如何使用for_each，transform，iterators和lambda的某种组合将字符串std :: set转换为小写c ++？

Question

I'm trying to convert a set of strings to lowercase without using low level loops like while and for(;;) because I'm practicing using the STL. I was thinking about using for_each and transform and lambda but I'm not really sure how they work.

#include <string>
#include <iterator>
#include <algorithm>
#include <set>
using namespace std;
int main()
{
    set<string> words;
    //insert a bunch of words using words.insert().....
    //convert everything in words to lower case
    return 0;
}

How would I convert each string in the set words using any combination of for_each, transform, iterators and lambdas?

I was thinking of doing something like: transform(words.begin(), words.end(), words.begin(), ??lambda??) however I don't know how to do the 4th parameter

Answer 1

You can't do something like transform(words.begin(), words.end(), words.begin(), ??lambda??), because words.begin() would be a const iterator.

You'll have to do something like this:

set<string> words;
set<string> new_words;
transform(words.begin(), words.end(), std::inserter(new_words, new_words.begin()), [](string s) {
        transform(s.begin(), s.end(), s.begin(), ::tolower); 
        return s;
    });
return new_words;

Answer 2

try

     std::for_each(words.begin(),words.end(),[](char & c) { c = tolower(c); });

and include cctype.