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;
        }
}