can you help explain me how to use std::transform ? I need to create a function that returns a string and has a string as parameter and use std::transform to convert all the uppercase char to lower and vice versa lowercase char to uppercase example: input = "aBc" output = "AbC"

and i want to do it with a lambda, not using other mehtod like toupper, etc.

​​​​​​this is what i have so far which doesnt work, it compiles and runs but it returns nothing/ empty string;

std::string func(std::string inputString){
        std::string result;
        std::transform(inputString.begin(), inputString.end(), result.begin(), [](char& c){
                if (c < 97) return c   32;
                if (c >= 97) return c - 32;
        });
        return result;
}

CodePudding user response：

You haven't allocated any space in result, so you are observing a pretty gentle case of undefined behavior (gentle because it's observably not working, rather than happening to work by pure luck).

So you either allocate such memory before calling std::transform, e.g. via

result.resize(inputString.size());

or you can use a back_inserter for result, instead of its begin iterator result.begin(), which will take care of the allocation; the page on std::transform has such an example. In this latter case, is still probably a good idea to reserve some space via

result.reserve/* not resize! */(inputString.size());

CodePudding user response：

The cause of the problem:
You haven't allocated memory for result.

You can cope with it in one of the following ways:

1. Allocate memory explicitly using std::string::resize before calling std::transform:

result.resize(inputString.length());

2. Use std::back_inserter instead of result.begin() when calling std::transform:

std::transform(inputString.begin(), inputString.end(), std::back_inserter(result), [](char& c) {

Note: your lambda's body can be simplified to:

    if (c < 97) return c   32;
    return c - 32;

It will also fix a compiler warning (MSVC at least complained that the lambda is not always returning a value).