Using function objects with transform() - C++ STL Algorithm

C++ examples for STL Algorithm:transform

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Description

Using function objects with transform()

Demo Code

#include <iostream>
#include <vector>
#include <algorithm>
#include <functional>
using namespace std;
// A function object that computes a reciprocal.
class reciprocal : unary_function<double, double> {
   public://w  ww  .  ja  v  a 2  s .  co  m
   result_type sum;
   result_type operator()(argument_type val) {
      if(val == 0.0) return 0.0;
      return 1.0 / val; // return reciprocal
   }
};
// A function object that finds the midpoint between two values.
class midpoint : binary_function<int, int, double> {
   public:
   result_type operator()(first_argument_type a, second_argument_type b) {
      return((a-b) / 2) + b;
   }
};
template<class T> void show(const char *msg, vector<T> vect);
int main(){
   int i;
   vector<double> v;
   // Put values into v.

   for(i=1; i < 10; ++i) v.push_back((double)i);

   show("Initial contents of v:\n", v);

   cout << endl;

   // First, demonstrate a unary function object.
   // Transform v by applying the reciprocal function object. Put the result back into v.

   cout << "Use a unary function object in calls to transform() to\n";

   cout << "compute reciprocals for v and store the results back in v.\n";

   transform(v.begin(), v.end(), v.begin(), reciprocal());

   show("Transformed contents of v:\n", v);

   cout << endl;
   // Transform v a second time, putting the result into a new sequence.
   cout << "Use a unary function object to transform v again.\n";

   cout << "This time, store the results in v2.\n";

   vector<double> v2(10);

   transform(v.begin(), v.end(), v2.begin(), reciprocal());

   show("Here is v2:\n", v2);

   cout << endl;

   vector<int> v3, v4, v5(10);

   for(i = 0; i < 10; ++i) v3.push_back(i);

   for(i = 10; i < 20; ++i) 
       if(i%2) 
          v4.push_back(i); 
       else 
          v4.push_back(-i);

   show("Contents of v3:\n", v3);
   show("Contents of v4:\n", v4);
   cout << endl;

   // Now, demonstrate a binary function object.
   cout << "Now, use a binary function object to find the midpoints\n";
   cout << "between elements in v3 and v4 and store the results in v5.\n";

   transform(v3.begin(), v3.end(), v4.begin(), v5.begin(), midpoint());

   show("Contents of v5:\n", v5);

   return 0;
}
template<class T> void show(const char *msg, vector<T> vect) {
   cout << msg;
   for(unsigned i=0; i < vect.size(); ++i)
      cout << vect[i] << " ";
   cout << "\n";
}

Result


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