MATLAB: Efficiently Swapping Columns in a Matrix

MATLABmatrixmatrix manipulationmex

Is there a more efficient/faster way to switch out two columns in a matrix than calling:
A(:,[i j]) = A(:,[j i]);
If anyone knew of faster implementation (possibly even using MEX), I would greatly appreciate them sharing.

Best Answer

Here is the mex code to swap columns in-place. Note that it is up to the user to make sure the input matrix is not shared with another variable, since every shared variable would have their columns swapped in that case.
/* swapcolumns.c  Swaps columns of X in-place.
 *                Relies on user to make sure X is not shared with another variable
 *                since there is no checking for this in the code.
 * Syntax:  swapcolumns(X,i,j)
 *          X = a matrix (any standard class ... no objects)
 *          i,j = columns to swap
 * Programmer:  James Tursa
 * Date:        June 24, 2016
 */
#include "mex.h"
void mexFunction(int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[])
{
    unsigned char b;
    unsigned char *data, *target, *source;
    size_t M, N, i, j, k, c, bytes;
      if( nrhs == 0 ) {
          mexPrintf("swapcolumns -> Swaps columns of X in-place.\n");
          mexPrintf("Relies on user to make sure X is not shared with another variable\n");
          mexPrintf("since there is no checking for this in the code.\n");
          mexPrintf("Syntax:  swapcolumns(X,i,j)\n");
          mexPrintf("         X = a matrix (any standard class ... no objects)\n");
          mexPrintf("         i,j = columns to swap\n");
          return;
      }
      if( nrhs != 3 ) {
          mexErrMsgTxt("Need exactly 3 inputs");
      }
      if( nlhs > 0 ) {
          mexErrMsgTxt("Too many outputs\n");
      }
      if( !(mxIsNumeric(prhs[0]) || mxIsChar(prhs[0]) || mxIsLogical(prhs[0]) || 
            mxIsCell(prhs[0])) ) {
          mexErrMsgTxt("1st argument needs to be standard class");
      }
      M = mxGetM(prhs[0]);
      N = mxGetN(prhs[0]);
      if( M == 0 || N == 0 ) {
          return;
      }
      i = (size_t) mxGetScalar(prhs[1]);
      j = (size_t) mxGetScalar(prhs[2]);
      if( i > N || j > N ) {
          mexErrMsgTxt("Column index(es) are too large");
      }
      if( i == 0 || j == 0 ) {
          mexErrMsgTxt("Column index(es) cannot be 0");
      }
      data = mxGetData(prhs[0]);
      bytes = M * mxGetElementSize(prhs[0]);
      c = 2;
      while( c-- ) {
          target = data + (i-1) * bytes;
          source = data + (j-1) * bytes;
          for( k=0; k<bytes; k++ ) {
              b = *target;
              *target++ = *source;
              *source++ = b;
          }
          if( mxIsNumeric(prhs[0]) && mxIsComplex(prhs[0]) ) {
              data = mxGetImagData(prhs[0]);
          } else {
              break;
          }
      }
  }
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