accumulate函数用法
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:
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
// accumulate example
#include <iostream> // std::cout
#include <functional> // std::minus
#include <numeric> // std::accumulate
int 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;
}
Output:
using default accumulate: 160 using functional's minus: 40 using custom function: 220 using 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.
http://www.cplusplus.com/reference/numeric/accumulate/?kw=accumulate