counting primes between 1 to x

[abir50]

Trying to get pi(x)=number of primes from 1 to x but i am totally lost after getting up to here.

#include <iostream>
#include <cmath>
using namespace std;
bool isPrime;
typedef unsigned long long ull;

ull pi(ull x){
    // pi(x)=number of prime between 1 to x
}

bool prime(int x){
    isPrime=true;
    
    }
    return isPrime;
}

int main(){
    int x;
    cout<<"Input a number: ";
    cin>>x;
    
    }
}

[Ganado]

Move most of your logic in main to pi. Have a counter (ex: int counter = 0;) and every time prime(d) is true, increment that counter. Then, return the count after the for loop.

In main, having something like:

int main()
{
    ull x;
    cout << "Input a number: ";
    cin >> x;
    ull n = pi(x);
    cout << n << endl;
}

[longberns]

#include <iostream>
//#include <cmath>
using namespace std;
// bool isPrime;
typedef unsigned long long ull;

bool isPrime(int x) {
	if (x == 4) {
		return false;
	}
	for (int i = 2; i < x / 2; ++i) {
		if (x % i == 0) {
			return false;
			break;
		}
	}
	return true;
}

ull primeNumbers(int x) {
	ull primeCount = 0;
	for (int d = 2; d <= x; ++d) {
		if (isPrime(d)) {
			primeCount++;
		}
	}
	return primeCount;
}

int main() {
	int limit;
	cout << "Input a number: ";
	cin >> limit;

	ull primeCount = primeNumbers(limit);
	cout << "Number of prime numbers between 1 and  "
		<< limit << " : " << primeCount << endl;
}

NB: Think of how x can be validated so you get a valid integer before passing into your primeNumbers

[abir50]

I changed it to something like that but the output was always 0, now i know what i did wrong ty.

[keskiverto]

We cannot point/explain your errors unless you don't post your latest version.


PS. Do not erase content from your earlier posts.

[abir50]

sorry removed it because prof doesn't want classmates copying each others code.

but the first code that i posted before basically printed the prime from 1 to x, i took that code and changed it to something like this which wasn't working properly but i fixed it.

#include <iostream>
#include <cmath>
using namespace std;
bool isPrime;
typedef unsigned long long ull;
bool prime(int x){
    isPrime=true;
    if(x==4){
        isPrime=false;
    }
    for(int i=2; i<x/2; ++i){
            if(x%i==0){
                isPrime=false;
                break;
            }
    }
    return isPrime;
}

ull pi(ull x) {
	ull count=0;
	for (int d=2; d<=x; ++d){
		if(isPrime==true) {
			count=1;
		cout<<count;	
		}
	}
}

int main(){
    int x;
    cout<<"Input a number: ";
    cin>>x;
    cout<<pi(x);
}

the code was something like this before i fixed it, i had some more stuff on it but don't remember.

[Ganado]

You should probably increment the count, not just assign 1 to it. (count++ instead of count=1).