Sparse Array - Special Command?

Hi,

I'm dealing with a sparse array. Given 2 arrays "col" and "val", I want to create a one-rowed sparse array of value val(i) in the column col(i), for all i.

What I did is:

if true
% val, col given
leng = length(val);
row = ones(1, leng);
result = sparse(row, col, val, 1, leng);
end

Is there a better way to do, without creating an array "row" in particular? Because getting the length and creating the row take a bit of time in my problem.

Thanks!

[Martin]

No ideas? :)

Answer 1

[Walter Roberson]

result = sparse(1, col, val);

Note: this is not exactly equivalent to what you wrote. What you wrote only allocates a 1 x leng array, but leng is the number of elements in col, not the maximum col.

For example, sparse(1, 10, 3, 1, 1) would try to create a 1 x 1 sparse matrix, but it needs a 1 x 10 sparse matrix.

[Martin]

Sorry, I made a mistake. I want to create an array whose width I know, say nCol.

In particular I have max(col) < nCol.

The line I really wrote in Matlab is

result = sparse(row, col, val, 1, nCol);

I wil try sparse(1, col, val, 1, nCol). Thanks for your answer anyway.

[Martin]

It works :), thanks Walter! It's faster.

[Walter Roberson]

Much faster or just "faster" ?

[Martin]

In my program, the time necessary to create the matrix itself is similar, but I saved the time to create the "row" array. In particular, calculating the length of this array took time.

I'd say I saved about 25% of time but I reckon this is very problem-specific.