我正在尝试编写欧几里得算法，但是当我在C代码中使用循环时程序崩溃

Question

I am trying to write the Euclidean Algorithm in C code. When I use the do while or for loop the program crashes:

#include<stdio.h>
int gcd(int a,int b);

int main()
{
    int x,r,y;
    printf("Enter two values:\n");
    scanf("%d%d",&x,&y);
    if(x<y)r=x,x=y,y=r;
    r=gcd(x,y);
    printf("\n\n   Result: %d",r);
    return 0;
}

int gcd(int a,int b)
{
    if(a==0||b==0)
    {
        if(a==0)return b;
        if(b==0)return a;
    }
    else
    {
        for(;;a>0||b>0)//also try with do while loop but problem same
        {
            int x;
            x=a%b;
            a=b;
            b=x;
        }
    }
}

Can you please explain me the reason behind the crash of the program? I use codeblocks with gcc

Answer 1

This:

for(;;a>0||b>0)

is an infinite loop, then end condition is the middle part, it should be:

for (; a > 0 || b > 0;)

I doubt the logic of that condition, shouldn't it be && ?

Answer 2

Instead of if(x<y)r=x,x=y,y=r; you will need to do:

if(x < y){
    r = x;
    x = y;
    y = r;
}

Also you will need to place condition in second statement of for loop. Instead of:

for(;;a>0||b>0)
{
    ...
}

you could do while(a > 0 && b > 0) . Depending on your familiarity you could use recursion like:

int hcf(int a, int b)
{
    if (b == 0)
        return a;
    else
       return hcf(b, a%b);

}

Also note that your hcf function does not have return statement in else block. So you will have to use conditional return:

if (a == 0) return b;
else return a;

at the end of hcf function

Answer 3

This statement

for(;;a>0||b>0)

does nothing so you have an endless loop.

What seem to happen is that b becomes zero and then you do a % 0 which is illegal and cause a crash.

Remember that it is the second statement in the for-loop that is the condition for terminating the loop - it's not the third. So maybe you wanted:

for( ; a>0||b>0; )