如何在if语句c ++中将字符串转换为较低的字符串

Question

Something like this:

if (string.toLower() == "string") {
    cout << "output";
}

I have tried using: transform(input.begin(), input.end(), input.begin(), toupper); with no results.

Answer 1

Indeed you can use std::tolower() with std::transform() to achieve this. For example:

#include <string>
#include <algorithm>
#include <cctype>
#include <iostream>

std::string to_lower(const std::string& s) {
    std::string lower{s};
    std::transform(lower.begin(), lower.end(), lower.begin(), 
                   [](unsigned char c){ return std::tolower(c); });
    return lower;
}

int main()
{
    std::string str{"UPPER_CASE"};

    if (to_lower(str) == "upper_case")
        std::cout << "String matched.";

    return 0;
}

Output:

String matched.

Note here that for std::tolower(int ch) : "If the value of ch is not representable as unsigned char and does not equal EOF , the behavior is undefined" . Hence the lambda taking an unsigned char as the 4th argument to std::transform .

Also note that upon NRVO for calls to to_lower() , should such optimization takes place, no extra std::string is being copied -- beyond once for the single one necessary to hold the lower case version.

Answer 2

Using C++17 if with initializer

#include <iostream>
#include <string>
#include <algorithm>

int main()
{
    std::string value {"Hello World"};

    if (
        std::string lower{value}, dummy{lower.end(), std::transform(
            lower.begin(), lower.end(), lower.begin(),
            [](unsigned char c){ return std::tolower(c); }
        )};
        !lower.compare("hello world")
    ) {
        std::cout << "equal" << std::endl;
    }
}

Answer 3

what about something like this :

string LowerCaseString(string  str) {
  string ret;
  for (auto chr : str) {
      ret += tolower(chr);
  }
  return ret;
}

if (LowerCaseString(somestring) == "string") {
     // do something
}