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# XPRSbtran

Purpose
Post-multiplies a (row) vector provided by the user by the inverse of the current basis.
Topic areas
Linear Algebra, LP
Synopsis
`int XPRS_CC XPRSbtran(XPRSprob prob, double vec[]);`
Arguments

prob | The current problem.
vec | Double array of length ROWS containing the values by which the basis inverse is to be multiplied. The transformed values will also be returned in this array.

Related controls
Double

ETATOL | Tolerance on eta elements.

Example
Get the (unscaled) tableau row z of constraint number irow, assuming that all arrays have been dimensioned.

    /* Minimum size of arrays:
     * y: nrow + ncol;
     * start: 2;
     * rowind, rowcoef: nrow.
     */

    /* set up the unit vector y to pick out row irow */
    for(i = 0; i < nrow; i++) y[i] = 0.0;
    y[irow] = 1.0;

    rc = XPRSbtran(prob,y);       /* y = e*B^{-1} */

    /* Form z = y * A */
    for(j = 0; j < ncol, j++) {
      rc = XPRSgetcols(prob, start, rowind, rowcoef,
                       nrow, &ncoefs, j, j);
      for(d = 0.0, ielt = 0, ielt < ncoefs; ielt++)
        d += y[rowind[ielt]] * rowcoef[ielt];
      y[nrow + j] = d;
    }

See also example tableau.c.
Further information
If the matrix is in a presolved state, XPRSbtran works with the basis for the presolved problem.
Related topics
XPRSftran.
Parent Topic Console and Library Functions

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