Simple program using transform and lambdas isn't working - c++

Question

I'm getting these errors. Why?
In file included from /usr/include/c++/4.7/algorithm:63:0,
from prog.cpp:2:
/usr/include/c++/4.7/bits/stl_algo.h: In instantiation of ‘_OIter std::transform(_IIter, _IIter, _OIter, _UnaryOperation) [with _IIter = __gnu_cxx::__normal_iterator >; _OIter = std::back_insert_iterator >; _UnaryOperation = main()::]’:
prog.cpp:10:98: required from here
/usr/include/c++/4.7/bits/stl_algo.h:4951:2: error: no match for call to ‘(main()::) (int&)’
prog.cpp:10:64: note: candidates are:
In file included from /usr/include/c++/4.7/algorithm:63:0,
from prog.cpp:2:
/usr/include/c++/4.7/bits/stl_algo.h:4951:2: note: int (*)(int, int)
/usr/include/c++/4.7/bits/stl_algo.h:4951:2: note: candidate expects 3 arguments, 2 provided
prog.cpp:10:79: note: main()::
prog.cpp:10:79: note: candidate expects 2 arguments, 1 provided
#include <algorithm>
#include <vector>
int main()
{
std::vector<int> v {1, 2, 3, 4, 5, 6};
std::vector<int> r;
std::transform(v.begin(), v.end(), std::back_inserter(r), [] (int a, int b) { return a + b; });
}
This code is supposed to add up each pair of numbers and put it into the r vector but it's not working. Why is that?

Answer 1

You've picked the overload of transform() that works on a single range - and thus expects a unary functor to be provided as the last argument.
If what you want is to work on two ranges (perhaps two "copies" of the same range?), then you should do:
std::transform(v.begin(), v.end(), v.begin(),
// ^^^^^^^^^
std::back_inserter(r), [] (int a, int b) { return a + b; });
So here is the full code:
#include <algorithm>
#include <vector>
int main()
{
std::vector<int> v {1, 2, 3, 4, 5, 6};
std::vector<int> r;
std::transform(v.begin(), v.end(), v.begin(),
std::back_inserter(r), [] (int a, int b) { return a + b; });
}
And here is a compiling live example.
However, unless you just wanted to practice with the overload of std::transform() that operates on two input ranges, you could follow Konrad Rudolph's advice from the comments, and write:
std::transform(v.begin(), v.end(),
std::back_inserter(r), [] (int a) { return a * 2; });
If, instead, you want to perform a sum of each pair of consecutive elements and store the result in the r vector while using a transform-like approach, then you may resort to Boost.Range, and write something like:
#include <boost/range.hpp>
#include <boost/range/adaptors.hpp>
#include <boost/range/algorithm.hpp>
// ...
namespace rng = boost::adaptors;
// This will fill r with values { 3, 7, 11 }
boost::transform(
v | rng::strided(2),
std::make_pair(v.begin() + 1, v.end()) | rng::strided(2),
std::back_inserter(r),
[] (int a, int b) { return a + b; });
Here is a live example.
Also, as pointed out by NeelBasu in the comments, instead of using std::make_pair() to define the second range, you could write a more expressive (and compact) version based on the sliced range adaptor:
boost::transform(
v | rng::strided(2),
v | rng::sliced(1, v.size()) | rng::strided(2),
std::back_inserter(r),
[] (int a, int b) { return a + b; });