Mapping a function onto a vector recursively - c++

Question

I have a class called mapTriple that has a method that takes an integer vector and multiplies all the values in vector by a private function of the mapTriple class (the function takes an int, and returns the int *3)
I have already set up the classes and the function that triples the integer. I am stuck on the mapTriple method. The method cannot be iterative, it must be recursive.
vector<int> MapTriple::map(vector<int> myVector)
{
if(myVector.size() == 1)
{
myVector[0] = f(myVector[0]);
return myVector;
}
else
{
map(myVector.erase(myVector.begin()+myVector.size()-1));
myVector[myVector.size()-1] = f(myVector[myVector.size()-1]);
return myVector;
}
}
int f (int a)
{
return (a*3);
}
It currently isnt compiling, it is say there is no matching call to map. I have all the .h files and main files etc

Answer 1

erase does not return the modified vector. It returns an iterator after the removed element (which will be end in your case, so you don't need that). Just pass the modified vector itself.
You currently don't re-add the erased element, so even if your code compiled, you would always be returning a vector of length 1 (and the remaining element would be tripled n times if the vector was originally size n).
The correct else branch should be:
else
{
// Store and remove the last element.
int currentElement = myVector.back();
myVector.erase(myVector.end()-1);
// Recursively process the remaining elements.
map(myVector);
// Process and re-add the above element.
myVector.push_back(f(currentElement));
return myVector;
}
However, instead of erasing elements and re-adding them, you could work with the iterators.
using Iterator = std::vector<int>::iterator;
void MapTriple::map(Iterator start, Iterator end)
{
// No elements remaining?
if (start == end)
return;
// Process first element.
*start = f(*start);
// Process remaining elements recursively.
map(start+1, end);
}
While this is pretty elegant, it would of course be even simpler to do this with a simple for loop:
for (auto& e : myVector) e = f(e);
or std::transform:
std::transform(myVector.begin(), myVector.end(), myVector.begin(),
[this](int e) -> { return f(e); });`
It should also be noted that map is probably a mediocre name for this method, if you did using namespace std; as seems to be the case (see also Why is "using namespace std" considered bad practice?).