Question

Good morning experts,

I have an array which contain integer numbers, and I have a list with the unique values that are in the array sorted in special order. What I want is to make another array which will contain the indexes of each value in the a array.

#a numpy array with integer values
#size_x and size_y: array dimensions of a
#index_list contain the unique values of a sorted in a special order.
#b New array with the index values

for i in xrange(0,size_x):
     for j in xrange(0,size_y):                    
         b[i][j]=index_list.index(a[i][j])

This works but it takes long time to do it. Is there a faster way to do it?

Many thanks for your help

German

Answer 1

The slow part is the lookup

index_list.index(a[i][j])

It will be much quicker to use a Python dictionary for this task, ie. rather than

index_list = [ item_0, item_1, item_2, ...]

use

index_dict = { item_0:0,  item_1:1, item_2:2, ...}

Which can be created using:

index_dict = dict( (item, i) for i, item in enumerate(index_list) )