Can anyone tell the difference between these two code snippets in C? - c

Question

I'm new to C programming, I'm practicing while loop using getchar(). But I'm confusing about these two while loop code.
This is the first code:
char c;
c=getchar();
while(c != 'X')
c=getchar();
printf("Thanks for finally inputting an X.\n");
I undestand that this function will printf until the user enter X.
Here is the second code:
char c;
while((c=getchar()) != 'X');
printf("Thanks for finally inputting an X");
Which did the same thing as the last function, but I don't really understand while((c=getchar()) != 'X'); It means to me that when the user enters other characters than X, it will be true and print the sentence.

Answer 1

Look closely at the while in the second snippet:
char c;
// here ----------------v
while((c=getchar()) != 'X');
printf("Thanks for finally inputting an X");
There is a ; immediately after the while. This means that the body of the loop is empty. So the loop will continue to run just the condition until it is false, i.e. when c is equal to 'X'.

Answer 2

Both approaches are equivalent, but the second is more concise and uses less code:
Evaluating the condition (c = getchar()) != 'X') first calls getchar() to read a byte from the standard input, store in into c, which by the way should be defined as an int, and compares this byte with 'X', continuing the loop until the user finally types an X (followed by a return) because the body of the while loop is an empty statement.
The main problem with this approach is the program does not test for EOF, so it will perform a costly infinite loop if the standard intput reaches the end of file without ever reading an X.
Here is a better alternative:
int c;
while ((c = getchar()) != EOF) {
if (c == 'X') {
printf("Thanks for finally inputting an X\n");
break;
}
}
To avoid the confusing syntax while((c=getchar()) != 'X'); some programmers would use an empty block:
while ((c = getchar()) != 'X') {
}
Possiblity with an obnoxious comment:
while ((c = getchar()) != 'X') {
/* do nothing */
}
I personally prefer a redundant continue:
while ((c = getchar()) != 'X')
continue;
Note also that this syntactic issue is also present in other languages inspired by C: C++, java, javascript, C#, Objective C, etc.

Answer 3

These two code snippets are equivalent in behavior.
In order to understand the second snippet, you need to remember that what exists in the inner parentheses in this case, will be evaluated first.
So, this:
while((c=getchar()) != 'X');
will have the expression:
c = getchar()
evaluated first. Now, as you already know, the method getchar will be executed, receive the character the user input, and it will assign it to the variable c.
Now our expression looks like this:
while((c) != 'X');
but since c is just a character, it's surrounding parentheses do not have an effect anymore, and we have right now:
while(c != 'X')
which is the same as the the condition of the while loop in the first code snippet, that you already understand.
Tip: Read about operators precedence, which will help you understand why the parentheses are needed - otherwise you'd get a different, most probably unwanted, behavior.
The reason is that the != operator has higher precedence than the assignment operator. Using parentheses imposes the order of evaluation we desire.