2d array and for loops - c

Question

I did this code to try the basic operations on bidimensional arrays (2d arrays), but it doesn't print
on screen the letter 'A'.
Can someone help me figure this out?
Thanks
#include <stdio.h>
#define ROWS 2
#define COLS 3
int main()
{
int x;
int y;
int i;
int j;
int Field[ROWS][COLS];
printf("Enter two values: ");
scanf("%d %d", &x, &y);
Field[y][x] = 1;
for(i = ROWS - 1; i > 0; i--){
for(j = 0; j < COLS; j++){
if(Field[i][j] == 1){
printf("A");
}
}
}
return 0;
}

Answer 1

In the outer loop
for(i = ROWS - 1; i > 0; i--){
the range of indices is (in the descending order) [ROWS-1, 1], So the index 0 is not processed in this loop.
It is better such a loop to write the following way
for ( i = ROWS; i != 0; i-- ){
for(j = 0; j < COLS; j++){
if(Field[i-1][j] == 1){
printf("A");
}
}
}
In this case the variable i can have even an unsigned integer type as for example size_t and the code will be valid even when ROWS is equal to 0.
Otherwise the expression ROWS - 1 can yield the maximum value of the unsigned type when ROWS is equal to 0.

Answer 2

as mentioned i>0 should be i>=0 other wise you won't traverse the array completely.
also note that even after you you corrected that statement , it's better to check input x and y , because if they are greater or equal to ROWS and COLS , than by passing boundaries of your array here Field[y][x] = 1; your program will lead to undefined behavior.
so I suggest this
scanf("%d %d", &x, &y);
if (x < ROWS && y < COLS)
Field[y][x] = 1;
else
return 0;
//rest of your code with i>=0 in this loop for(i = ROWS - 1; i >= 0; i--)

Answer 3

This: for(i = ROWS - 1; i > 0; i--){ fails to traverse the necessary array location, and does not clearly convey to future maintainers of your software what exactly the intent is.
However, to do what you have described in your problem description, unless there is a compelling reason to set up a decrementing index (--), a normal incrementing (++) set of for loops is adequate and more idiomatic :)
for(i = 0; i < ROWS; i++)
{
for(j = 0; j < COLS; j++)
{
if(Field[i][j] == 1)
{
printf("A");
}
}
}

Answer 4

This code adds some protections to out of bounds arrays and prints 'A'. Should be a good starting point for you.
#include <stdio.h>
#define ROWS 2
#define COLS 3
int main()
{
int x,y,i,j = 0;
int Field[ROWS][COLS]; // 2 Rows 3 Column array
printf("\nEnter row value less than %i: ",ROWS);
scanf("%d", &y);
printf("\nEnter column value less than %i: ",COLS);
scanf("%d", &x);
if(x >= COLS || y>= ROWS)
{
printf("\nOut of bounds inputs!");
return 0;
}
Field[y][x] = 1;
for(i = ROWS - 1; i > 0; i--)
{
for(j = 0; j < COLS; j++)
{
if(Field[i][j] == 1)
{
printf("A");
}
}
}
return 0;
}