C program : Does not execute beyond 1000 or just hangs - c

Question

This program is to find three four digit prime numbers, such that they are permutations of each other and have a difference of 3320 between them. Now the following program correctly produces the prime numbers but execution stops or just hangs after the value 1000. Why this happens and how can I correct it?
#include<stdlib.h>
#include<stdio.h>
#define LIMIT 1000000
long int arr[LIMIT];
int Bsearch(long int *primes,long int low,long int high,long int search)
{
while(low<=high)
{
long int mid=(low+high)/2;
if(search<primes[mid])
{
high=mid+1;
}
else if(search>primes[mid])
{
low=mid+1;
}
else if(search==primes[mid])
return 1;
}
return 0;
}
int checkPermu(long int *primes,long int i,long int j,long int k)
{
long int prod1=1;
long int prod2=1;
long int prod3=1;
for(int i=0;i<3;i++)
{
prod1*=primes[i%10];prod2*=primes[j%10];prod3*=primes[k%10];
i/=10;j/=10;k/=10;
}
prod1*=primes[i];prod2*=primes[j];prod3*=primes[k];
if(prod1==prod2==prod3)
return 1;
else
return 0;
}
int main()
{
for(long int i=1;i<LIMIT;i++)
{
arr[i]=1;
}
for(long int i=2;i<LIMIT;i++)
{
if(arr[i]==1)
{
for(long int j=2;i*j<LIMIT;j++)
{
arr[i*j]=0;
}
}
}
long int index=0;
long int count=0;
//count number of primes
for(long int i=1;i<LIMIT;i++)
{
if(arr[i]==1) count++;
}
long int primes[count];
for(long int i=2,index=1;i<LIMIT;i++)
{
if(arr[i]==1)
primes[index++]=i;
}
for(long int i=1000;i<9999;i++)
{
printf("\nCurrent num : %ld\n",i);
if(Bsearch(primes,1,count-1,i))
{
printf("Hello1!");
if(Bsearch(primes,1,count-1,i+3330))
{
printf("\nHello2!");
if(Bsearch(primes,1,count-1,i+3330+3330))
{
printf("\nHello3!");
if(checkPermu(primes,i,i+3330,i+3330+3330))
{
printf("\nFinal Hello!");
printf("required sequence : %ld%ld%ld",i,i+3330,i+3330+3330);
exit(0);
}
}
}
}
}
return 0;
}

Answer 1

There are many bugs in your code.
In the function Bsearch, you are searching for a value in the array. What if its not present? In your case, 1000 is not a prime number. So, you can't find that in the array. You are coming out of the loop only if the value matches exactly. which won't happen in this case ending up in an infinite loop
In the function, checkPermu, there are two variables declared as i. which means you are shadowing the i that is passed as parameter. To avoid it, give it some other name.
Now, to avoid the first case, you can use an inbuilt function in C++, lower_bound , or else modify the code correspondingly. Here is your edited code.
#include <stdlib.h>
#include <stdio.h>
#define LIMIT 1000000
long int arr[LIMIT];
int Binary_Search(long int a[], int low, int high, long int e)
{
if ( low < 0) return 0;
if (low>=high )
{
if ( e <= a[low] ) return low;
return low+1;
}
int mid=(low+high)/2;
if ( e> a[mid])
return Binary_Search(a,mid+1,high,e);
return Binary_Search(a,low,mid,e);
}
int Bsearch(long int *primes,long int low,long int high,long int search)
{
int idx = Binary_Search(primes, low, high+1,search);
if(primes[idx]==search)
return 1;
return 0;
}
int checkPermu(long int *primes,long int ii,long int j,long int k)
{
long int prod1=1;
long int prod2=1;
long int prod3=1;
for(int i=0;i<3;i++)
{
prod1*=primes[ii%10];prod2*=primes[j%10];prod3*=primes[k%10];
ii/=10;j/=10;k/=10;
}
prod1*=primes[ii];prod2*=primes[j];prod3*=primes[k];
if(prod1==prod2==prod3)
return 1;
else
return 0;
}
int main()
{
for(long int i=1;i<LIMIT;i++)
{
arr[i]=1;
}
for(long int i=2;i<LIMIT;i++)
{
if(arr[i]==1)
{
for(long int j=2;i*j<LIMIT;j++)
{
arr[i*j]=0;
}
}
}
long int index=0;
long int count=0;
//count number of primes
for(long int i=1;i<LIMIT;i++)
{
if(arr[i]==1) count++;
}
long int primes[count];
for(long int i=2,index=1;i<LIMIT;i++)
{
if(arr[i]==1)
primes[index++]=i;
}
//Bsearch(primes,1,count-1,1000);
for(long int i=1000;i<9999;i++)
{
printf("\nCurrent num : %ld\n",i);
if(Bsearch(primes,1,count-1,i))
{
printf("Hello1!\n");
if(Bsearch(primes,1,count-1,i+3330))
{
printf("\nHello2!\n");
if(Bsearch(primes,1,count-1,i+3330+3330))
{
printf("\nHello3!\n");
if(checkPermu(primes,i,i+3330,i+3330+3330))
{
printf("\nFinal Hello!\n");
printf("required sequence : %ld%ld%ld",i,i+3330,i+3330+3330);
exit(0);
}
}
}
}
}
return 0;
}

Answer 2

First, the reason for hang is probably due to this line:
long int primes[count];
count here is 78499 and you are allocating 313996 bytes on the stack. The stack size is probably smaller than this and the stack overflow seems to be causing problem with bash or cmd.exe in my case (blind guess, it's cmd.exe).
Changing that to
long int *primes = malloc(count * sizeof *primes);
resolves the issue.
The actual problem with your code however is that your binary search never terminates. So fixing the previous bug, your code is in an infinite loop, so you don't see any output. I recommend trying to figure out why your binary search doesn't terminate for practice, but in real life, use bsearch from the standard library.