Ask the eigs function to return the largest magnitude eigenvalue.
% Generate a vector of eigenvalues suitable for the 'randcorr' option of the gallery function
x = rand(1, 100);
x = 100*x./sum(x);
% Generate a matrix whose eigenvalues are the elements of x
A = gallery('randcorr', x);
% Compute the largest magnitude eigenvalue
largest = eigs(A, 1, 'lm')
% Check: this should be small
largest - max(x)
If you're using release R2017b or later, you might want to use 'largestabs' instead of 'lm' in the eigs call. The old options are still accepted, but 'largestabs' is more descriptive of what it's doing than 'lm'.
