Prime Number Check 1-100 (C) [closed] - c

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The code below is to read input from the user to check if an int [1-100] is a prime number or not. (If out of range, will print "Done). If non prime, will output that to the console and the number divisible.
Right now this program is running correctly for 1-10 except for 3 and 9... Any suggestions?
#include <stdio.h>
int main()
{
int num, i;
printf("Number [1-100]:? \n");
scanf("%d", &num);
while(num>0 && num <101)
{
if (num==1||num==2)
printf("Prime\n");
for (i=2; i<=num/2; ++i)
{
if (num%i==0)
{
printf("Non-prime,divisible by %d\n",i);
break;
}
else {
printf("Prime\n");
break;
}
}
printf("Number[1-100]:? \n");
scanf("%d",&num);
}
printf("Done\n");
}

Answer 1

First, make sure your code has appropriate whitespace. This will help you realize when things aren't lined up like you think they are.
#include <stdio.h>
int main()
{
int num, i;
printf("Number [1-100]:? \n");
scanf("%d", &num);
while(num>0 && num <101){
if (num==1||num==2)
printf("Prime\n");
for(i=2; i<=num/2; ++i)
{
if (num%i==0)
{
printf("Non-prime,divisible by %d\n",i);
break;
}
else {
printf("Prime\n");
break;
}
}
printf("Number[1-100]:? \n");
scanf("%d",&num);
}
printf("Done\n");
}
Now you should realize that your else statement happens on the first check! So when 3 is not divisible by 2, it prints "prime."
And then it breaks out of the loop.
And this happens for EVERY number. All your program is doing is checking to see if numbers are divisible by 2.
If you wrote "Odd" instead of "Prime" it would at least be correct there.
This is the kind of problem where setting a flag might be useful (there are other ways to do this, but this is one way).
So you could set a flag, say int isPrime = 1;
Now, if you find out that the number is not prime, you simply set isPrime = 0;.
Finally, at the end of the for loop (let me repeat: AFTER the for loop finishes), you need to check that variable.
And you can say,
if (isPrime == 1)
{
printf("Prime\n");
} else
{
printf("Non-prime.");
}
I'll let you figure out how to print the divisor :)
(For reference, correctly using the flag would look like this -- and for clarity I removed the 'feature' in which it continuously looped)
#include <stdio.h>
int main()
{
int num, i;
int isPrime = 1;
printf("Number [1-100]:? \n");
scanf("%d", &num);
for(i=2; i<=num/2; ++i)
{
if (num%i==0)
{
isPrime = 0;
break;
}
}
if (isPrime == 1)
{
printf("Prime\n");
} else
{
printf("Non-prime.");
}
printf("Done\n");
}

Answer 2

The reason why 3 is behaving differently is that the for logic never reaches 3. For "(i=2; i <= num/2; ++i)", if num equals 3, then the i (being 2) is no longer less than 3/2, which is 1 (after rounding off). So, you should add "num==3" check to the "if (num==1||num==2)".

Answer 3

You're not checking the entire range between 2 and num/2. You need a while loop and a prime flag.
Something like this.
while(num>0 && num <101)
{
unsigned char prime = 1; // set prime flag
i = 2;
while( i < (num/2)+1)
{
if(num%i == 0)
prime = 0;
i++;
}
if(num == 1)
prime = 0;
if(prime == 0)
printf("%d is nonprime\n", num);
else
printf("%d is prime\n", num);
prime = 1;
printf("Number[1-100]:? \n");
scanf("%d",&num);
}