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pythonarraysnumpy
Manoj
Manoj

Reputation: 971

Multiplying each element of a numpy array and summing it up

I have two numpy arrays, X and y. X is of size m, and y is of size n. I need to multiply each element of y by every element of X, and then sum up.

Something like [sum(X[0]*y) sum(X[1]*y) sum(X[n]*y)]

This is what I mean

np.sum(X[:, np.newaxis] * y, axis=1)

However typically X and y are really large and so doing

X[:, np.newaxis] * y

creates a huge temporary array, which blows up stuff. Is there a better way of implementing this?

Upvotes: 2

Views: 2298

Answers (1)

Alex Riley
Alex Riley

Reputation: 176750

If you're multiplying each element of y by every element of X, just multiply all the elements of X together first, then use multiply the array y by this number and sum:

num = X.prod()

(num * y).sum()

Edit: the array you specify can be obtained by multiplying the array X by the sum of the elements of y:

X * y.sum()

Upvotes: 4

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