C, scanning and printing Matrix using Pointers - c

Question

void input(int (*a)[3]){
int i, j;
for(i = 0; i < 3; i++){
a += i;
for(j = 0; j < 3; j++)
scanf("%d",*a+j);
}
}
void print_matrix(int (*a)[3]){
int i, j;
for(i = 0; i < 3; i++){
a += i;
for(j = 0; j < 3; j++)
printf("%d ",*(*a+j));
printf("\n");
}
putchar('\n');
}
int main(){
int matrix[3][3];
input(matrix);
printf("Printing matrix using function: \n");
print_matrix(matrix);
int i, j;
for(i = 0; i < 3; i++){
for(j = 0; j < 3; j++)
printf("%d ", matrix[i][j]);
printf("\n");
}
return 0;
}
If I enter matrix like this:
1 1 1
2 2 2
3 3 3
Printing matrix using function produces correct output: 1 1 1 2 2 2 3 3 3.
But printing matrix using two loops produces output like this:
1 1 1
2 2 2
-5555 0 -22222
So I obviously have some error, but I can't figure out where. What I don't understand here is why this won't work for third row in matrix.
Note: I know this can be done without using pointers and functions, but that is not the point here, I want to do it exactly like this, just can't figure out what I did wrong.

Answer 1

replace a += i; with a += 1; in both the functions void input(int (*a)[3]) and void print_matrix(int (*a)[3]).
The reason is when i is 2 it goes out of the boundary. You are adding 2.
Also, move a += 1 after the 2nd for loop.

Answer 2

The problem is in how you're manipulating a in both input and print_matrix.
In input:
a += i; // problem is here
for(j = 0; j < 3; j++)
scanf("%d",*a+j);
On the first iteration, a is fine since 0 is added. It's also fine on the second, as 1 is added to move it to the next row. But on the third, it is incremented by 2, moving it outside of the bounds of the matrix.
You could simply replace a += i with a++, but if you do then you'll be writing everything one row down from where it should be.
Sample output with a += i replaced with a++:
[dbush] /tmp/x1
1 1 1
2 2 2
3 3 3
Printing matrix using function:
1 1 1
2 2 2
3 3 3
0 0 -668873792
1 1 1
2 2 2
For this to work properly, move a++ to the bottom of the outer for loop.
In input:
for(i = 0; i < 3; i++){
for(j = 0; j < 3; j++)
scanf("%d",*a+j);
a++;
}
In print_matrix:
for(i = 0; i < 3; i++){
for(j = 0; j < 3; j++)
printf("%d ",*(*a+j));
printf("\n");
a++;
}