Question

I am solving a problem on USACO. In the problem, I have to take two strings as inputs and calculate the numerical values modulo 47. If the values are same, then GO is to be printed otherwise STAY has to be printed. The initial numerical value will be calculated by taking the product of the numerical values of the alphabets ( 1 for A and similarily 26 for Z ) and then the final number will be calculated by using modulo.

My program is being compiled withour any error and running well on my computer. However, it is showing a segmentation fault as the execution error by the grader computer. The program and the output is as follows:-

Program:-

#include<stdio.h>
#include<string.h>
main()
{
    int cal(char *ptr);
    char *comet,*group;
    int a,b;
    scanf("%s",comet);
    a=cal(comet);
    scanf("%s",group);
    b=cal(group);
    if(a==b)
        printf("GO");
    else
        printf("STAY");
    return 0;
}
int cal(char *ptr)
{
    int i=0,c,prod=1,mod;
    while(ptr[i]!='\0')
        {
            if(ptr[i]>='A'&&ptr[i]<='Z')
            {
                c=ptr[i]-'@';
                prod=prod*c;
                i++;
            }
        }
    mod=prod%47;
    return mod;
}

OUTPUT:-

My question is how to pinpoint the segmentation fault. I have read about this fault but don’t know what to do in this program. Any help would be great.

Answer 1

char *comet,*group;
int a,b;
scanf("%s",comet);

comet pointer is uninitialized. You need to allocate memory and makes comet points at this allocated memory otherwise scanf will write bytes in a random location which will likely crash your system.