - 4, 14 - 5]
Would give:
[9, 9, 9, 9, 9]
So your code works.
If you make arrave
to be:
[0, 1, 2, 3, 4]
All the final values would be 10
.