break Exits the Loop

Are you done with the read_int exercise? I imagine you could've written something like this:

while (isdigit(c = getchar())) {
	result = result * 10 + (c - '0');
}

and if so, great job! You could have written a longer version too, and it's completely fine. Something like this will work:

finished = 0;

while (!finished) {
	c = getchar();
	if (c >= '0' && c <= '9') {  /* same as isdigit(c) */
		result = result * 10 + (c - '0');
	} else {
		finished = 1;
	}
}

This is the second time we write a while loop this way:

while (!finished) {
	...
}

The finished variable gets eventually assigned to 1 inside the loop, and the loop ends after that.

There is an easier way to do the same thing: the break instruction. break; means "exit the loop right now". Here I'll use while (1), which is an infinite loop – or, better to say, it would've been an infinite loop if I hadn't used break inside:

while (1) {
	c = getchar();
	if (c >= '0' && c <= '9') {  /* same as isdigit(c) */
		result = result * 10 + (c - '0');
	} else {
		break;
	}
}

Since break exits the loop, it sometimes makes sense to check the exit condition first:

while (1) {
	c = getchar();
	if (!isdigit(c)) {
		break;
	}
	result = result * 10 + (c - '0');
}

But break is useful not only for would-be-infinite loops, but for regular loops as well. Let's look at the example on the next page!