Assignments = Expressions

When you assign a value, you also return that value.

So, you can do things like this:

int c;
while ((c = getchar()) != EOF){
	putchar(c);
}

c will get value from user value, then compare that given value with EOF. note that assignment matters a lot refer to Operator Precedence

It shortens code which would otherwise be like this:

int c;
c = getchar();
while (c != EOF){
	c = getchar();
	putchar(c);
}