macOS Need some help

Hey guys, rookie programmer here, need some help for a program due. Excuse me for the triviality of this program, remember we all have to start somewhere.

The topic: Write a program that asks the user to enter a fraction of two nature numbers, and then
reduces the fraction to lowest terms:
Enter a fraction of natural numbers: 18/6
The lowest term is: 3/1
Note: You’re required to check the validity of the input, i.e. the input should be non-negative
and the denominator should ≠ 0. If not, print a hint and let the user re-input.
Hints: 1. Use a loop to check the numbers until they are valid.
2. To reduce a fraction, first compute the greatest common divisor (GCD). A classic
algorithm for computing GCD is the Euclidean algorithm. It goes as follows: Let and be the
two numbers, . Compute the remainder when is divided by . Copy into and
copy the remainder into . If is 0, then stop and contains the GCD. Otherwise, repeat this
process. For detail, please refer to http://en.wikipedia.org/wiki/Euclidean_algorithm


I've never used a loop before, but right now here's what i got. :


Kind of get lost here. I know what I want to do, just not entirely sure on how to get there. Right now I'm just trying to calculate the GCD, not really working on anything else till i get that. I have c=a%b, this is the remainder, then c=a%c, so something like 10/6, c=4, then c=2, then if i did a/c, would give me 5, meaning c is zero, then breaking the loop? excuse me once more, some help is needed, just a general push in the right direction would be appreciated!

EDIT: now i'm thinking it could go something like:


This did not work, but am I headed in the right track?

not sure what you mean by that. I understand how I can get the GCD, but I cannot figure out how write a program for it. Right now I'm looking at,


c=a%b;
while(c!=0){ //while c is not equal to zero
a=b;
b=c;
c=a%b;
}





I'm not sure how to make my loop work to how i want it.

for something like 9/6, 9=a, 6=b. c would equal 3(not zero), then would go to the loop body, a is now 6, b is now 3, c is zero, so now c is equal to zero, thus leaving the loop. how do i leave the loop with c being equal to 3?

a hint was posted by the TA:

1. Initialize integer m and n
2. Do: m / n , and save remainder
3. Do: m = n, n = remainder
4. If n = 0, end and return m as the GCD
5. Else: repeat step 2 ~ 4

so now i changed to:

while(b!=0){
c=a%b;
a=b;
b=c;
}

so for something like a=24, b=10 i want the loop to go like this:
loop1: b!=0
c=14
a=10
b=14
loop2: since b!= 0
c=10
a=14
b=10
loop3:
c=4
a=10
b=4
loop4:
c=2
a=4
b=2
loop5: now b=0, thus exit from loop, leaving b as the GCD equaling to 2.
c=0
a=2
b=0

when i did this, then tried compiling, gave me b = 0

while(b!=0){
c=a%b;
a=b;
b=c;
}
printf("%d",a);


wooooh, a is the GCD!

so now my problem is that it alters the values of my orginal a and b, this is a problem since the reduced fraction would be a(orginal)/a(gcd). how can i make this happen? i thought i read that you can define variables that are used just inside the loop?

Right now I'm at,


#include <stdio.h>

main(void)
{


int numerator, denominator, remainder, numerator2, denominator2, reduced_numerator, reduced_denominator;


printf("Enter a fraction of natural numbers (such as 18/6): ");
scanf_s("%d/%d", &numerator,&denominator);


if(denominator!=0){

if(numerator/denominator>=0){

numerator2=numerator;
denominator2=denominator;

while(denominator2!=0){
remainder=numerator2%denominator2;
numerator2=denominator2;
denominator2=remainder;
}

reduced_numerator=numerator/numerator2;
reduced_denominator=denominator/numerator2;

printf("The lowest term is: %d/%d", reduced_numerator,reduced_denominator);
}
else { printf("Please enter a positive fraction");
}
}
else{ printf("Invalid denominator, please enter a non-zero denominator");}
return 0;

}




everything works perfectly, but in the assignment, there is a "Note: You’re required to check the validity of the input, i.e. the input should be non-negative and the denominator should ≠ 0. If not, print a hint and let the user re-input"

how do i do this? the chapter this assignment is based off includes break, continue, goto, and null statements.