Question

Possible Duplicate:
Why doesn't getchar() wait for me to press enter?

I continue to learn C, and at this stage I have something that is not clear to me.
When I write a program, that has several printf();, at the end, where I ask a user to press Enter key to finish the program, I have to write getchar(); twice, because when I write it once it does not work. I use getchar(); only at the end of the program, nowhere else.

I work on Ubuntu. I write in C.

here is my latest work:

#include<stdio.h>
main()
{
int m,n,r,k,q,l;
printf("This program will help you to find GCD & LCM of 2 non-negative integers\n");
printf("Now, you'll be asked to enter your integers, press Enter to continue");
getchar();
printf("Enter first integer:");
scanf("%i", &m);
printf("Enter second integer:");
scanf("%i", &n);
while(m<0 || n<0)
{
printf("The integers cannot be negative! You'll be asked to enter integers again.\n");
printf("Enter first integer:");
scanf("%i", &m);
printf("Enter second integer:");
scanf("%i", &n);
}
while(m==0 && n==0)
{
printf("Both of the integers cannot be zero at the same time! You'll be asked to enter integers again.\n");
printf("Enter first integer:");
scanf("%i", &m);
printf("Enter second integer:");
scanf("%i", &n);
}
if(n>m)
{
int b;
b=n;
n=m;
m=b;
}
r=m%n;
if(r==0)
{
printf("The GCD of these integers is %i\n", n);
printf("The LCM of these integers is %i\n", m);
printf("Press Enter to finish");
getchar();
getchar();
return 0;
}
k=n%r;
while(k>0)
{
r=k;
k=q;
}
l=(m*n)/r;
printf("The GCD of these integers is %i\n", r);
printf("The LCM of these integers is %i\n", l);
printf("Press Enter to finish");
getchar();
getchar();
return 0;
}

Answer 1

The reason you need 2 getchar() is because

1. the last scanf() call left the ENTER waiting in the buffer
2. the 1st getchar() “ate” that ENTER
3. the 2nd getchar() waits for input.

---

To properly deal with user input, use fgets() and sscanf() instaed of the simpler scanf(). Define a buffer for these functions, for example

char buffer[1000];

and then replace your scanf() calls with the pair

fgets(buffer, sizeof buffer, stdin);
sscanf(buffer, "%d", &n);

---

In the future you might want to also check the return value of sscanf() to detect invalid inputs, like foo42ENTER

if (sscanf(buffer, "%d", &n) != 1) /* invalid input */;

---

Edit (using strtol() rather than sscanf() is even better — thanks to @Scooter)

char buffer[1000];
char *err;
/* ... */
fgets(buffer, sizeof buffer, stdin); /* error checking ommited */
n = strtol(buffer, &err, 10);
/* error checking ommited */
/* ... */