C program to calculate largest gap between prime numbers in a range

You will have to write the program by yourself. But we can share the basic idea. Let's discuss the most easiest way:
1. Make the list of all prime number in the range
2. Get the difference of all prime number
3. Find the max

No, it's your homework, and we do not do your homework: it is set for a reason. It is there so that you think about what you have been told, and try to understand it. It is also there so that your tutor can identify areas where you are weak, and focus more attention on remedial action.

Try it yourself, you may find it is not as difficult as you think! Start by generating att the prime numbers in the range - that's relatively simple, and probably a task you have done before. Then loop through them, looking for gaps. Keep track of the largest so far, and update it when you find a bigger one. After the loop you have the interval you need.

If you meet a specific problem, then please ask about that and we will do our best to help. But we aren't going to do it all for you!

Monolithic programs is a bad idea, it just make things more complicated to you. As suggested in S3, making a function handling if a number is a prime or not will simplify the code.

    for(x=2;x<y;++x)
        a=y%x;

if(a==0) break;

Your code don't work, but I get your idea, the code is overkill, to check if 97 is a prime, it will check every number from 2 to 96. When you check by hand, do you stop at 96 or do you stop earlier?
The rule you apply to stop earlier should appear in you code.

The usage of {} matters in C, you should learn to use them as soon as possible.

Quote:

I mean (29-23-1)=5.

Are you sure the gap between 23 and 29 is 5 ?
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Your code do not behave the way you expect, or you don't understand why !

There is an almost universal solution: Run your code on debugger step by step, inspect variables.
The debugger is here to show you what your code is doing and your task is to compare with what it should do.
There is no magic in the debugger, it don't know what your code is supposed to do, it don't find bugs, it just help you to by showing you what is going on. When the code don't do what is expected, you are close to a bug.
To see what your code is doing: Just set a breakpoint and see your code performing, the debugger allow you to execute lines 1 by 1 and to inspect variables as it execute.
Debugger - Wikipedia, the free encyclopedia[^]

Mastering Debugging in Visual Studio 2010 - A Beginner's Guide[^]
Basic Debugging with Visual Studio 2010 - YouTube[^]
1.11 — Debugging your program (stepping and breakpoints) | Learn C++[^]
The debugger is here to only show you what your code is doing and your task is to compare with what it should do.

#include <iostream>
using namespace std;
int prime(int n)
{
   int d=0;
   for(int j=1;j<=n;j++){
       if(n%j==0)
       d++;
   }
   if(d==2)
   return true;
   else
   return false;
    
}
int main()
{
    int doo,gap=0,naj=0,x=0;
    cin>>doo;
    for(int i=1;i<=doo;i++){
        if(prime(i)){
            cout<<i<<" gap is "<<gap<<endl;        
            
            if(naj<gap){
            naj=gap;
            x=i;
                
            }
            gap=1;
        }
        else
        gap++;
        
    }
    cout<<endl<<endl<<"Biggest gap is "<<naj<<"  "<<x;
    return 0;
}