What is the most efficient way to replace \\n with \n

Question

I ask this because I am using SFML strings. sf::String does not insert a new line in the presence of a \n.

I can't seem to figure out a way without using 3/4 STL algorithms.

std::replace_if(str.begin(), str.end(), [](const char&c](return c == '\\n'), '\n'}); does not work. The string remains the same.

I have also tried replacing the \\ occurrence with a temporary, say ~. This works, but when I go to replace the ~ with \, then it adds a \\ instead of a \

I have produced a solution by manually replacing and deleting duplicates after \n insertion :

for (auto it = str.begin(); it != str.end(); ++it) {
    if (*it == '\\') {
        if (it + 1 != str.end()) {
            if (*(it + 1) != 'n') continue;
            *it = '\n';
            str.erase(it + 1);
        }
    }
}

Then at least add the code that you tried, and describe how it doesn't work. — cigien, Aug 18 '20 at 00:34
Have you considered just looping over the string manually? `for (auto& c : str) { ... }` — Tas, Aug 18 '20 at 00:38
`'\\n'` [isn't what you would like it to be](https://en.cppreference.com/w/cpp/language/character_literal#Notes). — user4581301, Aug 18 '20 at 00:38
A [mcve] would be useful, because it sounds like you have multiple bugs. — Eljay, Aug 18 '20 at 00:42
'\\n' is a multi-character literal you certainly don't want that. ) Also I am not sure you want to do what you think you need to do. — drescherjm, Aug 18 '20 at 00:54
[https://stackoverflow.com/questions/3960954/multicharacter-literal-in-c-and-c](https://stackoverflow.com/questions/3960954/multicharacter-literal-in-c-and-c) — drescherjm, Aug 18 '20 at 00:58
Always enable all warnings. You'll definitely get one for `'\\n'` — phuclv, Aug 18 '20 at 01:44

Jarod42 · Answer 1 · 2020-08-18T00:47:18.647

3

You might do:

str = std::regex_replace(str, std::regex(R"(\\n)"), "\n")

Demo

edited Aug 18 '20 at 00:47

answered Aug 18 '20 at 00:42

Jarod42

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My compiler does not let me do this. I get an error for some reason? – expl0it3r Aug 18 '20 at 00:45
3

All of this back-and-forth could be reduced immensely by asking the right question: include a [mcve] in your question (it should be like 5 lines long!), mention what compiler you're using and IDE. There's no set rule on what version of C++ people who answer questions use, though it's typically _at least_ C++11 (which is 10 years old). If you only have support for an older version like C++03 you should specify – Tas Aug 18 '20 at 00:47
Added demo link and a "typo". – Jarod42 Aug 18 '20 at 00:47

score 1 · Answer 2 · answered Aug 18 '20 at 00:57

The problem is that '\\n' is not a single character but two characters. So it needs to be stored in a string "\\n". But now std::replace_if doesn't work because it operates on elements and the elements of a std::string are single characters.

You can write a new function to replace sub-strings within a string and call that instead. For example:

std::string& replace_all(std::string& s, std::string const& from, std::string const& to)
{
    if(!from.empty())
        for(std::string::size_type pos = 0; (pos = s.find(from, pos) + 1); pos += to.size())
            s.replace(--pos, from.size(), to);
    return s;
}

// ...

std::string s = "a\\nb\\nc";

std::cout << s << '\n';

replace_all(s, "\\n", "\n");

std::cout << s << '\n';

It might be two characters `{'\\', 'n'}`... it might be three characters `{'\\', '\\', 'n'}` ... hard to say for sure. — Eljay, Aug 18 '20 at 01:09

What is the most efficient way to replace \\n with \n

2 Answers2