CODEWITHSUNDEEP
c++vectorunary-function
EliSquared
EliSquared

Reputation: 1559

Passing an iterarator to a unary predicate with std::find_if

I am trying to find the index, i, of an element of a vector, v, that satisfies: v[i] <= x < v[i + 1], where x is a given arbitrary value. I am trying to use the find_if function, but it appears that find_if passes the value from the iterator and not the iterator, because of this I cannot figure out a way to perform the x < v[i + 1] comparison. Is there a way to do this comparison with a unary predicate set up the way that I have it below: 

#include <vector>
#include <iostream>
#include <algorithm>

//Create predicate for find_if
template<typename T>
struct eq {
    eq(const T _x) : x(x) { };

    //Does not work
    bool operator()(typedef std::vector<T>::iterator it) const {  //
        return *it <= x && x < *(++it);
    }
private:
    T x;
};

//Make vector
std::vector<double> vDouble;
vDouble.push_back(1.5);
vDouble.push_back(3.1);
vDouble.push_back(12.88);
vDouble.push_back(32.4);

double elemVal = *std::find_if(vNumeric.begin(), vNumeric.end(), eq<double>(13.0));

Upvotes: 1

Views: 1117

Answers (1)

Jarod42
Jarod42

Reputation: 217810

With std::adjacent_find, you may simply do:

const auto x = 13.0;
auto it = std::adjacent_find(v.begin(), v.end(),
                             [x](double lhs, double rhs){ return lhs <= x && x < rhs; });

Demo

Upvotes: 4

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