Is +#+#++#5,2++++ a valid revision?

Yes. The revision +#+#++#5,2++++ starts with a + followed by the character # to use as a delimiter, and extends to the next occurrence of #. That Add command +#+# says to add the one character + contained between the delimiters.

The next character + starts another Add command. The + that follows it is the character to use as a delimiter, and the text #5,2 up to the next occurrence of the delimiter + is the text to be added by that Add command ++#5,2+.

The next character + starts another Add command. The + that follows it is the delimiter, and the next occurrence of that delimiter comes right afterward. This final Add command +++ says to add no text since there is no text between the delimiters. (If would be foolish for createRevision to ever produce a command like +// or +++ or +== since it needlessly makes the revision string longer, but your revise function must still correctly handle such a command if it's in the revision string.)

If createRevision is called with an older file containing the 17 characters "abcdefghi\njklmno\n" and a newer file containing the 15 characters cdefghi\njklmxyz" (no final newline), would the 11 characters "#2,12+/xyz/" be one possible correct revision file?

Yes. Here are three examples of correct revision files for that example:

• "#2,12+/xyz/" (copy "#2,12", add "+/xyz/")
• "#2,12+/xyz/\n" (copy "#2,12", add "+/xyz/", do-nothing)
• "#2,12\n+/xyz/\n" (copy "#2,12", do-nothing, add "+/xyz/", do-nothing)

An incorrect revision file would be "#2,12+/xyz\n/" (copy "#2,12", incorrectly add "+/xyz\n/"); it's incorrect because the original new file did not end with a newline, yet this revision file has an instruction that would make a newline the last character (after the z).

How do I use std::hash to hash a string?

If str is an expression of type string or something convertible to string, then if you've included <functional>, the expression hash<string>()(str) hashes the string and yields an unsigned integer in the range 0 to at least 4 billion. Notice the extra set of parentheses.

(For the insatiably curious, hash<string>() default-constructs a temporary object of type hash<string>, a class that has a member function named operator() that takes a string and returns an unsigned integer. If t is an object of a class that has such a member function, then the call t.operator()(str) can be written t(str), similarly to how for an object x of a class that has a member function named operator[], like string and vector, the expression x.operator[](i) can be written x[i].)

Project 4. Homework 5. What to do?

First of all, notice that Homework 5 contributes 3.8% to your total score, but should take at most an hour or two to do. With such a high return on your time investment, you'd be wise to switch your focus off Project 4 for a brief time to be sure you do Homework 5.

That said, if you haven't started on Project 4 yet, you should immediately do so, starting by implementing only a very simple revise function that works only for revision files consisting of a single valid Add command (and possibly Do-nothing commands after it); it doesn't have to work for anything else. Such a revision need not do anything with the fold argument (since there are no Copy commands), and will simply write to the fnew argument the text between the delimiters of the Add command.

What's the point of this? Well, we've found that any time we give an assignment that involves reading or writing files, some people spend an inordinate amount of time wrestling with the mechanics of opening a file. For example, perhaps they ignored or misread the part of the File I/O writeup that says where the file is expected to be on a Windows or Mac or Linux machine, so they wonder why their program can't find the file. Whatever the problem is, it's not really a C++ issue, but until it's solved it holds the person back from getting any work done. The reason to start on the simple revise function is to make sure that if you have this problem, you get past it as soon as possible, when there's plenty of office hours support, because this kind of environment problem is most easily dealt with in person.