Replace multiple spaces with one space in a string

How would I do something in c++ similar to the following code:

//Lang: Java
string.replaceAll("  ", " ");

This code-snippet would replace all multiple spaces in a string with a single space.

Solution 1:

bool BothAreSpaces(char lhs, char rhs) { return (lhs == rhs) && (lhs == ' '); }

std::string::iterator new_end = std::unique(str.begin(), str.end(), BothAreSpaces);
str.erase(new_end, str.end());   

How this works. The std::unique has two forms. The first form goes through a range and removes adjacent duplicates. So the string "abbaaabbbb" becomes "abab". The second form, which I used, takes a predicate which should take two elements and return true if they should be considered duplicates. The function I wrote, BothAreSpaces, serves this purpose. It determines exactly what it's name implies, that both of it's parameters are spaces. So when combined with std::unique, duplicate adjacent spaces are removed.

Just like std::remove and remove_if, std::unique doesn't actually make the container smaller, it just moves elements at the end closer to the beginning. It returns an iterator to the new end of range so you can use that to call the erase function, which is a member function of the string class.

Breaking it down, the erase function takes two parameters, a begin and an end iterator for a range to erase. For it's first parameter I'm passing the return value of std::unique, because that's where I want to start erasing. For it's second parameter, I am passing the string's end iterator.

Solution 2:

So, I tried a way with std::remove_if & lambda expressions - though it seems still in my eyes easier to follow than above code, it doesn't have that "wow neat, didn't realize you could do that" thing to it.. Anyways I still post it, if only for learning purposes:

bool prev(false);
char rem(' ');
auto iter = std::remove_if(str.begin(), str.end(), [&] (char c) -> bool {
    if (c == rem && prev) {
        return true;
    }
    prev = (c == rem);
    return false;
});
in.erase(iter, in.end());

EDIT realized that std::remove_if returns an iterator which can be used.. removed unnecessary code.

Solution 3:

A variant of Benjamin Lindley's answer that uses a lambda expression to make things cleaner:

std::string::iterator new_end = 
        std::unique(str.begin(), str.end(),
        [=](char lhs, char rhs){ return (lhs == rhs) && (lhs == ' '); }
        );
str.erase(new_end, str.end());

Solution 4:

Why not use a regular expression:

boost::regex_replace(str, boost::regex("[' ']{2,}"), " ");