[SciPy-User] "inverting" an array

[Guyer, Jonathan E. Dr.]

> On Feb 4, 2014, at 2:33 PM, "nicky van foreest" <vanforeest at gmail.com> wrote:
> 
> Hi,
> 
> I am wondering whether a shortcut exists in numpy/scipy for the following problem.  The values in an array represent the number of customers that arrive in a certain time slot, e.g.,
> 
> a = [0,4,7,3,1,5, 0,0,0,] 
> 
> means that in time slot 1 4 customers arrive, in time slot 2 seven arrive, and so on. Now I want to "invert" this array to compute the arrival time of the i-th customer. Thus, customer 2 arrives in time slot 1, customer 6 in time slot 2, and so on. For this problem I wrote the following function:
> 
> a = [0,4,7,3,1,5, 0,0,0,] 
> A = np.cumsum(a)
> 
> def invert(A):
>     Ainv = np.empty(A[-1])
>     aprev=0
>     for i, a in enumerate(A):
>         Ainv[aprev:a] = i
>         aprev = a
>     return Ainv
> 
> 
> Ainv= invert(A)
> 
> print a
> print A
> print Ainv
> 
> The output is
> 
> [0, 4, 7, 3, 1, 5, 0, 0, 0]
> [ 0  4 11 14 15 20 20 20 20]
> [ 1.  1.  1.  1.  2.  2.  2.  2.  2.  2.  2.  3.  3.  3.  4.  5.  5.  5.
>   5.  5.]
> 
> Does anybody know whether this code can be made faster, or whether a numpy/scipy function exists that establishes this in one go? 
> 
> thanks
> 
> Nicky
> 
> 
> 
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