CODEWITHSUNDEEP
c++namespacesprimesiostream
Naman Verma
Naman Verma

Reputation: 64

i'm writing a cpp program to print all prime numbers between two numbers . Program is running successfully but it is not printing anything

#include <iostream>
using namespace std;
bool isPrime(int num){
    for(int i=2;i<=num;i++){
        if(num%i==0){
            return false;
        }
    }
    return true;
}
int main()
{
    int a,b;
    cin>>a>>b;
    for(int i=a;i<=b;i++){
        if(isPrime(i)){
            cout<<i;
        }
    }
    return 0;
}

Please tell me my mistake, program is running successfully but not printing anything.

Upvotes: 1

Views: 393

Answers (4)

Ajay Singh Bisht
Ajay Singh Bisht

Reputation: 1

#include <iostream>
#include <cmath>
using namespace std;

bool isPrime(int num) {
    if (num <= 1) return false; // 0 and 1 are not primes
    if (num <= 3) return true; // 2 and 3 are primes
    if (num % 2 == 0) return false;
    for (int i = 3; i <= sqrt(num); i += 2) {
        if (num % i == 0) {
            return false;
        }
    }
    return true;
}

int main() {
    int a, b;
    cout << "Enter two integers (a < b): ";
    cin >> a >> b;
    if (a >= b || a < 2) {
        cout << "Invalid input. 'a' should be less than 'b' and greater than or equal to 2." << endl;
        return 1; 
    }
    cout << "Prime numbers between " << a << " and " << b << " are:" << endl;
    for (int i = a; i <= b; ++i) {
        if (isPrime(i)) {
            cout << i << " ";
        }
    }
    cout << endl;
    return 0;
}

Upvotes: 0

shubham danodia
shubham danodia

Reputation: 1

bool isPrime(int num){

      if(num<2){
          return false; 
       }
          else {
                  int x = num/2;
                   for(int i=2;i<=x;i++){
                     if(num%i==0){
                     return false;
                     }
                     }
                     }
            return true;

}

Upvotes: 0

f4h1m
f4h1m

Reputation: 18

for(int i=2;i<num;i++)

you've used i<=num where the logic should be i<num in your isPrime function. cause all the primes are divided by themselves. but you should not count that while finding primes

Upvotes: 0

Tom Karzes
Tom Karzes

Reputation: 24062

The problem is you're checking all divisors from 2 to num, inclusive. Since every positive integer divides itself, you're not finding any primes. The minimal fix is to change the loop exit condition to exclude num:

    for(int i=2;i<num;i++)

Note that this will work, but it's very inefficient. Here's a much faster version. This version also rejects anything less than 2, since 0 and 1 aren't prime:

bool isPrime(int num){
    if (num < 2){
        return false;
    }
    if (num == 2){
        return true;
    }
    if (num % 2 == 0){
        return false;
    }
    for (int i=3; i*i<=num; i+=2){
        if(num%i==0){
            return false;
        }
    }
    return true;
}

This version makes a special case check for 2, and for even numbers. After that, it only checks for odd divisors. Once the divisor exceeds sqrt(num), it can stop (the loop exits when i*i > num).

Upvotes: 3

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