C++ Code optimization

Question

I have created my custom function to turn a wstring into lower case. However, it is pretty slow in DebugMode. Yes, I know ReleaseMode is what counts, but anyway it is pretty unnerving.

wstring wstringToLower(wstring u) { wstring s; for (int i=0;i<u.size();i++) { wstring sChar; sChar=u.substr(i,1); int iChar=static_cast<int>(sChar[0]); int iNewChar=charCodeToLower(iChar); wstring sNewChar=wstring(1,iNewChar); s.append(sNewChar); } return s; }

Does anybody see anything obvious that I could improve to speed up the code, even in DebugMode?

Thank you!

stackoverflow.com/questions/313970/stl-string-to-lower-case - I'm afraid your function is about 50 times too complicated for what it does. — Mat
– Mat, Commented Nov 25, 2013 at 18:47
@0x499602D2 I need to handle Unicode characters. Standard functions only handle ASCII. — tmighty
– tmighty, Commented Nov 25, 2013 at 18:48

Community · Accepted Answer · 2017-05-23 11:56:07Z

There's no need to make temporary strings.

So, for start, instead of:

 wstring sNewChar=wstring(1,iNewChar); s.append(sNewChar);

This should do the trick:

 s.push_back(iNewChar);

Then, instead of:

 wstring sChar; sChar=u.substr(i,1); int iChar=static_cast<int>(sChar[0]);

This should work:

 int iChar=static_cast<int>(u[i]);

And, of course, as noted by Marcel, you can do everything on the passed copy, avoiding the extra string allocation.

Also, as noted in the comments: How to convert std::string to lower case? . Also, read all answers (and comments) here: how to Make lower case letters for unicode characters :

#include <algorithm> #include <string> #include <iostream> using namespace std; int main() { ::setlocale(LC_ALL,""); std::wstring data = L"НЕМАЊА БОРИЋ"; // Wide chars std::transform(data.begin(), data.end(), data.begin(), ::towlower); // prints немања борић std::wcout << data << std::endl; return 0; }

http://en.cppreference.com/w/cpp/string/wide/towlower

There is actually no need for s at all. He could do all the operations on u and return it and avoid the extra copy.
@ZacHowland you're totally right, I don't know how I missed it - just saw it in Marcel's answer.
@NemanjaBoric Thank you. ::setlocale(LC_ALL,""); You are great!

Marcel Blanck · Accepted Answer · 2014-02-20 20:11:46Z

First of all I would avoid to allocate memory for variables each run, since allocating is a heavy operation.

Then do not call u.size() in the for-loop declaration. It will be called every loop otherwise. Every function call less that you call in a loop is a good win for performance.

Next everything Nemanja Boric said in the other answer.

And since the variable u is passed as copy, you can use it as return value and operate directly on it.

wstring wstringToLower(wstring u) { int size = u.size(); for (int i = 0; i < size; ++i) { u[i] = charCodeToLower(static_cast<int>(u[i])); } return u; }

Conclusion: Basically avoid to allocate memory or calling functions in loops. Do just as much as you really have to.

Zac Howland · Accepted Answer · 2013-11-25 19:15:55Z

There is actually no need for the wstringToLower function at all. You can use <algorithm> to do most of the work:

std::wstring str = "Some String"; std::transform(str.begin(), str.end(), str.begin(), ::towlower);

If you are trying to localize it, you may want to modify it slightly:

std::wstring str = "Some String"; std::locale loc; // set your locale std::transform(str.begin(), str.end(), str.begin(), [](wchar_t c) { return use_facet<ctype<wchar_t>>(loc).tolower(c); });

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