accumulate函数

function template

<numeric>

std::accumulate

sum (1)

template <class InputIterator, class T>   T accumulate (InputIterator first, InputIterator last, T init);

custom (2)

template <class InputIterator, class T, class BinaryOperation>   T accumulate (InputIterator first, InputIterator last, T init,                 BinaryOperation binary_op);

Accumulate values in range

Returns the result of accumulating all the values in the range [first,last) to init.

The default operation is to add the elements up, but a different operation can be specified as binary_op.

The behavior of this function template is equivalent to:

123456789

template <class InputIterator, class T>   T accumulate (InputIterator first, InputIterator last, T init){  while (first!=last) {    init = init + *first;  // or: init=binary_op(init,*first) for the binary_op version    ++first;  }  return init;}

 

Parameters

first, last

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

init

Initial value for the accumulator.

binary_op

Binary operation taking an element of type T as first argument and an element in the range as second, and which returns a value that can be assigned to type T.
This can either be a function pointer or a function object.
The operation shall not modify the elements passed as its arguments.

Return value

The result of accumulating init and all the elements in the range [first,last).

Example

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// accumulate example#include <iostream>     // std::cout#include <functional>   // std::minus#include <numeric>      // std::accumulateint myfunction (int x, int y) {return x+2*y;}struct myclass {int operator()(int x, int y) {return x+3*y;}} myobject;int main () {  int init = 100;  int numbers[] = {10,20,30};  std::cout << "using default accumulate: ";  std::cout << std::accumulate(numbers,numbers+3,init);  std::cout << '\n';  std::cout << "using functional's minus: ";  std::cout << std::accumulate (numbers, numbers+3, init, std::minus<int>());  std::cout << '\n';  std::cout << "using custom function: ";  std::cout << std::accumulate (numbers, numbers+3, init, myfunction);  std::cout << '\n';  std::cout << "using custom object: ";  std::cout << std::accumulate (numbers, numbers+3, init, myobject);  std::cout << '\n';  return 0;}

Edit & Run

Output:

using default accumulate: 160using functional's minus: 40using custom function: 220using custom object: 280

Complexity

Linear in the distance between first and last.

Data races

The elements in the range [first,last) are accessed (each element is accessed exactly once).

Exceptions

Throws if any of binary_op, the assignments or an operation on an iterator throws.
Note that invalid arguments cause undefined behavior.