Filling a two-dimensional array correctly - c

Question

I'm stuck in place. I cannot come up with the algorithm and will fill up the square matrix as below:
"Gets a natural number n from the user n not more than 20. Fills in a square table as below. The numbers below the main diagonal (a21; a31; a32, etc.) are given by the user."
#include <stdio.h>
int main() {
int tab[20][20] = { {0},{0} };
int size = 0;
printf("Enter the natural number n not more than 20: ");
while (scanf_s("%d", &size) != 1 || size < 0 || size >20 || getchar() != '\n')
{
while (getchar() != '\n');
printf("Error. Correct!");
}
for (int x = 0; x < size; x++)
{
for (int y = 0; y < size; y++)
{
if (y==x)
{
tab[x][y]=1; // what's next?
}
}
}
for (int x = 0; x < size; x++)
{
for (int y = 0; y < size; y++)
{
printf("%d ",tab[x][y]);
}
printf("\n");
}
return 0;
}

Answer 1

This is what you want.
You do not need to use if statement.
#include <stdio.h>
int main()
{
int tab[20][20]={ {0},{0} };
int size = 6,n=0;
for (int x = 0; x < size; x++)
{
n=0;
for (int y = x; y < size; y++)
{
n++;
tab[x][y]=n; //fill the upper part of the matrix including diagonal
}
}
for (int x = 1; x < size; x++)
{
for (int y = 0; y < x; y++)
{
tab[x][y]=8; //fill the lower part of the matrix
//or ask for user input
}
}
for (int x = 0; x < size; x++)
{
for (int y = 0; y < size; y++)
{
printf("%d ",tab[x][y]);
}
printf("\n");
}
return 0;
}

Answer 2

i think this would work for just above the main diagonal..
#include <stdio.h>
int main() {
int tab[20][20] = { {0},{0} };
int size = 0;
printf("Enter the natural number n not more than 20: ");
while (scanf("%d", &size) != 1 || size < 0 || size >20 || getchar() != '\n')
{
while (getchar() != '\n');
printf("Error. Correct!");
}
for (int x = 0; x < size; x++)
{
printf("%d. diagonal value:", x+1);
scanf("%d", & tab[0][x]);
}
for (int x = 0; x < size; x++)
{
for (int y = x; y < size; y++)
{
if (y==x)
{
tab[x][y]=1; // what's next?
}
else if(x>0){
tab[x][y]=tab[x-1][y-1];
}
}
}
for (int x = 0; x < size; x++)
{
for (int y = 0; y < size; y++)
{
printf("%d ",tab[x][y]);
}
printf("\n");
}
return 0;
}
output: