Using scanf in a while loop

In C, 0 is evaluated to false and everything else to true. Thus, if scanf returned EOF, which is a negative value, the loop would evaluate to true, which is not what you'd want.

Since scanf returns the value EOF (which is -1) on end of file, the loop as written is correct. It runs as long as the input contains text that matches %d, and stops either at the first non-match or end of file.

It would have been clearer at a glance if scanf were expecting more than one input....

while (scanf("%d %d", &x, &y)==2) { ... }

would exit the loop when the first time it was unable to match two values, either due to end of file end of file (scanf returns EOF (which is -1)) or on input matching error (e.g. the input xyzzy 42 does not match %d %d so scanf stops on the first failure and returns 0 without writing to either x or y) when it returns some value less than 2.

Of course, scanf is not your friend when parsing real input from normal humans. There are many pitfalls in its handling of error cases.

Edit: Corrected an error: scanf returns EOF on end of file, or a non-negative integer counting the number of variables it successfully set.

The key point is that since any non-zero value is TRUE in C, failing to test the return value correctly in a loop like this can easily lead to unexpected behavior. In particular, while(scanf(...)) is an infinite loop unless it encounters input text that cannot be converted according to its format.

And I cannot emphasize strongly enough that scanf is not your friend. A combination of fgets and sscanf might be enough for some simple parsing, but even then it is easily overwhelmed by edge cases and errors.

You understood the C code correctly.

Sometimes the reason for testing the number of items read is that someone wants to make sure that all items were read instead of scanf quitting early when it the input didn't match the expected type. In this particular case it didn't matter.

Usually scanf is a poor choice of functions because it doesn't meet the needs of interactive input from a human user. Usually a combination of fgets and sscanf yield better results. In this particular case it didn't matter.

If later chapters explain why some kinds of coding practices are better than this trivial example, good. But if not, you should dump the book you're reading.

On the other hand, your substitute code isn't exactly a substitute. If scanf returns -1 then your while loop will execute.