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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

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Use numpy sorting, fancy indexing or whatever you need. (Or for example np.unique with its optional returns) It should be much much faster then the dictionary based method as well. – seberg Sep 11 '12 at 10:16

2 Answers

up vote 2 down vote accepted

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) )
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Hayden Many thanks for your help, it works very well compared to my code, big big difference on run time. Best regards German – gerocampo Sep 11 '12 at 9:53
up vote 1 down vote

Didn't try, but as this is pure numpy, it should be much faster then a dictionary based approach:

# note that the code will use the next higher value if a value is
# missing from index_list.
new_vals, old_index = np.unique(index_list, return_index=True)

# use searchsorted to find the index:
b_new_index = np.searchsorted(new_vals, a)

# And the original index:
b = old_index[b_new_index]

Alternatively you could simply fill any wholes in index_list.

Edited code, it was as such quite simply wrong (or very limited)...

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I have a problem with this, it works for consecutive values in the index_list (i.e. [0,1,2,3] , when I have non consecutive values (i.e. [0,1,3,5] it doesn't work,please have a look: import numpy as np a = np.random.random((11, 13))*100 a=a.astype(int) list_colors=np.unique(a) print "list colors",list_colors new_vals = np.argsort(list_colors) print "new vals",new_vals b = new_vals[a] – gerocampo Sep 13 '12 at 12:14
Have reworked to use searchsorted... the old code would have needed to fill the holes, yeah... – seberg Sep 13 '12 at 12:46

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