The Sieve of Eratosthenes in C using arrays

Question

I'ma beginner in C and I have to convert The Sieve of Eratosthenes algorithm into C code. This is the algorithm given: *START Initialize the array is_prime so that all the values of the elements will be TRUE. set the value of is_prime[1] to be FALSE (since 1 is NOT prime.) For I=2 until sqrt(N) execute: set all the multiples of I to FALSE, starting with I*I until N. Print all the indexes of is_prime that hold the value TRUE. END*

Here is my code so far:

#include <stdio.h>        
#include <math.h>

#define N 300             

void displayPrime (bool checkPrime);         
bool checkPrime (int num);

main()                    
{
     bool is_prime[N+1];         

     displayPrime(is_prime);

     getchar();      
}

void displayPrime (bool check)          
{
     int I;

     for(I=1; I<N; I++)
     {
              checkPrime(is_prime[I]);
              if(is_prime[I]==TRUE)
              {
                      printf("%d\n", I);
              }
              else if(is_prime[I]==FALSE)
              {
                      printf("");
              }
     }
}

bool checkPrime (int num)
{
     int num;

     is_prime[1]=FALSE;
     for(I=2; I<=sqrt(N); I++)
     {
              for(num=I; num<=N/num; num=num*I)
              {
                      is_prime[num]=FALSE;    
              }         
              return(is_prime[I]);
     }

}

The program does not compile, and I'm wondering what is wrong with the program. Thank you

Answer 1

I am not looking into the algorithm. This is the working code (Using visual studio):

#include <stdio.h>        
#include <math.h>

#define N 300             

void displayPrime (bool checkPrime);         
bool checkPrime (int num);
bool is_prime[N+1];


void displayPrime (bool check)          
{
     int I;

     for(I=1; I<N; I++)
     {
              checkPrime(is_prime[I]);
              if(is_prime[I]==true)
              {
                      printf("%d\n", I);
              }
              else if(is_prime[I]==false)
              {
                      printf("");
              }
     }
}

bool checkPrime (int num)
{
     int I;
     is_prime[1]=false;
     for(I=2; I*I<=N; I++)
     {
              for(num=I; num<=N/num; num=num*I)
              {
                      is_prime[num]=false;    
              }         
              return(is_prime[I]);
     }

}

void main()                    
{

     displayPrime(is_prime);

     getchar();      
}

I have changed these things:

1. You cannot declare a variable twice.
2. Every variable can be used within scope {}
3. A function called should be declared before you call.
4. TRUE should be true and FALSE should be false.
5. use return type in main function.

Anyway Sieve has a lot of good implementation. Try them out.