[SciPy-User] "inverting" an array

[nicky van foreest]

HI,

Thanks. What an elegant solutions!

bye

Nicky



On 4 February 2014 20:54, Eric Hermes <ehermes at chem.wisc.edu> wrote:

>
> On 2/4/2014 1:52 PM, Warren Weckesser wrote:
>
>
>  On Tue, 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
>>
>>
>  You can use `np.repeat`:
>
> In [10]: a
> Out[10]: [0, 4, 7, 3, 1, 5, 0, 0, 0]
>
> In [11]: np.repeat(np.arange(len(a)), a)
> Out[11]: array([1, 1, 1, 1, 2, 2, 2, 2, 2, 2, 2, 3, 3, 3, 4, 5, 5, 5, 5,
> 5])
>
>
>  Warren
>
>    I came up with a version that only uses python intrinsics:
>
> def invert(a):
>     ainv = []
>     for i, n in enumerate(a):
>         ainv += [i]*n
>     return ainv
>
> Eric
>
>
>
>
>>
>>
>>
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>>
>
>
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>
> --
> Eric Hermes
> J.R. Schmidt Group
> Chemistry Department
> University of Wisconsin - Madison
>
>
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