Which is the best way to multiply a large and sparse matrix with its transpose?

Question

I currently want to multiply a large sparse matrix(~1M x 200k) with its transpose. The values of the resulting matrix would be in float. - I tried loading the matrix in scipy's sparse matrix and by multiplying each row of first matrix with the second matrix. The multiplication took ~2hrs to complete. What is the efficient way to achieve this multiplication? Because I see a pattern in the computation. - The matrix being large and sparse . - The multiplication of a matrix with its transpose. So, the resulting matrix would be symmetric. I would like to know what libraries can achieve the computation faster. It can be in Python, R, C, C++ or any other one.

Accepted Answer

I suppose your main need is to save memory. First as you multiply a matrix with its transpose, you do not need any memeory for the transpose : all of its cells are directly accessible through first matrix (tA[i,j] = A[j,i]). Almost 1/3 of memory saved.

I can see that computation time cannot be neglected too. As the resulting matrix will be symetric, you can compute only one half and directly store the other. Near half of computation time saved.

And if you are sure that you initial matrix is really sparse, and so can hope the resulting one will be too, you can directly store the result in a scipy sparse matrix, COO format : only three lists to store the non null values.

But ... I do not know any library to do that and you will have to code it yourself in your prefered language (probably python as you spoke of scipy).

Python code example (matrix = A[M][N])

```
I = []
J = []
V = []
for i in range(M):
    for j in range(i:M) :
        X = 0.0
        for k in range(N):
            X += A[i ][k] * A[k][j]
        if X != 0.0 # or abs (X) > epsilon if floating point accuracy is a concern ... 
            I.append (i )
            J.append(j)
            V.append(X)
            I.append (j )
            J.append(i)
            V.append(X)
```

And I, J, V are what is needed for a scipy COO sparse matrix via :

```
RESULT = sparse.coo_matrix((V,(I,J)),shape=(N, N))
```

Which is the best way to multiply a large and sparse matrix with its transpose?

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