Sort vector of vector using variable index

Question

I'm having some problems using the std sort function. I want to sort a vector of vector without using always the same index. I know I can use something like:

sort(dataset.begin(), dataset.end(), myfunction);
// or
std::sort(dataset.begin(), dataset.end(),
    [](const std::vector<int>& a, const std::vector<int>& b) {
        return a[2] < b[2]);
    });

In the first case I don't know how to include the specified index as an input of myfunction. In the second case, I though of including the index as an input of the function signature but I can't even make it compile as shown above!

The error:

main.cpp:28:39: error: expected expression sort(dataset.begin(), dataset.end(), [](const vector& a, const vector& b)

Is it always the same index for one run? Or do you intend to compare for example element 1 of vector 0 with element 20 of vector 1? — Daniel Jour
– Daniel Jour, Commented Oct 3, 2015 at 19:08

LogicStuff · Accepted Answer · 2015-11-16 16:11:00Z

1

lambda function - you can capture the index variable:

std::size_t index = 2;
std::sort(dataset.begin(), dataset.end(),
    [index](const std::vector<int>& a, const std::vector<int>& b) {
        return a[index] < b[index]);
    });

function - if you have

bool myfunction(const std::vector<int>& a, const std::vector<int>& b, std::size_t index)

you can std::bind index to the third parameter:

using namespace std::placeholders;
std::size_t index = 2;
std::sort(dataset.begin(), dataset.end(), std::bind(myfunction, _1, _2, index));

Consider this solution inferior to lambdas, use it only when they're not available, or you'll be treated badly.

edited Nov 16, 2015 at 16:11

answered Oct 3, 2015 at 19:08

LogicStuff

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4 Comments

Daniel Jour Over a year ago

Don't use int ... use std::size_t.

Puppy Over a year ago

I feel compelled to downvote you for mentioning that bind crap. It's superfluous and inferior to lambdas in the vast majority of cases, including this one.

LogicStuff Over a year ago

Well, OP asked about that. He/she can also choose between the two.

Puppy Over a year ago

He didn't ask about that, he just had a generic need to capture or bind a variable. There's nothing std::bind specific in it.

AlexD · Accepted Answer · 2015-10-03 19:11:10Z

1

You could capture index and use it within the lambda function:

std::vector<std::vector<int>> dataset = ...

std::size_t index = 2;
std::sort(dataset.begin(), dataset.end(),
    [index](const std::vector<int>& a, const std::vector<int>& b) {
    return a[index] < b[index];
});

edited Oct 3, 2015 at 19:11

answered Oct 3, 2015 at 19:04

AlexD

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