C++STL-算法transform

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本文详细介绍了 C++ STL 中的 transform 函数用法，包括单输入和双输入两种形式，展示了如何使用自定义操作符对序列进行转换，并提供了具体的代码示例。

template < class InputIterator, class OutputIterator, class UnaryOperator >
  OutputIterator transform ( InputIterator first1, InputIterator last1,
                             OutputIterator result, UnaryOperator op );

template < class InputIterator1, class InputIterator2,
           class OutputIterator, class BinaryOperator >
  OutputIterator transform ( InputIterator1 first1, InputIterator1 last1,
                             InputIterator2 first2, OutputIterator result,
                             BinaryOperator binary_op );

Apply function to range

The first version applies op to all the elements in the input range ([first1,last1)) and stores each returned value in the range beginning at result.

The second version uses as argument for each call to binary_op one element from the first input range ([first1,last1)) and one element from the second input range (beginning at first2).

The behavior of this function template is equivalent to:

template < class InputIterator, class OutputIterator, class UnaryOperator >
  OutputIterator transform ( InputIterator first1, InputIterator last1,
                             OutputIterator result, UnaryOperator op )
{
  while (first1 != last1)
    *result++ = op(*first1++);  // or: *result++=binary_op(*first1++,*first2++);
  return result;
}

The function allows for the destination range to be the same as one of the input ranges to make transformationsin place.

Parameters

first1, last1

Input iterators to the initial and final positions of the first sequence. The range used is [first1,last1), which contains all the elements between first1 and last1, including the element pointed by first1 but not the element pointed by last1.

first2

Input iterator to the initial position of the second range. The range includes as many elements as[first1,last1).

result

Output iterator to the initial position of the range where function results are stored. The range includes as many elements as [first1,last1).

Unary function taking one element as argument, and returning some result value. This can either be a pointer to a function or an object whose class overloads operator().

binary_op

Binary function taking two elements as argument (one of each of the two sequences), and returning some result value. This can either be a pointer to a function or an object whose class overloads operator().

Return value

An iterator pointing to the element that follows the last element written in the result sequence.

Example

// transform algorithm example
#include <iostream>
#include <algorithm>
#include <vector>
using namespace std;

int op_increase (int i) { return ++i; }
int op_sum (int i, int j) { return i+j; }

int main () {
  vector<int> first;
  vector<int> second;
  vector<int>::iterator it;

  // set some values:
  for (int i=1; i<6; i++) first.push_back (i*10); //  first: 10 20 30 40 50

  second.resize(first.size());     // allocate space
  transform (first.begin(), first.end(), second.begin(), op_increase);
                                                  // second: 11 21 31 41 51

  transform (first.begin(), first.end(), second.begin(), first.begin(), op_sum);
                                                  //  first: 21 41 61 81 101

  cout << "first contains:";
  for (it=first.begin(); it!=first.end(); ++it)
    cout << " " << *it;

  cout << endl;
  return 0;
}