Beginner question - Loop invariants (Specifically Ch.3 of "Accelerated C++")
- by Owen
Hi - as I said, a complete beginner question here. I'm currently working my way through "Accelerated C++" and just came across this in chapter 3:
// invariant:
// we have read count grades so far, and
// sum is the sum of the first count grades
while (cin >> x) {
++count;
sum += x;
}
The authors follow this by explaining that the invariant needs special attention paid to it because when the input is read into the variable x, we will have read count+1 grades and thus the invariant will be untrue. Similarly, when we have incremented the counter, the variable sum will no longer be the sum of the last count grades (in case you hadn't guessed, it's the traditional program for calculating student marks).
What I don't understand is why this matters. Surely for just about any other loop, a similar statement would be true? For example, here is the book's first while loop (the output is filled in later):
// invariant: we have written r rows so far
while (r != rows) {
// write a row of output
std::cout << std::endl;
++r;
}
Once we have written the appropriate row of output, surely the invariant is false until we have incremented r, just as in the other example?
It's probably something really obvious, anyone could enlighten me as to what makes these two cases different, that'd be great - and thanks in advance for taking the time to answer such a complete novice question.
Owen
