[HOMEWORK] How to check if a number is prime or not

[td0g03]

Hello, the code below is what I got so far. I would like the loop to check numbers to 2-10000 and stop at 10000 and begins at 2. The question I have how do you make a function into a loop for.

/*    This program checks if a number is prime or not.
*/
 
 
#include <stdio.h>
 
int isPrime (int n, int f);
 
//    Function Declarations 
int main (void)
//    Local Definitions 
{
int n,f, isprime;
//    Statements 
 
   for ( n = 2; n<=10000; n++)
    {   
    if(n < 2)
    break;
    if(n>=2)
    n = f;
    if(n>=2)
    isPrime(int n,int f)
    }
   return 0;
}
 
/*    ==================== isPrime ====================
    This function checks if a number is prime or not
    using a simple algorithm that tries to find a divisor.
    It stop searching at 1+ the integer part of the squre
    root of the number being tested.
       Pre   num
       Post  returns 1 if num is prime, 0 otherwise
*/
int isPrime(int n,int f)
{
     if(n==f)
          printf("1");
 
     if(n%f==0 || n==1)
          printf("0");
 
 return;
 
}
 
 

[RedSon]

You have a for loop and a function call already in your program. What are you asking about? You want to loop through all the numbers between 2 and 10000, but you already do that. You want it to loop back from 10000 to 2 when it finally reaches 10000, that can be easily done by checking if the current value is >= 10000 and if it is assigning a value of 2 to start all over again.

If you need other answers you are going to have to describe your question in more detail.

[RedSon]

/*    ==================== isPrime ====================
    This function checks if a number is prime or not
    using a simple algorithm that tries to find a divisor.
    It stop searching at 1+ the integer part of the squre
    root of the number being tested.
       Pre   num
       Post  returns 1 if num is prime, 0 otherwise
*/
int isPrime(int n,int f)
{
     if(n==f)
          printf("1");
 
     if(n%f==0 || n==1)
          printf("0");
 
 return;
 
}
 
 

Also your isPrime method does not return anything. I would assume that the compiler has complained about that to you already. If it hasn't I would suggest using another compiler.

[td0g03]

You have a for loop and a function call already in your program. What are you asking about? You want to loop through all the numbers between 2 and 10000, but you already do that. You want it to loop back from 10000 to 2 when it finally reaches 10000, that can be easily done by checking if the current value is >= 10000 and if it is assigning a value of 2 to start all over again.

If you need other answers you are going to have to describe your question in more detail.

Sorry, let me try to make it more clear. I want to make a loop that is in main that loops from 2 to 10000. The loop will go through one function "isPrime". isPrime will check to see if a number is a prime or not and will return 1 if it is and if not return 0.

[td0g03]

Also your isPrime method does not return anything. I would assume that the compiler has complained about that to you already. If it hasn't I would suggest using another compiler.

I notice that and changed it to

int isPrime(int f, int temp)
{
 
     if(f==temp)
          return 1;
     else
     if(f%2==0 || f==1)
          return 0;
 
 

The question I was asked was

Write a function named checkPrime that checks if a number is prime or not. In main() call checkPrime in a loop, to display all prime numbers between 2 and 10000. An integer is prime if its only positive divisors are itself and 1. For instance: 2, 3, 5, 7, 11, 13, 17, 19, 23 are prime numbers, and 4, 10,15, 22, 30 are not prime. A simple way of checking if a number is prime is to try to find a divisor other than 1 and itself; if there is at least one divisor the number is not prime, otherwise the number is prime. When to stop searching for divisors? For example, if we are checking if 47 is prime, we can stop at 7 which is 1 + the integer part of the square root of 47.

[RedSon]

Sorry, let me try to make it more clear. I want to make a loop that is in main that loops from 2 to 10000. The loop will go through one function "isPrime". isPrime will check to see if a number is a prime or not and will return 1 if it is and if not return 0.

Your main method already has a loop in it that starts at 2 and goes to 10000 and it calls isPrime. Do you have more questions about that?

As for your isPrime method, it looks like you have got a good setup but it is going to require a bit more design in order to work properly.

[td0g03]

Your main method already has a loop in it that starts at 2 and goes to 10000 and it calls isPrime. Do you have more questions about that?

Yeah. Am I correctly checking if its a prime or is it wrong? The result I get is completely wrong.

[RedSon]

Yes the code you have in your isPrime method does not really do anything useful, perhaps it might be a good idea to spend some time trying to design a good algorithm for this. I do not recommend you just jump right in and try to program it straight away. Take some time to think about it first, and then post your results.

[DeMan]

When to stop searching for divisors? For example, if we are checking if 47 is prime, we can stop at 7 which is 1 + the integer part of the square root of 47.

Exactly.....

(Also, just to be pedantic, your spec says you need a method checkPrime and your using isPrime....)

[RedSon]

Exactly.....

(Also, just to be pedantic, your spec says you need a method checkPrime and your using isPrime....)

I saw that but I wasn't going to say anything, you are just splitting hairs there.

[td0g03]

Exactly.....

(Also, just to be pedantic, your spec says you need a method checkPrime and your using isPrime....)

Actually, my teacher told me to use isPrime instead of checkPrime.

[RedSon]

Actually, my teacher told me to use isPrime instead of checkPrime.

Well it doesn't really matter, as Shakespeare said "A rose by any other name..."

[meatstu]

I cant quite see what youre trying to do with your bit of code but if you want to find if a number is prime why dont you try something like this:

1. Declare a variable, call it 'div' and initialise equall to 3.

2. Get the user to type in a number to be checked.

3. Divide that number by 2. If there is no remainder the number is not prime, Yes? If there is a remainder go to step 3.

4. Divide the number by the variable 'div'. If there is no remainder the number is not prime, if there is a remainder increment 'div' by 2 and repeat step 4.

Continue step 4 untill you either (a) you find the number is not prime or (b) div becomes larger than or equall to the number input by the user, in which case it is a prime number.

Hope it makes sense.
Stu