Prime Number Function

[SweetLeftFoot]

Hello, i have designed some code that works out the first 250 prime numbers and prints them to the screen. However i need to implement 2 functions, one of which returns a 1 if the number is a prime and 0 if it isn't. I have spent ages just getting this code to work and i'm worried i need to rip it to bit to get this function to work ! The function is:

int prime(int number)

here is my code so far:

#include <stdio.h>    //standard header file
#include <conio.h>    //header file
#define maxp 250    //defines "maxp" as a value of 250 //
 
int main()            //start of main program
{
  int factor;        //declares "factor" as an integer
  int primefound;    //declares "primefound" as an integer
  int i;            //declares "i" as an integer
  int l;            //declares "l" as an integer
  int number;        //declares "number" as an integer (the number to be tested)
 
  int primearray[maxp];//declares an array of size determined by "maxp" (250)
  int c;        //declares c as an integer (which is used has a counter)
  c=0;                //initialises c = 0
  number=1;        //initialises the number = 1
 
while(c<maxp)    // while is set up so that the loop executes whilst 
        / c is lesss than 250(i.e. until 250 have been found)                            
  { 
    number++;        //increments the number under test
    primefound=1;        //initialises primefound = 1
    for(i=2;i<number;i++)    //this for loop increments i whilst checking to see if prime
    {
    factor = number%i;    //by checking the remainder determines if prime or not
    if(factor ==0)        //if no remainder ever occurs then the number is not prime, so: 
          {primefound=0;}    //set primefound to 0
    }
 
    if (primefound!=0)    //if there is a prime number found then
    {
       primearray[c]=number; //put the number into the array in the order found.
       ++c;    // increment c ready for the next prime number to be found
    }
  }
  for(l=0;l<maxp;l++)    //this loop is used so that the whole array can be printed
  {
 
   printf("%d\n",primearray[l]);//prints the values in the array in order to the screen
 
  }
 
  getch();
}

help please !

the second function is
void displayprime(int prime[])

which has to display the prime numbers in columns on the screen

can someone point me in the right direction ?

[willakawill]

Hello, i have designed some code that works out the first 250 prime numbers and prints them to the screen. However i need to implement 2 functions, one of which returns a 1 if the number is a prime and 0 if it isn't. I have spent ages just getting this code to work and i'm worried i need to rip it to bit to get this function to work ! The function is:

int prime(int number)

here is my code so far:

#include <stdio.h>    //standard header file
#include <conio.h>    //header file
#define maxp 250    //defines "maxp" as a value of 250 //
 
int main()            //start of main program
{
  int factor;        //declares "factor" as an integer
  int primefound;    //declares "primefound" as an integer
  int i;            //declares "i" as an integer
  int l;            //declares "l" as an integer
  int number;        //declares "number" as an integer (the number to be tested)
 
  int primearray[maxp];//declares an array of size determined by "maxp" (250)
  int c;        //declares c as an integer (which is used has a counter)
  c=0;                //initialises c = 0
  number=1;        //initialises the number = 1
 
while(c<maxp)    // while is set up so that the loop executes whilst 
        / c is lesss than 250(i.e. until 250 have been found)                            
  { 
    number++;        //increments the number under test
    primefound=1;        //initialises primefound = 1
    for(i=2;i<number;i++)    //this for loop increments i whilst checking to see if prime
    {
    factor = number%i;    //by checking the remainder determines if prime or not
    if(factor ==0)        //if no remainder ever occurs then the number is not prime, so: 
          {primefound=0;}    //set primefound to 0
    }
 
    if (primefound!=0)    //if there is a prime number found then
    {
       primearray[c]=number; //put the number into the array in the order found.
       ++c;    // increment c ready for the next prime number to be found
    }
  }
  for(l=0;l<maxp;l++)    //this loop is used so that the whole array can be printed
  {
 
   printf("%d\n",primearray[l]);//prints the values in the array in order to the screen
 
  }
 
  getch();
}

help please !

the second function is
void displayprime(int prime[])

which has to display the prime numbers in columns on the screen

can someone point me in the right direction ?

Hi,

if(factor ==0)
 {primefound=0;}

in plain english; if there is no factor then this is not a prime number
should be;

if(factor ==0)        
{primefound=1;}

in plain english; if there is no factor then this is a prime number

[SweetLeftFoot]

Hi,

if(factor ==0)
 {primefound=0;}

in plain english; if there is no factor then this is not a prime number
should be;

if(factor ==0)        
{primefound=1;}

in plain english; if there is no factor then this is a prime number

this code works fine tho, i just need to implement the functions

[gurutheexpert]

Hello, i have designed some code that works out the first 250 prime numbers and prints them to the screen. However i need to implement 2 functions, one of which returns a 1 if the number is a prime and 0 if it isn't. I have spent ages just getting this code to work and i'm worried i need to rip it to bit to get this function to work ! The function is:

int prime(int number)

here is my code so far:

#include <stdio.h>    //standard header file
#include <conio.h>    //header file
#define maxp 250    //defines "maxp" as a value of 250 //
 
int main()            //start of main program
 
 
 
 
{
  int factor;        //declares "factor" as an integer
  int primefound;    //declares "primefound" as an integer
  int i;            //declares "i" as an integer
  int l;            //declares "l" as an integer
  int number;        //declares "number" as an integer (the number to be tested)
 
  int primearray[maxp];//declares an array of size determined by "maxp" (250)
  int c;        //declares c as an integer (which is used has a counter)
  c=0;                //initialises c = 0
  number=1;        //initialises the number = 1
 
while(c<maxp)    // while is set up so that the loop executes whilst 
        / c is lesss than 250(i.e. until 250 have been found)                            
  { 
    number++;        //increments the number under test
    primefound=1;        //initialises primefound = 1
    for(i=2;i<number;i++)    //this for loop increments i whilst checking to see if prime
    {
    factor = number%i;    //by checking the remainder determines if prime or not
    if(factor ==0)        //if no remainder ever occurs then the number is not prime, so: 
          {primefound=0;}    //set primefound to 0
    }
 
    if (primefound!=0)    //if there is a prime number found then
    {
       primearray[c]=number; //put the number into the array in the order found.
       ++c;    // increment c ready for the next prime number to be found
    }
  }
  for(l=0;l<maxp;l++)    //this loop is used so that the whole array can be printed
  {
 
   printf("%d\n",primearray[l]);//prints the values in the array in order to the screen
 
  }
 
  getch();
}

help please !

the second function is
void displayprime(int prime[])

which has to display the prime numbers in columns on the screen

can someone point me in the right direction ?

Hi ,

You can declare the function as You have done

int Prime (int TestNumber);
implementation:

int Prime (int TestNumber)
{
 
  int IsPrime; 
  // Here write the logic for testing a number to be prime and return 
 // set value of IsPrime accordingly in logic
 return IsPrime;
}
 

Second fuction is

 
void displayprime(int prime[], int nTotalPrimes)
{
  for (int i = 0; i< nTotalPrimes; i++
  prinf ("%d", prime[i]);
}
 
 

[DeMan]

Are you having problems with the writing method part or the logic part?

The logic for the method would be: given an integer x test whether it is divisible by any number below it (or if we're really keen, any number below the square root of it, in fact any prime number, but now we digress and get complicated).

In rough pseudocode this would be:

int isPrime(int x)
{
  int counter=2;
  int found=0;
  while((counter*counter)<=x &&(found==0))
  {
    if((x mod counter)==0) found =1; //Or if you don't mind 
    counter++
  }
return found;
}
 

How many columns did you want to display the primes in ?
Simplest way is tyo iterate through the array displaying 1 at a time adding "/n" to your print statement each time you need to start a new row and using "/t" to line numbers up on the screen....

And sorry, I was beaten to it, didn't realise,.....