Question

Consider an input file A2.txt. Each line of A2.txt consists of two numbers separated by space. Write a C++ program that reads the two numbers from each line and prints how many numbers are prime between them. Your program must have a user defined function that takes one number as input parameter and detects whether its prime or not. Show your output in a file named B2.txt using the following format: Sample Input: 3 17 42 91 Sample Output: 6 11 (For further clarification, the first line of the sample output = 6 because from 3 to 17 there are 6 prime numbers which are 3,5,7,11,13,17)

I realize there is already another answer to this question on Chegg, however there are multiple ways in which the code can be written but I was hoping someone could provide a easier, understandable and more basic code.

Answer 1

PROGRAM

#include<iostream>
#include<fstream>
using namespace std;
//prime method
bool isPrime(int n)
{
    int i,c=0; //c is a variable count the number of factors
    //loop from 2 to n-1
    for(i=2;i<=n-1;i++)
    if(n%i==0) //if divisible then i is a factor
    {
        c++; //count the number of factors
        break; //terminate the loop
       }
    if(c==0) //if c==0 then its a prime number
    return true;
    else
    return false; //return false when its not a prime number
}

int main()
{
   //declare the input file object
   ifstream infile;
   //declare the output file object
   ofstream outfile;
   //variable declarations
   int a,b,i,count=0;
   //open the input file
   infile.open("D:/A2.txt");
   //if unable to open the file then display the messge
   if(!infile)
{
cout<<endl<<"Unable to open the source file";
exit(0);
}
//open the file for writing purpose
outfile.open("D:/B2.txt");
//block when unable to open the file for writing purpose
if(!outfile)
{
cout<<endl<<"Unable to open the target file";
infile.close();
exit(0);
}
//loop to read the data from input file
   while(!infile.eof())
   {
      count=0;//set the count to 0
      infile>>a>>b;//Read the data from file and store them in two variables
      for(i=a;i<=b;i++) //loop from starting value to ending value
          if(isPrime(i)) //check for prime or not
              count++; //count when the number is prime
       outfile<<count<<" "; //write the count into the file
   }
}

Output

Input file

Out file