CODEWITHSUNDEEP
pythonarrayslistnumpyindexing
gerocampo
gerocampo

Reputation: 105

Making a index array from an array in numpy

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

Upvotes: 0

Views: 143

Answers (2)

seberg
seberg

Reputation: 8975

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)...

Upvotes: 1

Andy Hayden
Andy Hayden

Reputation: 375485

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) )

Upvotes: 2

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