Question

I have four std::vector containers that all might (or might not) contain elements. I want to determine which of them has the most elements and use it subsequently.

I tried to create a std::map with their respective sizes as keys and references to those containers as values. Then I applied std::max on the size() of each vector to figure out the maximum and accessed it through the std::map.

Obviously, this gets me into trouble once there is the same number of elements in at least two vectors.

Can anyone think of a elegant solution ?

Answer 1

You’re severely overthinking this. You’ve only got four vectors. You can determine the largest vector using 3 comparisons. Just do that:

std::vector<blah>& max = vector1;
if (max.size() < vector2.size()) max = vector2;
if (max.size() < vector3.size()) max = vector3;
if (max.size() < vector4.size()) max = vector4;

EDIT:

Now with pointers!

EDIT (280Z28):

Now with references! 🙂

EDIT:

The version with references won’t work. Pavel Minaev explains it nicely in the comments:

That’s correct, the code use
references. The first line, which
declares max, doesn’t cause a copy.
However, all following lines do cause
a copy, because when you write max =
vectorN, if max is a reference, it
doesn’t cause the reference to refer
to a different vector (a reference
cannot be changed to refer to a
different object once initialized).
Instead, it is the same as
max.operator=(vectorN), which simply
causes vector1 to be cleared and
replaced by elements contained in
vectorN, copying them.

The pointer version is likely your best bet: it’s quick, low-cost, and simple.

std::vector<blah> * max = &vector1;
if (max->size() < vector2.size()) max = &vector2;
if (max->size() < vector3.size()) max = &vector3;
if (max->size() < vector4.size()) max = &vector4;