Question

I’m not trying to ask you guys to help me to do homework because i’ve do much research and also try to program it myself but still i encounter problem and i think so far i’ve know where the problem is but still no solution can be figure out by me :

The Code

#include <iostream>
#include <string>
#include <cmath>

int main(void)
{
 using namespace std;

 int num;
 int max;
 string answer = "";

 cin >> num;

 for(int i = 2 ; i < num ; i++)
 {
     max = sqrt(i);

     if(max < 2) // This must be done beacuse sqrt(2) and sqrt(3) 
     {           // is 1 which will make it become  nonprime.
         answer += i;
         answer += ' ';
         continue;
     }

     for(int j = 2 ; j <= max ; j++) // Trial division ,divide each by integer
     {                               // more than 1 and less than sqrt(oftheinteger)
         if(i % j == 0)
             break;

         else if(j == max)
         {
             answer += i + " ";
             answer += ' ';
         }
     }

  }

         cout <<"The answer is " << answer ;


     return 0;
 }

The Question

1.)This program will prompt for a number from user and return all the prime number before it(e.g if user input 9 : then the answer is 2 , 3 , 5 , 7).

2.)I think the wrong part is the string and integer concatenation , till now i still puzzle how to concat string and integer in C++(Previous Javascript programmer so i’m accustomed to using + as string-int concat operator)

3.)Beside the problem i mention above , so far i’ve go through the code and find none of other problem exist.If any expert manage to find any , mind to point it out to enlighten me??

4.)If there’s any mistake in terms of coding or algorithm or anything done by me , please don’t hesitate to point it out , i’m willing to learn.

Thanks for spending time reading my question

Answer 1

The usual way to perform formatting in C++ is to use streams.

In this situation, you can use a std::stringstream to accumulate the results, and then convert it into a string when you do the final printing.

Include sstream to get the required type and function declarations:

#include <sstream>

Declare answer to be a std::stringstream instead of a std::string:

stringstream answer;

and then wherever you have:

answer += bla;

, replace it with:

answer << bla;

To get a std::string out of answer, use answer.str():

cout << "The answer is " << answer.str();