Thread: Passing an array of strings to a function.

Hi,

I have actually (sort of) figured out how to do this. By passing wach string in the array separately to a function that accepts a string. But I actually want it to pass a whole array of char[x][y] to a function in one smack and have my loops inside the function instead. I think i have been confused by the pointers somehow.

This works, but its not exactly what I want to do:

Code:

#include <stdio.h>

void printmatrixline(char *matrix);

char first[20][21] = {
	"....................",
	"....................",
	"....................",
	"....................",
	"....................",
	"....................",
	"....................",
	"....................",
	"....................",
	"....................",
	"....................",
	"....................",
	"....................",
	"....................",
	"....................",
	"....................",
	"....................",
	"....................",
	"....................",
	"....................",
};

int main () {
	int i;
	for(i=0; i < 20; i++) {
		printmatrixline(first[i]);
	}
	printf("\n");
}

void printmatrixline(char *matrix) {
	int i;
	for (i=0; i < 20; i++) {
		printf("%c", matrix[i]);
	}
	printf("\n");
}

This is what I want to do, but it doesn't work.

Code:

#include <stdio.h>

void printmatrix(char *matrix);

char first[20][21] = {
	"....................",
	"....................",
	"....................",
	"....................",
	"....................",
	"....................",
	"....................",
	"....................",
	"....................",
	"....................",
	"....................",
	"....................",
	"....................",
	"....................",
	"....................",
	"....................",
	"....................",
	"....................",
	"....................",
	"....................",
};

int main () {
	printmatrix(first);
	printf("\n");
}

void printmatrix(char *matrix) {
	int i,j;
	for (i=0; i < 20; i++) {
		for (j=0; j < 20; j++) {
                printf("%c", matrix[i][j]);
                }
	}
	printf("\n");
}

I can successfully pass a pointer to a string using *string but I cant pass an array of strings OR and array of an array of characters by using *array. Any help?

Thanks for your help, the second option worked.

I am curious as to how the first one works. As i think this is supposed to be the best practice. I assume that the code you wrote goes in the main function and not the void function.

char *ptray[20];
for (i=0;i<20;i++) { ptray[i]=first[i]; }
printmatrix(ptray);

I also assume that this code creates another variable that points to an array in an array(in this case a string in an array as my first ends each line in a '\0'.

So when "void printmatrix(char **matrix);" is used then the first * points to an array called ptray which is the array of array of characters, and the second * points to the char in the array. amirite?

This is quite confusing.

Try this version:

Code:

#include <stdio.h>

void printmatrix (char **matrix);

char *first[20] = {
"....................",
"....................",
"....................",
"....................",
"....................",
"....................",
"....................",
"....................",
"....................",
"....................",
"....................",
"....................",
"....................",
"....................",
"....................",
"....................",
"....................",
"....................",
"....................",
"....................",
};

int main (int argc, char *argv[])
{
   printmatrix (first);
   printf("\n");
}

void  printmatrix (char **matrix)
{
   int  i, j;
   
   for (i=0; i < 20; i++)  {
      for (j=0; j < 20; j++)  {
         printf ("%c", matrix[i][j]);
      }
      printf("\n");
   }
}

---

EDIT: Ah, bithub did the same thing.

1) Yes, that for() loop would go somewhere OUTSIDE of printmatrix(**matrix). I would not say this is the best practice, it is more of a clue (but it is not too bad either). Normally you would declare one **matrix, then allocate and populate each of it's rows, so that you would not have to have two variables. None of that is necessary if the parameter to printmatrix() is always the same size ([20][21]); it is only necessary if printmatrix() is going to accept any 2D matrix of any size -- printmatrix(**matrix).

I dont really want to write a loop around the function every time I want to call it.
I had written a loop around my function and passed a pointer to the strings in the first instance. I have no trouble understanding this. I may do myself some justice doing some reading up on pointers in C.