Lambda function, arguments and logic in c++ [duplicate] - c++

Question

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How does comparator function of c++ STL sort work?
(1 answer)
how does sort function in c++ work? [closed]
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Closed 1 year ago.
I am new to using lambda functions in C++. I have been researching the web, and found several articles, explaining the syntax and purpose of lambda function, but I have not come found articles which are clearly giving an explaining how to write the inner logic of a lambda function.
For example
During sorting a vector in c++ in decreasing order:
sort(v1.begin(), v1.end(), [](const int &a, const int &b){return b < a;});
I write the above code. Here, I have a few questions:
Why do I only provide two parameters in the lambda function? Why not three? or why not I give all the n parameter(n is size of vector) and do a logic? I am not trying to find maximum of two elements, I am trying to sort a vector, why should I only consider two values?
Why does a > b gives descending order? Why not b > a? Are there any kind of ordering inside the lambda function?
The return value in the above lambda function is either false(0) or true(1)? Why do I only have to return false(0) or true(1) to sort? Why can't I return a character to sort, like let's suppose for return value 'a' it is ascending and return value 'd' it is descending?
Again
During finding the max even element
itr = max_element(v1.begin(), v1.end(), [](const int &a, const int &b){
if (isEven(a) && isEven(b)) {
return (a < b);
} else
return false;
}
);
I am returning b > a. Rather than a greater than b. ???
Any suggestion would be greatly appreciated.

Answer 1

Your question has nothing to do with lambdas, but with the std::sort function.
Indeed, if you read the documentation about the third parameter (the comparison function, the lambda in your case), it says:
comparison function object which returns ​true if the first argument is
less than (i.e. is ordered before) the second.
The signature of the comparison function should be equivalent to the
following:
bool cmp(const Type1 &a, const Type2 &b);
Indeed, there is not need to create a lambda to pass it as the third parameter. You can pass any function object that receives two arguments of type T (the one of your container's elements) and returns bool.
For example, you can do something like this:
#include <vector>
#include <algorithm>
#include <iostream>
struct {
bool operator () (int a, int b){
return a > b;
}
}my_comp;
int main(){
std::vector<int> v={1,2,3};
std::sort(v.begin(), v.end(), my_comp);
for(auto e:v) std::cout << e << " ";
std::cout << std::endl;
}