Printing the value of 2d array stored in another function - c

Question

i currently have a array function that reads in values of user input and I want to print out the array using another function.
this is my code
Function to read in user input and store in 2d array
void readMatrix(){
int arr[3][4];
int *ptr;
for (int i = 0; i < 3; i++)
{
for (int j = 0; j < 4; j++)
{
printf("row %d, col %d: ", i + 1, j + 1);
scanf("%d", &arr[i][j]);
}
ptr = &arr;
}
}
function to print array stored in readMatrix() function
void printMatrix(){
for (int i = 0; i < 3; ++i)
{
for (int j = 0; j < 4; ++j)
{
printf("row %d, col %d = %d\n", i + 1, j + 1, arr[i][j]);
}
}
}
int main(){
//function to read matrix
readMatrix();
//function to print matrix
printMatrix();
return 0;
}

Answer 1

int arr[3][4]; is a local variable only valid inside that function, so you can't pass it to anyone else. The easiest and best solution is to use caller allocation instead. You can also rewrite the functions to work with variable amounts of rows/cols.
With just a few tweaks to your code:
#include <stdio.h>
void readMatrix (int rows, int cols, int arr[rows][cols]){
for (int i = 0; i < rows; i++)
{
for (int j = 0; j < cols; j++)
{
printf("row %d, col %d: ", i + 1, j + 1);
scanf("%d", &arr[i][j]);
}
}
}
void printMatrix (int rows, int cols, int arr[rows][cols]){
for (int i = 0; i < rows; ++i)
{
printf("{ ");
for (int j = 0; j < cols; ++j)
{
printf("%2.d ", arr[i][j]);
}
printf("}\n");
}
}
int main (void)
{
int arr[3][4];
readMatrix(3,4,arr);
printMatrix(3,4,arr);
return 0;
}
Input:
1 2 3 4 5 6 7 8 9 10 11 12
Output:
{ 1 2 3 4 }
{ 5 6 7 8 }
{ 9 10 11 12 }
If you don't understand why this works, then "array decay" is the next best thing to study. What is array to pointer decay?

Answer 2

The problem is that you are trying to access the array from outside the scope of the function that created it. You need to pass it as a parameter to your print function. You can also return a pointer to the array from your read function and then pass the pointer to your print function. Here is an example of the second option:
#include <stdio.h>
int *readMatrix(int *arr);
void printMatrix(int *arr);
int main()
{
int arr[3][4];
readMatrix(&arr[0][0]);
printMatrix(&arr[0][0]);
return 0;
}
int *readMatrix(int *arr)
{
for (int i = 0; i < 3; ++i)
{
for (int j = 0; j < 4; ++j)
{
printf("row %d, col %d: ", i + 1, j + 1);
scanf("%d", &arr[i * 4 + j]);
}
}
return arr;
}
void printMatrix(int *arr)
{
for (int i = 0; i < 3; ++i)
{
for (int j = 0; j < 4; ++j)
{
printf("row %d, col %d = %d\n", i + 1, j + 1, arr[i * 4 + j]);
}
}
}