Lab 10: Tuesday May 1, 2014

Programming problem

Work on this in groups!

1. The OpenMP code $UWHPSC/labs/lab10/array_omp.f90 contains some bugs. Find the bugs and fix them so that it runs and gives output like this:

   $ gfortran -fopenmp array_omp.f90
   $ ./a.out
    nthreads =            6
    b and bt should be equal
    b=
       0.270000D+02
       0.330000D+02
       0.390000D+02
       0.450000D+02
       0.510000D+02
    bt=
       0.270000D+02
       0.330000D+02
       0.390000D+02
       0.450000D+02
       0.510000D+02
   

2. If \(A\) is an \(n \times n\) matrix and \(x\) is a vector of length \(n\), then \(x^TAx\) is a scalar, a “quadratic form” since it is the sum of terms of the form \(a_{ij}x_ix_j\) that are quadratic in the elements of \(x\) .

   Write an OpenMP code to compute this for a given matrix and vector. Write out the matrix-vector multiplies as loops and use “omp parallel do” loops to compute first the vector \(Ax\) and then the inner product of this with the vector x. Test your code using the \(10 \times 10\) identity matrix for \(A\) and \(x_i = i\), in which case the correct answer can be determined to be 385 from the formula

   \(\sum_{i=1}^n i^2 = \frac{n(n+1)(2n+1)}6.\)

3. There is a quiz for this lab.