C++ simple If statement making the rest of the program not execute - c++

Question

I have an assignment where I must read from a file and perform various calculations on it and write the answer to an output file. Everything was going great until I came to this step:
"Reread the file and compute the sum of the integers in the file as long as the sum does not exceed 1000. Use a flag controlled loop structure."
My code snippet is as follows:
dataFile2.close();
dataFile2.clear();
dataFile2.open("J:\\datafile2.txt");
sum = 0;
while(sum < 1000)
{
dataFile2 >> num;
sum = sum + num;
if(sum > 1000)
sum = sum - num;
}
answers << "The sum of the integers not exceeding 1000 is " << sum << endl;
cout << "The sum of the integers not exceeding 1000 is " << sum << endl;
return 0;
My variables have already been declared. when I take out the if statement the sum adds the last number and the sum then exceeds 1000. When the If statement is left in, the answers and cout statements are not executed and there are no compiler warnings or errors.
Any help on this would be greatly appreciated.
-ThePoloHobo

Answer 1

Since no one seems to want to give you a correct answer... (and
to be fair, it's hard to give a correct answer without actually
doing your work for you).
There are two issues in you code. The first is the requirement
that you use a flag. As I said in my comment, the idiomatic
solution would not use a flag, but there's no problem using one.
A flag is a boolean variable which will be tested in the
while, and will be set in a conditional in the loop, when you
find something that makes you want to leave the loop.
The second issue is that you are using num without checking
that the input has succeeded. You must check after the >>
operator. The idiomatic way of checking (and the only thing
that should ever be used by someone not experienced in the
language) is to treat the stream as if it were a boolean:
dataFile2 >> num;
if ( dataFile2 ) {
// Input succeeded...
} else {
// Input failed for some reason, maybe end of file
}
Since all operations on a stream return a reference to the
stream, it is usual to merge the test and the input:
if ( dataFile2 >> num ) {
// succeeded
} else {
// failed
}
(Personally, I find the idea of modifying state in the condition
of an if or a while horrible. But this idiom is so
ubiquitous that you should probably use it, for the simple
reason that that's what everyone expects.)
In pedagogical environments, it's probably acceptable to
consider any failure to be end of file, and just move the test
up into the while (except, of course, that you've been asked
to use a flag). In other contexts, you'll want to take into
account the fact that the failure could be due to a syntax error
in the input—someone inserted "abc" into the file where
you were expecting a number. There are a number of ways of
handling this, all of which are beyond the scope of what you are
trying to do, but be aware that after you've detected failure,
you can interogate the stream to know why. In particular, if
dataFile2.eof() is true, then the failure was (probably) due
to you having read all of the data, and everything is fine. (In
other words, failure to read a data is not necessarily an error.
It can be simply end of file.)

Answer 2

You don't seem to be using a flag variable, which could help in this case. Something like this should fix it:
sum = 0;
bool sumUnder1000 = true; //Or the C++ equivalent, I'm a bit rusty
while(sumUnder1000)
{
if(!dataFile2.good()){
sumUnder1000 = false; //We've reached end of file or an error has occurred
return;
}
dataFile2 >> num;
sum = sum + num;
else if(sum > 1000){
sum = sum - num;
sumUnder1000 = false;
}
}