# std::transform_exclusive_scan

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< cpp‎ | algorithm

Defined in header `<numeric>` |  |
template< class InputIt, class OutputIt, class T, class BinaryOperation, class UnaryOperation> OutputIt transform_exclusive_scan( InputIt first, InputIt last, OutputIt d_first, T init, BinaryOperation binary_op, UnaryOperation unary_op); | (1) | (since C++17)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2, class T, class BinaryOperation, class UnaryOperation > ForwardIt2 transform_exclusive_scan( ExecutionPolicy&& policy, ForwardIt1 first, ForwardIt1 last, ForwardIt2 d_first, T init, BinaryOperation binary_op, UnaryOperation unary_op ); | (2) | (since C++17)

Transforms each element in the range `[first, last)` with `unary_op`, then computes an exclusive prefix sum operation using `binary_op` over the resulting range, with `init` as the initial value, and writes the results to the range beginning at `d_first`. "exclusive" means that the i-th input element is not included in the i-th sum.

Formally, assigns through each iterator `i` in [d_first, d_first + (last - first)) the value of the generalized noncommutative sum of `init, unary_op(*j)...` for every `j` in [first, first + (i - d_first)) over `binary_op`,

where generalized noncommutative sum GNSUM(op, a
1, ..., a
N) is defined as follows:

- if N=1, a
  1
- if N > 1, op(GNSUM(op, a
  1, ..., a
  K), GNSUM(op, a
  M, ..., a
  N)) for any K where 1 < K+1 = M ≤ N

In other words, the summation operations may be performed in arbitrary order, and the behavior is nondeterministic if `binary_op` is not associative.

Overload (2) is executed according to `policy`, and does not participate in overload resolution unless std::is_execution_policy_v < std::decay_t <ExecutionPolicy>> is true.

`unary_op` and `binary_op` shall not invalidate iterators (including the end iterators) or subranges, nor modify elements in the ranges [first, last) or [d_first, d_first + (last - first)). Otherwise, the behavior is undefined.

### Parameters

first, last | - | the range of elements to sum
d_first | - | the beginning of the destination range, may be equal to `first`
policy | - | the execution policy to use. See execution policy for details.
init | - | the initial value
unary_op | - | unary FunctionObject that will be applied to each element of the input range. The return type must be acceptable as input to `binary_op`.
binary_op | - | binary FunctionObject that will be applied in to the result of `unary_op`, the results of other `binary_op`, and `init`.
Type requirements
- `InputIt` must meet the requirements of LegacyInputIterator.
- `OutputIt` must meet the requirements of LegacyOutputIterator.
- `ForwardIt1, ForwardIt2` must meet the requirements of LegacyForwardIterator.
- `T` must meet the requirements of MoveConstructible. All of `binary_op(init, unary_op(*first))`, `binary_op(init, init)`, and `binary_op(unary_op(*first), unary_op(*first))` must be convertible to `T`.

### Return value

Iterator to the element past the last element written.

### Complexity

O(last - first) applications of each of `binary_op` and `unary_op`.

### Exceptions

The overload with a template parameter named `ExecutionPolicy` reports errors as follows:

- If execution of a function invoked as part of the algorithm throws an exception and `ExecutionPolicy` is one of the standard policies, std::terminate is called. For any other `ExecutionPolicy`, the behavior is implementation-defined.
- If the algorithm fails to allocate memory, std::bad_alloc is thrown.

### Notes

`unary_op` is not applied to `init`.

### Example

Run this code

    #include <functional>
    #include <iostream>
    #include <iterator>
    #include <numeric>
    #include <vector>

    int times_10(int x)
    {
      return x * 10;
    }

    int main()
    {
      std::vector data {3, 1, 4, 1, 5, 9, 2, 6};

      std::cout << "10 times exclusive sum: ";
      std::transform_exclusive_scan(data.begin(), data.end(),
    				std::ostream_iterator<int>(std::cout, " "),
    				0, std::plus<int>{}, times_10);
      std::cout << "\n10 times inclusive sum: ";
      std::transform_inclusive_scan(data.begin(), data.end(),
    				std::ostream_iterator<int>(std::cout, " "),
    				std::plus<int>{}, times_10);
    }

Output:

    10 times exclusive sum: 0 30 40 80 90 140 230 250
    10 times inclusive sum: 30 40 80 90 140 230 250 310

### See also

partial_sum | computes the partial sum of a range of elements (function template)
exclusive_scan (C++17) | similar to std::partial_sum, excludes the ith input element from the ith sum (function template)
transform_inclusive_scan (C++17) | applies a functor, then calculates inclusive scan (function template)

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