[[11. 5.]
[24. 6.]
[39. 12.]
[14. 1.]
[25. 12.]]
That's my matrix, now I want to apply the following condition to it: 10 < a < 15 but only to the first column
As result I want an array with the indices [0. 3.]
My attempts with "where" failed, numpy applied it to both columns
The first column is a[:, 0]. np.flatnonzero is a version of np.nonzero (which is where with only one argument) that returns a flat array instead of a tuple of indices.
You can do something like
mask = (10 < a[:, 0]) & (a[:, 0] < 15)
idx = np.flatnonzero(mask)
Another way is
idx, = np.nonzero(mask)
Or even
idx, = np.where(mask)
Notice the comma after idx in the last two examples, which triggers argument unpacking.