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accumulate


Category: algorithms	Component type: function

Prototype

Accumulate is an overloaded name; there are actually two accumulate functions.

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

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

Description

Accumulate is a generalization of summation: it computes the sum (or some other binary operation) of init and all of the elements in the range [first, last). [1]

The function object binary_op is not required to be either commutative or associative: the order of all of accumulate's operations is specified. The result is first initialized to init. Then, for each iterator i in [first, last), in order from beginning to end, it is updated by result = result + *i (in the first version) or result = binary_op(result, *i) (in the second version).

Definition

Defined in the standard header numeric, and in the nonstandard backward-compatibility header algo.h.

Requirements on types

For the first version, the one that takes two arguments:

InputIterator is a model of Input Iterator.
T is a model of Assignable.
If x is an object of type T and y is an object of InputIterator's value type, then x + y is defined.
The return type of x + y is convertible to T.

For the second version, the one that takes three arguments:

InputIterator is a model of Input Iterator.
T is a model of Assignable.
BinaryFunction is a model of Binary Function.
T is convertible to BinaryFunction's first argument type.
The value type of InputIterator is convertible to BinaryFunction's second argument type.
BinaryFunction's return type is convertible to T.

Preconditions

[first, last) is a valid range.

Complexity

Linear. Exactly last - first invocations of the binary operation.

Example

int main()
{
  int A[] = {1, 2, 3, 4, 5};
  const int N = sizeof(A) / sizeof(int);

  cout << "The sum of all elements in A is " 
       << accumulate(A, A + N, 0)
       << endl;

  cout << "The product of all elements in A is "
       << accumulate(A, A + N, 1, multiplies<int>())
       << endl;
}

Notes

[1] There are several reasons why it is important that accumulate starts with the value init. One of the most basic is that this allows accumulate to have a well-defined result even if [first, last) is an empty range: if it is empty, the return value is init. If you want to find the sum of all of the elements in [first, last), you can just pass 0 as init.