find_if number present in container with given error tolerance

Question

There is container with numbers and it needs to find if there is a number in container that is equal to a query value within a given tolerance. Is there a way to implement it with similar to find_if method passing for comparison e.g. bool areEqual(double a, double b, double eps)?

something like ?http://en.cppreference.com/w/cpp/algorithm/find_first_of — billz, Mar 01 '13 at 07:27

Spook · Accepted Answer · 2013-03-01T07:43:50.737

4

If you can use C++11:

bool areEqual(double a, double b, double eps)
{
    return abs(a - b) < eps;
}

int main(int argc, char * argv[])
{
    std::vector<double> myvector;
    myvector.push_back(1.0);
    myvector.push_back(0.0);
    myvector.push_back(2.0);

    double eps = std::numeric_limits<double>::epsilon();
    double value = 0.0;

    std::vector<double>::iterator it = std::find_if (myvector.begin(), 
        myvector.end(), 
        [=](double d) -> bool
            {
                return areEqual(d, value, eps);
            });

    if (it != myvector.end())
        printf("Found value: %f\n", *it);
}

edited Mar 01 '13 at 07:43

answered Mar 01 '13 at 07:28

Spook

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Uh, I'm inclined to disagree, `std::numeric_limits::epsilon()` doesn't seem to be a good tolerance for comparing arbitrary doubles. – us2012 Mar 01 '13 at 07:30
@us2012 It depends on what values you are trying to compare. In my case it should work. Otherwise, anoter epsilon value should be chosen. I used `std::numeric_limits::epsilon()` only for demonstration purposes. – Spook Mar 01 '13 at 07:32
is it possible to call areEqual from lambda expression rather than duplicating function contents? – Chesnokov Yuriy Mar 01 '13 at 07:40
1

@ChesnokovYuriy Sure, modified the example. – Spook Mar 01 '13 at 07:44

Karthik T · Answer 2 · 2013-03-01T08:25:27.220

3

struct CompareFuzzy{
    CompareFuzzy(double qVal, double tolerance = 0.0f):m_tolerance(tolerance),m_qVal(qVal){}

    bool operator()(double a) {return std::abs( a-qVal ) < m_tolerance;}
    private:
    double m_tolerance , m_qVal;    
}

std::find_if(container.begin(),container.end(),CompareFuzzy(5.0, 0.5));

Use an old-school functor, as shown above, or lambda if you use C++11.

edited Mar 01 '13 at 08:25

answered Mar 01 '13 at 07:26

Karthik T

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I think it needs to pass a reference to container instead of a single qVal. Since we have a container to find a value inside – Chesnokov Yuriy Mar 01 '13 at 08:02
I understood the logic. value to find is passed to constructor and all values from container are passed in () operator – Chesnokov Yuriy Mar 01 '13 at 08:21
1

@ChesnokovYuriy I would term it as each, instead of all, but yes that is it. This i essentially a `< C++11` method of doing what the lamda that others have shown does. – Karthik T Mar 01 '13 at 08:24
you may use class instead of struct and inherit from unary_function – Chesnokov Yuriy Mar 01 '13 at 08:31
@ChesnokovYuriy you can, maybe unnecessary though, unless you need it. – Karthik T Mar 01 '13 at 08:37
yes, it is not necessary to inherit. however in the literature they suggested to inherit from either unary_function or from binary_function – Chesnokov Yuriy Mar 01 '13 at 08:44

score 3 · Answer 3 · edited Mar 01 '13 at 07:33

3

you can bind the 2nd and 3rd parameter:

std::find_if(
   c.begin(),
   c.end(),
   boost::bind(
       &areEqual,
       _1,
       number,
       eps
   )
);

also C++11 std::bind should do the trick

edited Mar 01 '13 at 07:33

StilesCrisis

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answered Mar 01 '13 at 07:30

Maciek B

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If you don't want to use Boost, you can also use `std::tr1::bind` which is present and older compilers as well (I know it ships with GCC 4.1.2). – Björn Pollex Mar 01 '13 at 07:37
Also, I am not sure if this actually works. Don't you have to adapt the result of `bind` in order to use it with standard algorithms (because it is missing the required typedefs)? See [here](http://www.boost.org/doc/libs/release/libs/bind/bind.html#err_modeling_stl_function_object_concepts). – Björn Pollex Mar 01 '13 at 07:38
In C++11, prefer Lambdas over bind, for clarity and readability. – Arne Mertz Mar 01 '13 at 07:38
yes, I would not like to use boost. what is the logic for std::bind? – Chesnokov Yuriy Mar 01 '13 at 07:38
there is a comparison function already defined. it would be superfluous to move its contents to lambda expression – Chesnokov Yuriy Mar 01 '13 at 07:39
@BjörnPollex standard algorithms need no typedefs on the predicat/comparator function objects you pass them. They just copy and call what ever you throw in. – Arne Mertz Mar 01 '13 at 07:40
1

@ChesnokovYuriy then use a lambda that just calls that function: `[=](double a){ return areEqual(a, number, eps); }` does the same as the bind in the answer. – Arne Mertz Mar 01 '13 at 07:43
@ArneMertz: You are right - the typedefs are only required for composing functors. – Björn Pollex Mar 01 '13 at 07:44
@ChesnokovYuriy std::bind is just standardized boost::bind pulled in to c++11 standard. The syntax is almost the same – Maciek B Mar 01 '13 at 07:48
A bit more on lambdas vs. bind can be found here: http://stackoverflow.com/questions/1930903/bind-vs-lambda – Arne Mertz Mar 01 '13 at 08:03
std::find_if(c.begin(), c.end(), bind(&areEqual,_1,number,eps)); ? – Chesnokov Yuriy Mar 01 '13 at 08:04

find_if number present in container with given error tolerance

3 Answers3