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Count Primes in Ranges in C++
We are given range variables START and END. The goal is to find the count of prime numbers in the range [START,END].
We will check if number i in range is prime by checking if any number other than 1 fully divides it and is between 1 and i/2. If it is prime. Increment count.
Let’s understand with examples.
Input
Start=1 End=20
Output
Primes in Ranges : 8
Explanation
Primes between 1 and 20 are: 2,3,5,7,11,13,17,19.
Input
Start=100 End=200
Output
Primes in Ranges : 21
Explanation
Primes between 100 and 200 are: 101 103 107 109 113 127 131 137 139 149 151 157 163 167 173 179 181 191 193 197 199
Approach used in the below program is as follows
We take range variables as START and END.
Function countPrimes(int strt,int end) returns the count of primes in range.
Take the initial variable count as 0.
Traverse using for loop from i=strt to i <=end
Take each number i and check if it is prime using isprime(i).
Function isprime(int num) returns 0 if the number is non prime and 1 if it is prime.
After the end of loop, return count as result.
Example
#include <bits/stdc++.h>
using namespace std;
int isprime(int num){
if (num <= 1)
return 0;
for (int i = 2; i <= num/2; i++){
if (num % i == 0)
{ return 0; }
}
return 1; //if both failed then num is prime
}
int countPrimes(int strt,int end){
int count=0;
for(int i=strt;i<=end;i++){
if(isprime(i)==1)
{ count++; }
}
return count;
}
int main(){
int START=10, END=20;
cout <<endl<<"Primes in Ranges : "<<countPrimes(START,END);
return 0;
}Output
If we run the above code it will generate the following output −
Primes in Ranges : 4
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