Write a program that ask user to guess a prime number in 5 tries?

[Ehtisham Ul Hassan]

#include<iostream>
using namespace std;
//I tried this but it does not run. How should I check guess is prime number or not?

bool isprime(int guess)
{

for (int i=0; i<=guess; i++)

if((guess%i != 0)||(i != 0)||(i!=guess))
{
return false;
}
else
{
return true;
}
}
int main()
{
const int limit = 5;
int attempt;
for(attempt = 0; attempt < limit; attempt++)
{
cout<<"Please enter any Prime number brtween 1 to 100. You have "<<(limit-attempt)<<" attempts remaining"<<endl; >
int guess;
cin>>guess;
if(isprime(guess))
break;
}
if(attempt == limit)
cout<<"You loose"<<endl; >
else
cout<<"You Winner"<<endl; >
Improve solution   Permalink        Posted 41 mins ago
Ehtisham Ul Hassan
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[John Matthews]

Hi Ehtisham - please could you use code tags so we can read your code? There's help on how to use them here:
https://coderanch.com/wiki/659781/Code-Tags

[Junilu Lacar]

The problem is here:

bool isprime(int guess)
{

   for (int i=0; i<=guess; i++)
      if ((guess%i != 0)||(i != 0)||(i!=guess))
      {
          return false;
      }
      else
      {
         return true;
      }
}

[John Matthews]

Thanks Junilu.

So Ehtisham, I suggest you work through your code with a test value, say 7 which we know is a prime.

In isprime() argument guess is 7, and when i=0, the condition (i!=guess) is true so the function returns false; not a good start

You need to modify your test(s), and check them by hand until you are happy they give the correct result for a few known primes and non-primes. For starters, you don't know if a number is prime or not until you have tested for division by all numbers in the for() range, so I don't think you can return true inside the loop.

[Campbell Ritchie]

Welcome to the Ranch

There is more wrong with your loop. Don't start by dividing by 0 (see lines 4‑5 in Junilu's post). Don't loop up to ≤ i, which is an out by one error. Work out how many times the loop will executed.
Anyway, you don't need to test all numbers to see whether they divide into a number. You need to test prime numbers ≤ √i. Even if you don't have a supply of prime numbers to iterate, you can use 2 and then all subsequent odd numbers. That will speed up execution a lot.

[Junilu Lacar]

The for-loop isn't guaranteed to execute at all so there's a possibility that execution will not go through the if-else statement inside the for-loop and hit one of the return statements. In that case, there is no statement after the for-loop that would return a boolean value. Unlike with Java which would produce a compile-time error, C/C++ is more permissive and will let that code compile. I don't know what happens at runtime though if the for-loop doesn't execute at least one iteration.

[Ehtisham Ul Hassan]

I have completed this problem. Thanks for helping ❤

#include <iostream>
using namespace std;
bool is_prime(int guess)
{
if ( guess < 2 || guess > 100)
return false;
int n=2;
while(n<guess)
{
if ( (guess % n ) == 0)
return false;
n++;
}
return true;
}
int main()
{
const int limit = 5;
int attempt ;
while ( attempt < limit)
{
cout << "Please enter any Prime number between 1 and 100, you have " <<
(limit- attempt) << " tries remaining." << endl;
int guess;
cin >> guess;
attempt++;
if ( is_prime(guess) ) break;
}
if (attempt == limit)
cout << "You loose noob" << endl;
else
{
cout << "Winner Winner Chicken Dinner" << endl;
cout<<"You guess the Prime number by taking "<<attempt<<" tries.";}
}