libsvmwrite.c

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#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "mex.h"

#ifdef MX_API_VER
#if MX_API_VER < 0x07030000
typedef int mwIndex;
#endif
#endif

void exit_with_help()
{
  mexPrintf(
    "Usage: libsvmwrite('filename', label_vector, instance_matrix);\n"
  );
}

void libsvmwrite(const char *filename, const mxArray *label_vec, const mxArray *instance_mat)
{
  FILE *fp = fopen(filename, "w");
  int i, k, low, high, l;
  mwIndex *ir, *jc;
  int label_vector_row_num;
  double *samples, *labels;
  mxArray *instance_mat_col; // instance sparse matrix in column format

  if (fp == NULL)
  {
    mexPrintf("can't open output file %s\n", filename);
    return;
  }

  // transpose instance matrix
  {
    mxArray *prhs[1], *plhs[1];
    prhs[0] = mxDuplicateArray(instance_mat);
    if (mexCallMATLAB(1, plhs, 1, prhs, "transpose"))
    {
      mexPrintf("Error: cannot transpose instance matrix\n");
      return;
    }
    instance_mat_col = plhs[0];
    mxDestroyArray(prhs[0]);
  }

  // the number of instance
  l = (int) mxGetN(instance_mat_col);
  label_vector_row_num = (int) mxGetM(label_vec);

  if (label_vector_row_num != l)
  {
    mexPrintf("Length of label vector does not match # of instances.\n");
    return;
  }

  // each column is one instance
  labels = mxGetPr(label_vec);
  samples = mxGetPr(instance_mat_col);
  ir = mxGetIr(instance_mat_col);
  jc = mxGetJc(instance_mat_col);

  for (i = 0; i < l; i++)
  {
    fprintf(fp, "%g", labels[i]);

    low = (int) jc[i], high = (int) jc[i + 1];
    for (k = low; k < high; k++)
      fprintf(fp, " %ld:%g", ir[k] + 1, samples[k]);

    fprintf(fp, "\n");
  }

  fclose(fp);
  return;
}

void mexFunction(int nlhs, mxArray *plhs[],
                 int nrhs, const mxArray *prhs[])
{
  // Transform the input Matrix to libsvm format
  if (nrhs == 3)
  {
    char filename[256];
    if (!mxIsDouble(prhs[1]) || !mxIsDouble(prhs[2]))
    {
      mexPrintf("Error: label vector and instance matrix must be double\n");
      return;
    }

    mxGetString(prhs[0], filename, mxGetN(prhs[0]) + 1);

    if (mxIsSparse(prhs[2]))
      libsvmwrite(filename, prhs[1], prhs[2]);
    else
    {
      mexPrintf("Instance_matrix must be sparse\n");
      return;
    }
  }
  else
  {
    exit_with_help();
    return;
  }
}