C++ issue with lambda function - c++

Question

So I have a map with a key as a string, and I want to sort that map by the first value in the strings.
I want to sort the strings like this:
10 - 20
40 - 50
60 - 80
200 - 400
420+
I have a couple of more maps that I sort alphabetically. But this doesn't work for this case, because it would sort on the first letter, so it will become:
10 - 20
200 - 400
40 - 50
60 - 80
420+
Which I obviously do not want.
I normally sort like this:
dropdowncategory.KeySort([](const string& A, const string& B) {
return A.Compare(B, ESearchCase::IgnoreCase) < 0;
});
Simple.
But in this case from the A and B I want to get the first number, convert it to int and sort on that.
So what I did is I wrote a function (Not able to test it yet, but it should get the first number from the string, this is not the problem.):
int GetIntFromString(string s) {
int value = 0;
string L, R;
if (s.Contains("-")) {
s.Split("-", &L, &R);
}
else{
s.Split("+", &L, &R);
}
value = FCString::Atoi(*L);
return value;
}
From this function I will get the first number in the string. Then I want to sort on that outcome.
What I tried is the following:
dropdowncategory.KeySort([](const string& A,const string& B) {
return GetIntFromString(A) < GetIntFromString(B);
});
and:
string SA, SB;
dropdowncategory.KeySort([SA,SB](string& A, string& B) {
return GetIntFromString(SA) < GetIntFromString(SB);
});
But I keep getting and error on the GetIntFromString function in the return value of the KeySort:
Error (active) an enclosing-function local variable cannot be referenced in a lambda body unless it is in the capture list.
I am not sure why though.
I guess I cannot use that function inside the KeySort because it is a local function, but I don't know how to fix that.

Answer 1

As stated in std::stoi documentation:
Discards any whitespace characters (as identified by calling isspace()) until the first non-whitespace character is found, then takes as many characters as possible to form a valid base-n (where n=base) integer number representation and converts them to an integer value.
(emphasis is mine) it does what you need already, so your lambda can be just:
dropdowncategory.KeySort([](const string& A,const string& B) {
return std::stoi(A) < std::stoi(B);
});
note, converting string to integer is not very simple operation so you may want to reorganize your data so you do not have to do it for that strings so many times.

Answer 2

You can try to express your intent: treat the entries as pairs of ints, and use existing sorting logic for that:
std::vector<std::tuple<int, int>> pairs;
std::transform(begin(dropdowncategory), end(dropdowncategory),
std::back_inserter(pairs),
[](auto s){
std::stringstream ss(s);
int i; ss >> i;
char dummy; ss >> dummy;
int j; ss >> j;
return std::make_tuple(i, j);
});
std::sort(begin(pairs), end(pairs)); // 30, 50 will come before 200, 210
// convert pairs back to strings?

Answer 3

FString SA, SB;
dropdowncategory.KeySort([SA,SB](FString& A, FString& B) {
return GetIntFromString(SA) < GetIntFromString(SB);
});
Here you have 2 local string vars, empty (I suppose, no idea what's FString). You provide a lambda as a sorting function. Usually such function should have signature bool(const Key& left, const Key& right), and your lambda almost satisfied this requirement (pay attention to const). left and right here will be two keys to compare, and you need to define this comparison to define your sorting order. So correct version would be:
dropdowncategory.KeySort([](const FString& A, const FString& B) {
return GetIntFromString(A) < GetIntFromString(B);
});
This still doesn't explain your error, mainly because I have no idea what the error is.
By the way, GetIntFromString should be a free function, or at least a static member function, because its implementation depends only on input parameters.
P.S. listen to #Slava in the question comments, which can lead to
dropdowncategory.KeySort([](const FString& A, const FString& B) {
return std::atoi(*A) < std::atoi(*B);
});