From cppreference.com
Defined in header
template<classInputIt,classOutputIt>OutputItpartial_sum(InputItfirst,InputItlast,OutputItd_first); (1)(constexpr since C++20)template<classInputIt,classOutputIt,classBinaryOp>OutputItpartial_sum(InputItfirst,InputItlast,OutputItd_first,BinaryOpop); (2)(constexpr since C++20)1) If [first, last) is empty, does nothing.
Otherwise, performs the following operations in order:
Creates an accumulator acc, whose type is the
of InputIt, and initializes it with *first.
Assigns acc to *d_first.
For each integer i in [1, std::distance(first,last)), performs the following operations in order:
a) Computes acc+*iter(until C++20)std::move(acc)+*iter(since C++20), where iter is the next ith iterator of first.
b) Assigns the result to acc.
c) Assigns acc
to *dest, where dest is the next ith iterator of d_first.
2) Same as (1), but computes op(acc,*iter)(until C++20)op(std::move(acc),*iter)(since C++20) instead.
Given binary_op as the actual binary operation:
If any of the following conditions is satisfied, the program is ill-formed:
The value type of InputIt is not constructible from *first.
acc is not
to d_first.
The result of binary_op(acc,*iter)(until C++20)binary_op(std::move(acc),*iter)(since C++20) is not implicitly convertible to the value type of InputIt.
Given d_last as the iterator to be
, if any of the following conditions is satisfied, the behavior is undefined:
binary_op modifies any element of [first, last) or [d_first, d_last).
binary_op invalidates any iterator or subrange in [first, last] or [d_first, d_last].
The actual value to be assigned is the result of the assignment in the previous step. We assume the assignment result is acc here.
Parameters
first, last - the pair of iterators defining the
of elements to sum d_first - the beginning of the destination range; may be equal to firstop - binary operation function object that will be applied. The signature of the function should be equivalent to the following:
Retfun(constType1&a,constType2&b);
The signature does not need to have const&.
The type Type1 must be such that an object of type std::iterator_traits<InputIt>::value_type can be implicitly converted to Type1. The type Type2 must be such that an object of type InputIt can be dereferenced and then implicitly converted to Type2. The type Ret must be such that an object of type InputIt can be dereferenced and assigned a value of type Ret.
Type requirements -InputIt must meet the requirements of
. -OutputIt must meet the requirements of
. Return value
Iterator to the element past the last element written, or d_first if [first, last) is empty.
Complexity
Given N as std::distance(first,last):
1) Exactly N-1 applications of operator+.
2) Exactly N-1 applications of the binary function op.
Possible implementation
template<classInputIt,classOutputIt>constexpr// since C++20OutputItpartial_sum(InputItfirst,InputItlast,OutputItd_first){if(first==last)returnd_first;typenamestd::iterator_traits<InputIt>::value_typesum=*first;*d_first=sum;while(++first!=last){sum=std::move(sum)+*first;// std::move since C++20*++d_first=sum;}return++d_first;// or, since C++14:// return std::partial_sum(first, last, d_first, std::plus<>());}
template<classInputIt,classOutputIt,classBinaryOp>constexpr// since C++20OutputItpartial_sum(InputItfirst,InputItlast,OutputItd_first,BinaryOpop){if(first==last)returnd_first;typenamestd::iterator_traits<InputIt>::value_typeacc=*first;*d_first=acc;while(++first!=last){acc=op(std::move(acc),*first);// std::move since C++20*++d_first=acc;}return++d_first;}Notes
acc was introduced because of the resolution of
. The reason of using acc rather than directly summing up the results (i.e. *(d_first+2)=(*first+*(first+1))+*(first+2);) is because the semantic of the latter is confusing if the following types mismatch:
the value type of InputIt
the writable type(s) of OutputIt
the types of the parameters of operator+ or op
the return type of operator+ or op
acc serves as the intermediate object to store and provide the values for each step of the computation:
its type is the value type of InputIt
it is written to d_first
its value is passed to operator+ or op
it stores the return value of operator+ or op
enumnot_int{x=1,y=2};chari_array[4]={100,100,100,100};not_inte_array[4]={x,x,y,y};into_array[4];// OK: uses operator+(char, char) and assigns char values to int arraystd::partial_sum(i_array,i_array+4,o_array);// Error: cannot assign not_int values to int arraystd::partial_sum(e_array,e_array+4,o_array);// OK: performs conversions when needed// 1. creates “acc” of type char (the value type)// 2. the char arguments are used for long multiplication (char -> long)// 3. the long product is assigned to “acc” (long -> char)// 4. “acc” is assigned to an element of “o_array” (char -> int)// 5. go back to step 2 to process the remaining elements in the input rangestd::partial_sum(i_array,i_array+4,o_array,std::multiplies<long>{});Example
Run this code
#include<functional>#include<iostream>#include<iterator>#include<numeric>#include<vector>intmain(){std::vector<int>v(10,2);// v = {2, 2, 2, 2, 2, 2, 2, 2, 2, 2}std::cout<<"The first "<<v.size()<<" even numbers are: ";// write the result to the cout streamstd::partial_sum(v.cbegin(),v.cend(),std::ostream_iterator<int>(std::cout," "));std::cout<<'\n';// write the result back to the vector vstd::partial_sum(v.cbegin(),v.cend(),v.begin(),std::multiplies<int>());std::cout<<"The first "<<v.size()<<" powers of 2 are: ";for(intn:v)std::cout<<n<<' ';std::cout<<'\n';}Output:
The first 10 even numbers are: 2 4 6 8 10 12 14 16 18 20 The first 10 powers of 2 are: 2 4 8 16 32 64 128 256 512 1024 Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
DR Applied to Behavior as published Correct behavior
C++98 op could not have side effects it cannot modify the ranges involved
C++98 the type requirements needed for the result
evaluations and assignments to be valid were missing added See also