CODEWITHSUNDEEP
c++boostblasublas
Andreas Mueller
Andreas Mueller

Reputation: 28748

Applying Boost UBLAS blas-1 to Matrices

How do I apply level 1 blas on a boost::numeric::ublas matrix? For example I want to compute the maximum entry or the sum of all entries. Using norm_1 or norm_inf on a matrix gives no compiler error but returns (as it seems to me) arbitrary results. I am using boost 1.42

Upvotes: 2

Views: 1068

Answers (2)

hannes
hannes

Reputation: 964

A minimal example looks like this:

    #include<iostream>
    #include<boost/numeric/ublas/matrix.hpp>
    using namespace boost::numeric::ublas;
    int main(){
            int l = 100;
            matrix<double> m(l,l);
            for (int i = 0; i < l; ++i) {
                    for (int j = 0; j < l; ++j) {
                            m(i,j)=i*l+j;
                    }
            }
            std::cout << norm_inf(m)<<std::endl;
            return 0;
    }

It should give 99, but yields 994950.

This is a one-liner that at least solves the given task:

    float infnorm = accumulate(m.data().begin(),m.data().end(),0,std::max<double>);

Upvotes: 1

Stephen Canon
Stephen Canon

Reputation: 106167

norm_inf on a matrix computes the matrix norm induced by the infinity-norm on the underlying vector space. This happens to be the maximum absolute row sum of the matrix.

If you look at hannes's example, the last row of the matrix (i=99, j=0...99) contains:

9900, 9901, 9902, ... , 9999

If you sum those entries, you get 994950, which is exactly what norm_inf produces.

Upvotes: 3

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