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typedef struct _double_matrix {    double *data ;    int xsize ;    int ysize ;} double_matrix ;matrix *double_matrix_new( int xsize, int ysize ){    matrix *m ;    m->data = calloc( xsize * ysize, sizeof(double) );    if( !m ) return( NULL );    m->xsize = xsize ;    m->ysize = ysize ;    /* Inutile avec calloc() mais juste pour l'exemple : */    for( x=0 ; x<xsize ; x++ ) {        for( y=0 ; y<size ; y++ ) {            double_matrix_set_value( m, x, y, (double)1 );        }    }}int double_matrix_set_value( matrix *m, int x, int y, double value ){    if( x<m->xsize || y<m->ysize ) {         m->data[ x*(m->xsize-1) + y ] = value ;    }}