Working with a vector of pair vectors? - c++

Question

I've been search around Google but I didn't find what I need. I'm trying to create a vector that allows me to add 3 (and after I'll need to store 4) variables, access and sort them.
I'm implementing the vector as follows for 3 variables:
std::vector<std::pair<std::string, std::pair<int, double> > > chromosomes;
To add information (variables), I'm doing:
chromosomes.emplace_back(dirp->d_name, std::make_pair(WSA, fault_percent));
How can I access each parameter and sort them based on the WSA and fault coverage? As in a vector of pair that I can do that using members first and second.
And for 4 variables, it would be as follows?
std::vector<std::pair<std::string, std::string>, std::pair<int, double> > > chromosomes;
chromosomes.emplace_back( std::make_pair(dirp->d_name, x), std::make_pair(WSA, fault_percent));`

Answer 1

As suggested here I think you should be using a vector of tuple<string, int, double>s or tuple<string, string, int, double>s respectively.
There is a defined tuple::operator< which uses the less-than-operator for each of it's composing types moving left to right. If a simple comparison of each element is sufficient then all you'll need to do is call sort:
sort(chromosomes.begin(), chromosomes.end());
If the tuple::operatior< does not provide a sufficient comparison for your needs sort provides an overload which takes a comparison lambda. Your lambda would need to do the following:
Take in 2 const references to the tuples
Return true if the first tuple is strictly smaller than the second tuple
Return false if the first tuple is greater or equal to the second tuple
In the end your call would look something like this:
sort(chromosomes.begin(), chromosomes.end(), [](const auto& lhs, const auto& rhs) {
// Your comparison between the two goes here
});
If you're not familiar with working with tuples you'll need to use the templated get method to extract either by index or type in the cases where there is not a duplicate type contained by the tuple.

Answer 2

First to access to the different elements:
for (auto& x :chromosomes)
cout <<x.first<<": "<<x.second.first<<" "<<x.second.second<<endl;
Next, to sort the elements on WSA:
sort(chromosomes.begin(), chromosomes.end(),
[](auto &x, auto &y) { return x.second.first<y.second.first;});
If you want to sort on several criteria, for example WSA and fault_percent, you just have to change the lambda function for comparison:
sort(chromosomes.begin(), chromosomes.end(),
[](auto &x, auto &y) { return x.second.first<y.second.first
|| (x.second.first==y.second.first
&& x.second.second<y.second.second );});
Here is an online demo
Remark
Now what puzzles me, is why you want to use pairs of pairs or even tuples, when you could use a clean struct which would be easier to store/retrieve, and access its members:
struct Chromosome {
string name;
int WSA;
double fault_percent;
};
vector <Chromosome> chromosomes;
It would be much more readable and maintainable this way:
sort(chromosomes.begin(), chromosomes.end(),
[](auto &x, auto &y) { return x.WSA<y.WSA
|| (x.WSA==y.WSA && x.fault_percent<y.fault_percent );});

Answer 3

It seems like you need a table-like data structure, that allows sorting by multiple columns. C++ isn't the easiest language to manipulate table/matrix data structures in, but here's a few links to help you get started.
An example Table class:
How to dynamically sort data by arbitrary column(s)
A vector/tuple solution, which is a slightly cleaner version of what you're currently working on:
sorting table in place using stl sort
A lengthy discussion of this problem, which might give you some additional ideas:
https://softwareengineering.stackexchange.com/questions/188130/what-is-the-best-way-to-store-a-table-in-c