How do scanf and getchar work in C? - c

Question

Here is something that confused me.
# include <stdio.h>
# include <limits.h>
int main()
{
int a;
int b;
printf("Enter two integers: ");
while(scanf("%d %d", &a, &b)!=2||b==0||a+b>INT_MAX||a*b>INT_MAX){
printf("Invalid entry, please re-enter: ");
while(getchar()=='\n');
}
printf("sum is %d, difference is %d, product is %lf, divid is %.2f, remainder is %d", a+b, a-b, (double)a*b, (float)a/b, a%b);
return 0;
}
With above code, if I enter "a 1" press ENTER:
Invalid entry, please re-enter:
pops up, then I enter "2 B" press ENTER, the last printf will execute.
However, if I change while(getchar()=='\n'); to while(getchar()!='\n');, and with the same entry (I enter "a 1" press ENTER)
Invalid entry, please re-enter:
pops up, then I enter "2 B" press ENTER), the last printf will not execute, and
Invalid entry, please re-enter:
pops up again.
What causes the difference here? How exactly do scanf and getchar work?

Answer 1

In the first case (while (getchar() == '\n');), the getchar() reads the a, but it isn't a newline, so the loop exits, leaving the space and the 1 in the input buffer. The repeated call to scanf() skips white space, reads 1, skips white space (newline included) and reads 2, leaving space and B in the input. Since the loop condition is now terminated (scanf() returned 2), the printf() is executed.
In the second case (while (getchar() != '\n');), this loop reads the a, the blank, the 1, and the newline before stopping. When you type 2 B, there aren't two numbers, so scanf() returns 1 and the main loop continues.
Note that your code will go into an infinite loop if you indicate EOF (usually Control-D on Unix, Control-Z on Windows) — at least, on most systems. You should always consider EOF. In context, you probably need something like:
int rc;
while (((rc = scanf(…)) != 2 && rc != EOF) || …)
You'd test rc against EOF before printing, too.

Answer 2

How do scanf and getchar work in C?
Sadly, they often do not work together well.
Code is a poor example of attempting to consume the rest of the input line when the input text is a problem.
If scanf("%d %d", &a, &b) failed to scan in 2 int, a prompt occurs and input is read with getchar until an Enter or '\n' happens.
If 2 int were scanned, various faulty tests are performed on a,b
// poor code
while(scanf("%d %d", &a, &b)!=2||b==0||a+b>INT_MAX||a*b>INT_MAX){
printf("Invalid entry, please re-enter: ");
while(getchar()=='\n');
}
The problem is that the code certainly intends to read a line of user input and then validate it. Unfortunately, scanf("%d %d" can consume multiple '\n' and leaves stdin with unclear contents when 2 int are not scanned. Code is an infinite loop on end-of-file.
Better to read a line of input with fgets(). Flushing stdout insures the prompt is seen before input is read.
char buf[80];
const char *prompt = "Enter two integers: ";
for (;;) {
fputs(prompt, stdout);
fflush(stdout);
if (fgets(buf, sizeof buf, stdin) == NULL) Handle_EndOfFile_or_Error();
prompt = "Invalid entry, please re-enter: ";
int n;
if (sscanf(buf, "%d%d %n", &a, &b, &n) != 2) continue; // not 2 ints
if (buf[n]) continue; // extra text
if ((a < 0) ? (b < INT_MIN - a) : (b > INT_MAX - a)) continue; // + overflow
... // other tests
else break;
}
other tests

Answer 3

scanf() and getchar() read what is in the input buffer (stdin for standard input). When stdin is empty, the program will wait for the user to write something with the keyboard. When you press ENTER, the text you just wrote is stored in stdin. Then scanf() or getchar() can read something in stdin.
Notice that pressing ENTER will store a '\n' character (newline) in stdin.
A code like while( getchar() != '\n' ); is asking to getchar() to read one character from stdin while the character is not '\n'. It compares the return of getchar() with '\n'. So this is a way to "clean" the input buffer otherwise your program can go crazy.
EDIT
With the code you posted :
scanf() is called, you write "a 1", it tries to read a first integer but it is a character ('a') so it stops reading. printf() ask you to re-enter, then getchar() is called and reads the 'a'. Now scanf() is called a second time, reads the 1, then you have to enter something, you enter "2 B" so the second %d in scanf() can read the 2. Now the return of scanf() is 2, b is not 0 and the others conditions are false so the loop is ending. Notice that the 'B' character is still in stdin :)

Answer 4

I try to summarize what you need to know.
First, about your question. You do know the function of scanf is to read what the input (number) we give right? getchar on the other hand, is like scanf except that it is used for character/sentence only. You cannot use getchar to input number.
Second, there are no problem in your code except that it is okay not to use some line like that getchar line. It is also okay not to use while. Unless you want to make some condition, you may combine while with if / else function.
Third, you cannot input the number like you did: "1 a", "2 b". Those are considered invalid. This is because you are using %d in your scanf which refer to decimal number only. It means, you can input number 1-9 only and not the letters. So when the program asked to enter the 2 integers, you just type for example: 4 5. That's 4, space, 5. That is the correct way.