Question

I have a std::vector with the element as pair and need to find all the elements with the first component within some ranges, for example, to find all elements such that abs(value of first component – reference)<0.01, I got the code from other post that

bool comp(pair<double, double> v1, pair<double, double> v2)
{
  return (abs(v1.first-v2.first)<0.001);
}

vector< pair<double, double> data;
for (int i=0; i<10; i++)
{
  double a, b; 
  // a & b are initialized randomly (the code was not shown here)
  data.push_back(make_pair(a, b));
}

// we sort the vector here based on the pair.first (code not shown)

// compval is used as reference value, i.e. we are going to find all elements with     abs(pair.first-ref.first)<0.01
pair<double, double> ref
ref.first = 0.5;
ref.second = 0.5;

std::pair< vector< pair<double, double>::iterator, vector< pair<double, double>::iterator> const range = std::equal_range(data.begin(), data.end(), ref, comp);

but if you look at all elements given by the resulting ranges, you will see that all elements of the data instead of the one satisfying abs(pair.first-ref.first)<0.01 will be returned. Is that anything I miss in the code? Thanks.

Answer 1

equal_range requires a sorted range.

The order implied by your comparison function does not match the order that you sorted the range into – and so the call to equal_range has undefined behaviour.

For example – say your list contained {{.5, 0}, {.6, 0}} (these are sorted), and you then applied std::bind(comp({.5,.5},_1)) to each element.

comp({.5,.5},{.5,0}) would return true.

comp({.5,.5},{.6,0}) would return false.

Your sort order is saying: “.5 is less than .6“, but at the same time your comp function is saying “.6 is less than .5” (because a there is a value which is less than .5 but not less than .6). This is contradictory, and the cause of your problems.

To actually find all the elements such that std::bind(comp({.5,.5}, _1)) returns true, you could use std::copy_if(data.begin(), data.end(), some_output_iterator, std::bind(comp(ref, std::placeholders::_1))) (although there are a variety of different ways of doing this, depending on your exact needs).