SCIP in C++11 ― 2.2.1節 その4

リストの写像と問題2.23

mapはSTLコンテナ名だから、ここではmappingにする。

algorithmのgenerateに似ているが、入出力リストの型を選ばない分、より一般的だな。

問題2.21、2.22はあまり興味ないのでパス。

----

// map

// map for single input list

template <typename ArgType, typename ReturnType>

const List mapping

(const function<ReturnType(ArgType)>& procedure,

 const List& listTree)

{  

    if(isNull(listTree)){return(makeList());}

    return(cons(procedure(value<ArgType>(car(listTree))),

                mapping(procedure,cdr(listTree))));

}

// for-each

template <typename ArgType>

void forEach(const function<void(ArgType)>& procedure,

             const List& listTree)

{

    if(isNull(listTree)){return;}

    procedure(value<ArgType>(car(listTree)));

    forEach(procedure, cdr(listTree));

}

//---------abstraction barrier---------

// scaling

template <typename FactorType>

const List scaleList

(const List& listTree, const FactorType& factor)

{

    function<FactorType(FactorType)> factoring

        =[factor](const FactorType& x){return(x*factor);};

    return(mapping(factoring,listTree));

}

int main(int argc, char** argv)

{

    const function<double(double)> absolute

        =[](const double x){return(abs(x));};

    auto list1(mapping(absolute,makeList(-10.0,2.5,-11.6,17.0)));

    cout<<"(map abs (list -10 2.5 -11.6 17)) = "

        <<listString(list1)<<endl;

   

    const function<int(int)> square=[](const int x){return(x*x);};

    auto list2(mapping(square,makeList(1,2,3,4)));

    cout<<"(map (lambda (x) (* x x))) (list 1 2 3 4) = "

        <<listString(list2)<<endl;

    auto list3(scaleList(list2,3));

    cout<<"(scale-list (list 1 4 9 16) 3) = "

        <<listString(list3)<<endl;

    cout<<endl<<"Excersize 2.23:"<<endl;

    cout<<"(for-each (lambda(x)(newline)(display x))"<<endl;

    cout<<"  (list 57 321 88)"<<endl;

    function<void(List)> display

        =[](const List& x){cout<<value<string>(x)<<endl;};

    forEach(display,makeList(57,321,88));

   

    return(0);

}

----

出力

----

(map abs (list -10 2.5 -11.6 17)) = (10 2.5 11.6 17)

(map (lambda (x) (* x x))) (list 1 2 3 4) = (1 4 9 16)

(scale-list (list 1 4 9 16) 3) = (3 12 27 48)

Excersize 2.23:

(for-each (lambda(x)(newline)(display x))

  (list 57 321 88)

57

321

88