(C++) std::transform

 

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

 

Prefer algorithms over hand-written loops [0][1][2]. View Exercise #9: No for-loops to learn how to remove hand-written loops .

 

std::transform does not use a predicate. Use Transform_if if a predicate is needed.

 

 

 

 

 

Example

 

The code below shows a simple way to multiply all elements in a std::vector by a certain value:

 

---

#include <vector> void Multiply(std::vector<int>& v, const int x) {   const int sz = static_cast<int>(v.size());   for (int i=0; i!=sz; ++i) {     v[i]*=x;   } }

 

std::transform can be used to replace the for-loop in the example below:

 

---

#include <algorithm> #include <numeric> #include <vector> void Multiply(std::vector<int>& v, const int x) {   std::transform(v.begin(),v.end(),v.begin(),     std::bind2nd(std::multiplies<int>(),x)); }

 

 

 

 

 

std::transform function definition

 

Simplified from the STL that ships with C++ Builder 6.0:

 

---

template   <     class InputIter,     class OutputIter,     class UnaryOperation   >   OutputIter transform(     InputIter first,     InputIter last,     OutputIter result,     UnaryOperation opr   ) {   for ( ; first != last; ++first, ++result)     *result = opr(*first);   return result; } template   <     class InputIter1,     class InputIter2,     class OutputIter,     class BinaryOperation   >   OutputIter transform(     InputIter1 first1,     InputIter1 last1,     InputIter2 first2,     OutputIter result,     BinaryOperation binary_op   ) {   for ( ; first1 != last1; ++first1, ++first2, ++result)     *result = binary_op(*first1, *first2);   return result; }

 

std::transform does not use a predicate. Use Transform_if if a predicate is needed.

 

 

 

 

 

References

 

• [0] Bjarne Stroustrup. The C++ Programming Language (3rd edition). ISBN: 0-201-88954-4. Chapter 18.12.1 : 'Prefer algorithms over loops'
• [1] 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.'
• [2] 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.'