find index of first element in array that matches some condition

Question

I want a pretty way to find the index of the first element of an np.array, that matches some condition. The condition is:

array>constant

This is my solution:

first_index = ((array<constant)*np.arange(len(array))).argmax()+1

Is there a prettier one-line way to do this in numpy or python?

Well that solution is one-line. Why the +1? Perhaps argwhere? — Dani Mesejo
– Dani Mesejo, Commented Feb 8, 2019 at 18:49
But it's not pretty, and I was wondering it there is a prettier way — Shay Lempert
– Shay Lempert, Commented Feb 8, 2019 at 18:50
Does your solution in the code actually works, it appears it returns the last index. — Dani Mesejo
– Dani Mesejo, Commented Feb 8, 2019 at 18:54
It returns the last index of the opposite condition(+1), I forgot to mention that — Shay Lempert
– Shay Lempert, Commented Feb 8, 2019 at 18:54

Mihirp · Accepted Answer · 2019-02-08 18:57:17Z

2

Argmax goes sequentially if you look at the document: so it will find the first true value

https://docs.scipy.org/doc/numpy-1.15.0/reference/generated/numpy.argmax.html

So you could use

np.argmax(array<val)

answered Feb 8, 2019 at 18:57

Mihirp

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2 Comments

Shay Lempert Over a year ago

Thanks, for my purposes I need np.argmin(), but the principle works. (I am looking for the opposite condition than the one in the expression).

anishtain4 Over a year ago

@ShayLempert argmax simply finds the first element that has met the condition and is True. If you want it the other way around, switch the direction of inequality instead of using argmin.

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