Opgaveløsning:
Tynde matricer

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#define BUF_MAX 100

void normal_matrix_to_sparse_matrix(char *matrix_in_file_name, 
                                    char *matrix_out_file_name){
  FILE *ifp, *ofp;
  char buf[BUF_MAX];
  int r, c, rows, cols;
  double value;

  // Open files:
  ifp = fopen(matrix_in_file_name, "r");
  ofp = fopen(matrix_out_file_name, "w");

  // Handle dimension. A line with two integers m n:
  fgets(buf, BUF_MAX, ifp);  
  fputs(buf, ofp);

  // Extract number of rows and number of colums to rows and cols:
  sscanf(buf, "%d %d", &rows, &cols);

  // Read normal matrix and print sparse matrix:
  for(r = 1; r <= rows; r++){
    for(c = 1; c <= cols; c++){ 
      fscanf(ifp, " %lf", &value);
      if (value != 0) fprintf(ofp, "%d %d %f\n", r, c, value);
    }
  }

  // Close files:
  fclose(ifp);
  fclose(ofp);
}

void sparse_matrix_to_normal_matrix(char *matrix_in_file_name,
                                    char *matrix_out_file_name){
  FILE *ifp, *ofp;
  char buf[BUF_MAX], *res;
  int rows, cols,
      row = -1, col = -1,   // Negative values signals no more 
      r, c;                 // entries in sparse representation.
  double value;

  // Open files:
  ifp = fopen(matrix_in_file_name, "r");
  ofp = fopen(matrix_out_file_name, "w");

  // Handle dimensions:
  fgets(buf, BUF_MAX, ifp);  
  fputs(buf, ofp);

  // Extract number of rows and number of colums:
  sscanf(buf, "%d %d", &rows, &cols);

  // Get first entry. Assign row, col and value.
  res = fgets(buf, BUF_MAX, ifp);
  if (res != NULL)
     sscanf(buf, "%d %d %lf", &row, &col, &value);
  else {row = -1; col = -1;}

  // Generate all entries, in a double for loop.
  // Read from sparse matrix along the way:
  for (r = 1; r <= rows; r++){
    for (c = 1; c <= cols; c++){
      if (r == row && c == col){
        fprintf(ofp, "%f ", value);

        // Get next entry: 
        if (res != NULL) res = fgets(buf, BUF_MAX, ifp);
        if (res != NULL)
           sscanf(buf, "%d %d %lf", &row, &col, &value);
        else {row = -1; col = -1;}
      }
      else fprintf(ofp, "%f ", 0.0);
   }
   fprintf(ofp, "\n");
  }

  // Close files:
  fclose(ifp);
  fclose(ofp);
}

int main(void) {
  normal_matrix_to_sparse_matrix("m1.dat", "ms.dat");
  sparse_matrix_to_normal_matrix("ms.dat", "m2.dat");
}