Question

Given a sequence (for example a string “Xa”), I want to get the next prefix in order lexicographic (i.e “Xb”). The next of “aZ” should be “b”

A motivating use case where this function is useful is described here.

As I don’t want to reinvent the wheel, I’m wondering if there is any function in C++ STL or boost that can help to define this generic function easily?
If not, do you think that this function can be useful?

Notes

• Even if the examples are strings, the function should work for any Sequence.
• The lexicographic order should be a template parameter of the function.

From the answers I conclude that there is nothing on C++/Boost that can help to define this generic function easily and also that this function is too specific to be proposed for free. I will implement a generic next_prefix and after that I will request if you find it useful.

I have accepted the single answer that gives some hints on how to do that even if the proposed implementation is not generic.

Answer 1

I’m not sure I understand the semantics by which you wish the string to transform, but maybe something like the following can be a starting point for you. The code will increment the sequence, as if it was a sequence of digits representing a number.

template<typename Bi, typename I>
bool increment(Bi first, Bi last, I minval, I maxval)
{
    if( last == first ) return false;
    while( --last != first && *last == maxval ) *last = minval;
    if( last == first && *last == maxval ) {
        *last = minval;
        return false;
    }
    ++*last;
    return true;
}

Maybe you wish to add an overload with a function object, or an overload or specialization for primitives. A couple of examples:

string s1("aaz");
increment(s1.begin(), s1.end(), 'a', 'z');
cout << s1 << endl;     // aba

string s2("95");
do {
    cout << s2 << ' ';  // 95 96 97 98 99
} while( increment(s2.begin(), s2.end(), '0', '9') );
cout << endl;