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CppStdFor_each.md

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(C++ ) std::for_each

 

Algorithm to perform a non-modifying function on the elements of a sequence (on a std::vector, for example). Use std::transform to perform modifying functions on the elements of a sequence.

 

Prefer algorithms over hand-written loops [1-3]. View Exercise #9: No for-loops to learn how to remove hand-written loops .

 

Note: std::for_each is supposed to be non-modifying [1], but I use it for modifying my sequences anyway.

 

There are two kinds of examples below. The first uses std::for_each combined with simple functions.

 

The second piece of code shows the use of functors for more advanced functionality. It is advised to use the latter [3], but I will show the first as an example. There are also STL functors.

 

 

 

 

 

Example: Use of plain functions

 

#include <algorith> #include <iostream> #include <vector> void SetToOne(int& i) {   i = 1; } void MultiplyByTwo(int& i) {   i*=2; } void SetToRandom(int& i) {   i = std::rand() % 10; } void CoutVector(const std::vector<int>& myVector) {   const int size = static_cast<int>(myVector.size());   for (int i=0; i!=size; ++i)   {     std::cout << i << " : " << myVector[i] << std::endl;   } } int main() {   const int size = 5;   std::vector<int> myVector(size);   std::for_each(myVector.begin(),myVector.end(), SetToOne);   CoutVector(myVector);   std::for_each(myVector.begin(),myVector.end(), MultiplyByTwo);   CoutVector(myVector);   std::for_each(myVector.begin(),myVector.end(), SetToRandom);   CoutVector(myVector);   std::for_each(myVector.begin(),myVector.end(), MultiplyByTwo);   CoutVector(myVector); }

 

 

 

 

 

Example: Use of non-STL functors

 

#include <algorith> #include <iostream> #include <vector> struct MyInitializer {   MyInitializer() : index(0) {}   template<class T>   void operator () (T & a)   {     a = index;     ++index;   } private:   int index; }; struct MyIndexCout {   MyIndexCout() : index(0) {}   template<class T>   void operator () (const T & a)   {     std::cout << index << " : " << a << std::endl;     ++index;   } private:   int index; }; struct MySquarer {   template<class T> void operator () (T & a)   {     a*=a;   } }; int main() {   const int size = 10;   std::vector<int> myVector(size);   std::for_each(myVector.begin(), myVector.end(), MyInitializer());   std::for_each(myVector.begin(), myVector.end(), MyIndexCout());   std::cout << "-----------" << std::endl;   std::for_each(myVector.begin(), myVector.end(), MySquarer());   std::for_each(myVector.begin(), myVector.end(), MyIndexCout()); }

 

 

 

 

 

References

 

  1. Bjarne Stroustrup. The C++ Programming Language (3rd edition). ISBN: 0-201-88954-4. Chapter 18.12.1 : 'Prefer algorithms over loops'
  2. Herb Sutter and Andrei Alexandrescu. C++ coding standards: 101 rules, guidelines, and best practices. ISBN: 0-32-111358-6. Chapter 84: 'Prefer algorithm calls to handwritten loops.'
  3. Herb Sutter and Andrei Alexandrescu. C++ coding standards: 101 rules, guidelines, and best practices. ISBN: 0-32-111358-6. Chapter 88: 'Prefer function objects over functions as algorithm and comparer arguments.'