'a' ---> 97
'A' ---> 65

 

#include <stdio.h>
int main()
{
    char a[] = "Hello World!";
    int i = 0;
    while(a[i] != '\0')
    {
        if(a[i] >= 'a' && a[i] <= 'z')
        {
            a[i] -= 32;
        }
        i++;
    }
    printf("%s\n", a);
}