Question

I have a program that has an array of pointers such as:

INTLIST* myArray[countOfRows];

each myArray pointer points to a linked list, basically it becomes an M*N matrix.

So for example:

3 2 1
2 1 0
1 9 8

So myArray[0] is a linked list with 3->2->1 and myArray[1] is a linked list with 2->1->0.. and so on. I have a function that prints the matrix which basically does this:

for(int i = 0; i<countOfRows; i++)
   list_print(myArray[i]);

And then my list_print function is like so:

 void list_print(INTLIST* list)
  { /* This function simply prints the linked list out */
     INTLIST* temp=NULL;  //temp pointer to head of list

     /* loops through each node of list printing its datum value */
     for(temp=list; temp!=NULL; temp=temp->next)
        {
           printf("%d ", temp->datum);         //print datum value
        }
     printf("\n");
  }

This is all great and working fine, (I know i could optimize and clean up and I promise to do so). Now my next question is I need to write this matrix out to a text file. So I tried this:

 char* outputFileName = "out.txt";
 FILE* ofp;

 ofp=fopen(outputFileName, "w");

 if(ofp == NULL) 
 {
        printf("Cannot open writeable file!");
        return -1;
 }

 for(i=0; i<fileLineCount; i++)
  {
     fprintf(ofp, list_print(aList[i]);
  } 

But I take it its not that simple. Can anyone show me how to print this out to a text file…

Thanks

Answer 1

Use the fprintf function in your list_print function using a FILE * parameter as shown in the example:

void list_print(INTLIST* list, FILE *fp)
  { /* This function simply prints the linked list out */
     INTLIST* temp=NULL;  //temp pointer to head of list

     /* loops through each node of list printing its datum value */
     for(temp=list; temp!=NULL; temp=temp->;next)
        {
           fprintf(fp, "%d ", temp->datum);         //print datum value
        }
     fprintf(fp, "\n");
  }

Would be done like this:

int main(){
   FILE *fp;
   fp = fopen("data.txt", "w");
   for(i=0; i<fileLineCount; i++)
   {
      list_print(alist[i], fp);
   }
   fclose(fp);
}

Or

If you wish to retain the binary dump of the linked list from memory to disk (this is the simple way of explaining it), top off my head:

for(i=0; i<fileLineCount; i++)
{
     fwrite(alist[i], sizeof(alist[i]), 1, ofp);
}

Then when reading from it,

for(i=0; i<fileLineCount; i++)
{
     fread(&alist[i], sizeof(alist[i]), 1, ofp);
}

Edit: I was hesitant to include the above, but paxdiablo pointed out a flaw in my fread(...) and fwrite(...) code above, and realized he is right. When doing the binary write of the linked list, the memory addresses references in the linked-list will be invalid when read in the next time. Thanks paxdiablo!