[Numpy-discussion] Find insertion point

[Warren Weckesser]

Nikolaus Rath wrote:
> Hello,
>
> I want to find the first i such that x[i] < y and x[i+1] >= y. Is there
> a way to do this without using a Python loop?
>
> I can't use np.searchsorted(), because my x array crosses y several
> times.
>
>
> Best,
>
>    -Nikolaus
>
>   

Here's one way:

In [32]: x
Out[32]: array([10,  3,  2,  5,  8,  7,  6,  5,  4,  7, 11])

In [33]: y = 4.1

In [34]: np.where((x[:-1] < y) & (x[1:] > y))[0][0]
Out[34]: 2

In [35]: y = 9.5

In [36]: np.where((x[:-1] < y) & (x[1:] > y))[0][0]
Out[36]: 9


(The second index [0] assumes that at least one such point is found.)


Warren