Python: Looking for a specific value in array column

Question

I have a specific integer value k and I want to find matching values in a column of a 2d numpy array (using a "for" loop). I thought I could use an if statement and directly compare the single array element with the integer value k. Here is my code:

for i in range(mock.shape[0]):

    n_cl = int(mock[i,0]/3500.)
    zcl = mock[i,5]
    pick = [np.random.random_integers(200, size=(n_cl))]
    print pick[0]   
    if(zcl <= 0.05):

        for k in range(len(pick)) :

            for j in range(z_001.shape[0]):
                n = z_001[j,1]
                if (int(n) == pick[k]):
                    binaries[j,7] = mock[i,0]   
                    binaries[j,8] = mock[i,1]
                    binaries[j,9] = mock[i,2]
                    binaries[j,10] = mock[i,3]
                    binaries[j,11] = mock[i,4]

I always get a ValueError about the truth value of an array with more than one element, which is ambiguous. I understand that the problem is in " int(n) == k", but I wonder where am I wrong, and how could I put things better and make this part of my code work.

likely n and k are arrays, not simple numbers. Check out this piece of code, that raises the same error: if (array([1,1])==array([1,3])): print 'foo' — gg349
– gg349, Commented Aug 22, 2014 at 16:29
They are. n is an element of a 2d array, k an element of a list. — Argentina
– Argentina, Commented Aug 22, 2014 at 16:42
It's very difficult to help when we don't know what pick, i, binaries, mock, etc. all are. You'll get much more help if you can get closer to a minimal working example. — Roger Fan
– Roger Fan, Commented Aug 22, 2014 at 16:43
How can I compare a single member of an array , let's say a[i,j] to an integer number? — Argentina
– Argentina, Commented Aug 22, 2014 at 16:45

felippe · Accepted Answer · 2014-08-22 22:35:05Z

3

Well, consider:

whenever you are using numpy, you probably don't want to use for loops
The error you get means that you are comparing an numpy array to a single value

So, one possible solution to the problem could be:

import numpy as np

# generating some fake data
x = np.repeat([[1,2,3,4]], 4, axis=0) 
x[2,2] = 200
x[3,1] = 200

# retrieving the indices where "x==200" using np.where
indices = np.where(x==200)
print indices

which gives:

(array([2, 3]), array([2, 1]))

so you can just index any other array (with appropriate shape) with indices and you will get the value of that array in the indices positions.

answered Aug 22, 2014 at 22:35

felippe

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1 Comment

code base 5000 Over a year ago

numpy is exactly what I thought to use. It provides the access,sorting, and querying one needs to accomplish the task.

Argentina · Accepted Answer · 2017-02-08 14:16:26Z

0

I resolved my problem by changing the pick list definition, from pick = [np.random.random_integers(200, size=(n_cl))] to pick = np.random.random_integers(200, size=(n_cl)).

The problem occurred because I had been comparing an array value to an integer value.

edited Feb 8, 2017 at 14:16

answered Aug 24, 2014 at 10:13

Argentina

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