Prime numbers in C - Overclock.net

loonyt7 · 10-04-09

I have the below program and it runs ok but when the program is suppose to be telling me if a number is prime or not it tells me my first fumer is prime and all others are not prime and that is not right. Can someone help me with this.

Code:

#include <stdio.h>

int isprime ( int number ); 

int main ( void ) {
int value;
int i = 0;
int number;
int modulo;
printf("Enter 10 integers\n");
printf("I will tell you whether each one is prime or not\n");


for ( i = 0 ; i < 9 ; i ++){
printf("Enter an int: ");
scanf("%d", &value);

if (i < isprime( number) ){
printf("%d is a prime number\n", value);
}
else
printf("%d is not a prime number\n", value);

}
system ("Pause");
return (0);
}

int isprime ( int number ) {
int i = 2;
int flag = 1;
for ( ; i < number / 2 ;  );
{
int modulo = number % i ;
if ( i = 0 ) flag = 0;
else flag = 1;
}

return flag;
}

gamervivek · 10-04-09

just tried running your code and the first error it commits is that the variable number is not initialised.So if it's working on your end i am guessing it's using a garbage value.you have to use value for that.
your isprime function seems to be borked.I understand that you are trying to get the remainder from the division of the number by all numbers from 2 to number/2.
However your for loop doesn't work correctly because of the semicolon at the end indicating that it finishes before it starts,so it's basically.....

just went through it again,the code seems to be broken everywhere,hopefully someone else can chime in.

edit: can you explain why you used the statements you did?

anyway here is a modified version of your code that works fine for me-

Code:

#include <stdio.h>

int isprime ( int number ); 

int main ( ) 
{
int value;
int i ;
int number;
int modulo;
printf("Enter 10 integers\n");
printf("I will tell you whether each one is prime or not\n");


for ( i = 0 ; i < 9 ; i ++)
 {
   printf("Enter an int: ");
   scanf("%d", &value);

if ( isprime(value) )
printf("%d is a prime number\n", value);

else
printf("%d is not a prime number\n", value);

}

system ("Pause");
return 0;
}

int isprime ( int number ) 
{
//int i = 2;
//int flag = 1;
int modulo;
for ( int i= 2; i <= (number / 2) ; i++  )
{
 modulo = number % i ;
 if ( modulo == 0 ) 
 return 0;
}

return 1;
}

loonyt7 · 10-04-09

The code I had posted was part of a code I was given to debug. What was there was all I had to work with. I had gotten through all of the problems except this last one that you helped me with. Thank you so much for the help on this.

Pooping^fish · 4 Weeks Ago

Code:

if(num == 1) return true;
for(x = 2; x < num; x++) {
    if(num % x == 0) return false; }
return true;

That should do fine.

The Bartender Paradox · 4 Weeks Ago

Quote:

Originally Posted by loonyt7

I have the below program and it runs ok but when the program is suppose to be telling me if a number is prime or not it tells me my first fumer is prime and all others are not prime and that is not right. Can someone help me with this.

Code:

#include <stdio.h>

int isprime ( int number ); 

int main ( void ) {
int value;
int i = 0;
int number;
int modulo;
printf("Enter 10 integers\n");
printf("I will tell you whether each one is prime or not\n");


for ( i = 0 ; i < 9 ; i ++){
printf("Enter an int: ");
scanf("%d", &value);

if (i < isprime( number) ){
printf("%d is a prime number\n", value);
}
else
printf("%d is not a prime number\n", value);

}
system ("Pause");
return (0);
}

int isprime ( int number ) {
int i = 2;
int flag = 1;
for ( ; i < number / 2 ;  );
{
int modulo = number % i ;
if ( i = 0 ) flag = 0;
else flag = 1;
}

return flag;
}

The problem you describe occurs because of this line:

Code:

...
if (i < isprime( number) ){
...

The function isprime returns 1 or 0. It is being compared to i, which is only less than 1 on the first iteration of the input loop. If you remove the comparison to i the code will no longer exhibit that problem:

Code:

...
 if (isprime( number) ){
...

I know the OP is two weeks old and his assignment is probably long done by now, but I figured it might be good to have the answer anyway.

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Prime numbers in C

10-04-09	#3 (permalink)
loonyt7 New to Overclock.net Join Date: Oct 2009 Posts: 5 Rep: 0 Unique Rep: 0 Trader Rating: 0	The code I had posted was part of a code I was given to debug. What was there was all I had to work with. I had gotten through all of the problems except this last one that you helped me with. Thank you so much for the help on this.

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