This is a text-only version of the following page on https://raymii.org:
---
Title : std::string to lowercase or uppercase in C++
Author : Remy van Elst
Date : 07-11-2019
URL : https://raymii.org/s/snippets/std_string_to_lowercase_or_uppercase_in_cpp.html
Format : Markdown/HTML
---
I'm using [codewars][9] to practice my development skills. Today I learned
a method to transform a std::string's casing, either to uppercase or lowercase.
It uses a lambda and does loop over all characters in the string. Researching it
further, I also found out how to do unicode strings with Boost. This
article also includes a mini howto on installing Boost on Windows 10 via `mingw`
for use with CLion.
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### Case transformation for ascii
The codewars assignment was to count unique lowercase characters in a string,
then return the character which was found the most. For the string "hello" this
would be `l` since it's found twice. To do this I first needed to convert the
string to lowercase. This is the code I used to lowercase the string for the
codewars practice:
int main() {
std::string inStr = "UPPERCASE";
std::transform(inStr.begin(), inStr.end(), inStr.begin(),
[](unsigned char c){ return std::tolower(c); });
std::cout << inStr << std::endl;
return 0;
}
Example output:
#C:\Users\Remy\CLionProjects\test1\cmake-build-debug\src\example.exe
uppercase
For uppercase:
int main() {
std::string inStr = "lowercase";
std::transform(inStr.begin(), inStr.end(), inStr.begin(),
[](unsigned char c){ return std::toupper(c); });
std::cout << inStr << std::endl;
return 0;
}
Example output:
#C:\Users\Remy\CLionProjects\test1\cmake-build-debug\src\example.exe
LOWERCASE
### Non-ASCII
Remember: everytime you assume text is ASCII, a kitten dies somewhere.
The code above does not work with [Emoji's][1]:
std::string inStr = "\U0001F4A9 ";
std::transform(inStr.begin(), inStr.end(), inStr.begin(),
[](unsigned char c){ return std::tolower(c); });
std::cout << inStr << std::endl;
This won't give the expected result. I'm using an image since your browser
will probably not render this correctly:
![img][3]
A unicode string like [a common german word][7] will also not
work, same kind of weird output.
But, with Boost and ICU you can get this to work. The setup is difficult, but
when you have it compiling and working, it's a pleasure to work with. You
can just pass entire strings instead of looping over every character.
### Boost
Boost is a set of libraries for C++ development, of which most end up in the
standard library after a few years.
To include Boost in your [cmake project][2], either install it with your package
manager or download it manually.
#### Installing Boost on Windows 10 or Ubuntu
On Ubuntu 18.04 it's as simple as:
apt-get install libboost-dev-all
TL;DR: On Windows 10 use [this mingw build][6] or __be warned. Here be dragons.__
It cost me multiple hours of troubleshooting and debugging. Appearantly `mingw`
and Boost on Windows are not the best of friends. Especially not if you also
need `Locale`, then `libICU` is required as well. If you use Visual Studio and
MSVC or CLion with MSVC instead of `Mingw` it all should be less problematic.
`libICU` provides downloads for `MSVC`, for `MinGW` you're on your own, good
luck with compiling.
Open a cmd, navigate to the folder and build Boost. If
you have visual studio installed you can use that, I use `mingw` so I have to
specify that and I have to run a `mingw` cmd prompt (via the .bat file provided
by mingw). Make sure to have `g++` available as command:
C:\Users\Remy\Downloads\boost_1_71_0\boost_1_71_0>g++ --version
g++ (i686-posix-sjlj, built by strawberryperl.com project) 4.9.2
Copyright (C) 2014 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
Bootstrap:
C:\Users\Remy\Downloads\boost_1_71_0\boost_1_71_0>bootstrap.bat gcc
Building Boost.Build engine
Generating Boost.Build configuration in project-config.jam for msvc...
Bootstrapping is done. To build, run:
.\b2
[...]
Build:
b2 toolset=gcc
[lots and lots of compiling later]
1 file(s) copied.
...failed updating 666 targets...
...skipped 204 targets...
...updated 1573 targets...
Install:
b2 toolset=gcc install
This will install into `C:\Boost` and the `findBoost` cmake package will detect it.
If you specify a different folder, you need to set `BOOST_ROOT` as environment variable
or pass it to cmake.
In your `CMakeLists.txt` file the following options might help with debugging
if you get errors:
set (Boost_DEBUG ON)
set (Boost_ARCHITECTURE "-x32")
set (Boost_USE_STATIC_LIBS ON)
set (Boost_USE_MULTITHREADED ON)
set (Boost_DETAILED_FAILURE_MSG ON)
Do note that I spent a few hours fiddling and trying to get the `boost.Locale`
library to compile. I ended with a linker error:
C:/PROGRA~2/MINGW-~1/I686-8~1.0-P/mingw32/bin/../lib/gcc/i686-w64-mingw32/8.1.0/
../../../../i686-w64-mingw32/lib/../lib/libiconv.a(localcharset.o):localcharset.c
:(.text+0x73): undefined reference to `_imp__GetACP@0'
Due to not having `libicu` installed. As stated earlier, I gave up due to ICU
as far as I could find only providing MSVC compatible builds, not `MinGW` builds.
Continue on with this guide on a Linux system if you want to follow along, or
use CLion with MSVC instead of MinGW.
__Update after another few hours of debugging:__ when using [this build of mingw][6]
by Stephan T. Lavavej, the code and cmake does compile and link without errors.
#### Boost in your CMakeLists file
If you've followed [my setup guide for CMake][2] then you should add this to
the main root-folder `CMakeLists.txt` file right before `include_directories`:
find_package(Boost REQUIRED COMPONENTS locale)
if(Boost_FOUND)
include_directories(${Boost_INCLUDE_DIR})
message("-- Boost found: ${Boost_VERSION_STRING}")
else()
message(FATAL_ERROR "Boost not found!")
endif()
In the `src/CMakeLists.txt` file, add the following at the bottom:
if(Boost_FOUND)
target_link_libraries (${BINARY} ${Boost_LIBRARIES})
message("-- Boost link to: ${Boost_VERSION_STRING}")
else()
message(FATAL_ERROR "Boost not found!")
endif()
If all went well, your CMake output should include the two new messages:
-- Boost found: 1.71.0
-- Boost link to: 1.71.0
-- Configuring done
-- Generating done
-- Build files have been written to: C:/Users/Remy/CLionProjects/test1/cmake-build-debug
### Boost locale conversion code
This is the code I used with Boost to convert uppercase to lowercase:
boost::locale::generator gen;
std::locale loc=gen("");
std::locale::global(loc);
std::cout.imbue(loc);
std::string grussen = "grussEN";
std::string poopla = "\U0001F4A9";
std::cout <<"Upper "<< boost::locale::to_upper(grussen) << std::endl
<<"Lower "<< boost::locale::to_lower(grussen) << std::endl
<<"Title "<< boost::locale::to_title(grussen) << std::endl
<<"Fold "<< boost::locale::fold_case(grussen) << std::endl
<<"Poop "<< boost::locale::to_lower(poopla) << std::endl;
It's mostly [example code from Boost][8].
My static site generator doesn't like the german S and U, it will not render it
correctly. Here's a picture of the code:
![boost1][4]
The result works as you would expect:
![boost2][5]
[1]: https://xn--ls8h.la/
[2]: /s/tutorials/Cpp_project_setup_with_cmake_and_unit_tests.html
[3]: /s/inc/img/boost-fail1.png
[4]: /s/inc/img/boost-fail3.png
[5]: /s/inc/img/boost-fail2.png
[6]: https://nuwen.net/mingw.html
[7]: https://en.wiktionary.org/wiki/gr%C3%BC%C3%9Fen
[8]: https://www.boost.org/doc/libs/1_71_0/libs/locale/doc/html/conversions_8cpp-example.html
[9]: https://www.codewars.com/r/KjbvJA
---
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