Thread: trouble with #define statement...

This code prompts the user to enter a number and returns the number with a statement if it is prime or not. It also includes "," to separate every 3 digits. I modified the code so that the user could have a choice of three separators(comma, space, period).

I thought there might be a way to use a control structure to use three different #define commands...something like...

Code:

if(choice == 1)
    #define GROUP_SEP ','
else if (choice == 2)
    #define GROUP_SEP ' '
else if (choice == 3)
    #define GROUP_SEP '.'

The code below works, but I couldn't use the above approach. It would crash my program. Any ideas why?

Code:

#include <iostream>
#include <cmath>
#include <cstdlib>
#include <sstream>

using namespace std;

string output_formatted_string(long long num);
string determine_separator(int choice);

int main() {
long long int  n;                 // Number to test for prime-ness, long long data type
int  i ;                                    // Loop counter
int  is_prime = true;   // Bool ean fl ag. . .
                                     // Assume true for now.

        // Get a number from the keyboard.
cout << "Enter a number and press ENTER: " << endl;
cin >> n;

// Test for pri me by checki ng for di vi si bi l i ty
//  by al l whol e numbers from 2 to sqrt(n) .
i = 2;
while (i <= sqrt(n) ) {   // Whi l e i i s <= sqrt(n) ,
if (n % i == 0)        //  If i di vi des n,
is_prime = false;  //    n i s not pri me.
i++;                   //  Add 1 to i .
}
// Pri nt resul ts
string s = output_formatted_string(n);
if (is_prime)
cout << s <<" is prime. " << endl ;
else
cout << s <<" is not prime." << endl;

system("PAUSE");
return 0;
}

//#define GROUP_SEP       ','
#define GROUP_SIZE        3

string output_formatted_string(long long num) {

    int choice;
    string GROUP_SEP;

    cout << "Choose digit separator (1) for comma, (2) for space, (3) for period: " << endl;
        cin >> choice;
        GROUP_SEP = determine_separator(choice);

    //Read data into string s.

    stringstream temp, out;
    temp << num;
    string s = temp.str();

    //Write first characters, in front of
    // first separator (GROUP_SEP).

    int n = s.size() % GROUP_SIZE;
    int i =0;
    if (n > 0 && s.size() > GROUP_SIZE) {
        out << s.substr(i, n) << GROUP_SEP;
        i += n;
            }
    // Handle all the remaining groups.

    n = s.size() / GROUP_SIZE -1;
    while (n-- > 0) {
        out << s.substr(i, GROUP_SIZE) << GROUP_SEP;
        i += GROUP_SIZE;
    }
    out << s.substr(i); //Write the rest of digits.
    return out.str();    //Convert stream -> string.
}

string determine_separator(int choice){

    switch(choice){

    case 1:
        return ",";
        break;

    case 2:
        return " ";
        break;

    case 3:
        return ".";
        break;
    }

}

# statements are preprocessor directives; they are evaluated before your code is even compiled, let alone run.

you aren't looking for a #defined variable, you are looking for a char.

generally speaking, don't use preprocessor directives, they don't work that well, and generally make your work harder to debug.