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up vote 1 down vote favorite

I am new to python, so please, bear with me!

This function:

def kerf(X,m):
        [n_samples, ]= X.shape
        n_sa, n_fe = m.shape
        ker = np.zeros((n_samples, n_sa))
        for i, x_i in enumerate(X):
            for j, m_j in enumerate(m):
                ker[i, j] = (np.dot(x_i, m_j))  # problem is here!!
        return ker

I call it like this:

Z=kerf(myarray[0,[0,1]],myarray[:,[0,1]])


ker[i, j] = (np.dot(x_i, m_j))
ValueError: setting an array element with a sequence.

myarray is basically the same matrix. Why?

share|improve this question
up vote 2 down vote accepted

When I replace the problem line with:

print(np.dot(x_i, m_j).shape)

it repeatedly prints (2,).

ker[i, j] takes 1 value; 2 values is sequence.

Please give us the dimensions of the arrays at various points, such as myarray (I guessed and tried a (3,4)), and at the problem point. print(...shape) is an essential debugging tool in numpy.

Do you need help on figure out why it's (2,)? May I suggest stepping through the loop in an interactive shell, looking at shapes at various points along the way.

the 2 inputs to the dot look like:

(1.0, array([ 1.,  1.]))

a scalar, and a 2 element array - so the dot is also a 2 element array.

You need to explain what size you expect these 2 arrays to be, and what size you expect the dot. Actually we can get the result - it's got to be (1,) or a scalar - 1 value to put in the one slot ker.

You can probably replace the double iteration with a single dot product (or if need be with an einsum call). But let's get this iteration working first.

share|improve this answer
    
myarray[0,[0,1]] contains just one row of these two numbers [1,233 0.6675] while myarray[:,[0,1]] contains like 12 rows of pair values like [1.879 0.5656]. I just wanna the dot product of the one row array with all the elements of the 12 rows array. – Th. Mtt. Oct 5 '15 at 12:04
    
With this one liner: ker = np.dot(myarray[0,[0,1]],np.transpose(myarray[:,[0,1]])) I can make it work, however i am not sure if it's quite good. – Th. Mtt. Oct 5 '15 at 12:23
    
Or shorter myarray[0, [0,1]).dot(myarray[:, [0,1].T), That looks like a good dot usage. Transpose if often used with matrix multipication to correctly pair dimensions. – hpaulj Oct 5 '15 at 16:49

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