std::transform

 

C++

Language

Standard library headers

Concepts

Utilities library

Strings library

Containers library

Algorithms library

Iterators library

Numerics library

Input/output library

Localizations library

Regular expressions library(C++11)

Atomic operations library(C++11)

Thread support library(C++11)

Filesystem library(C++17)

Technical Specifications

 

Algorithm library

Execution policies (C++17)
is_execution_policy	execution::seqexecution::parexecution::par_unseq	execution::sequenced_policyexecution::parallel_policyexecution::parallel_unsequenced_policy
Non-modifying sequence operations
all_ofany_ofnone_of (C++11)(C++11)(C++11) for_each for_each_n (C++17)	countcount_if mismatch equal lexicographical_compare	findfind_iffind_if_not (C++11) find_end find_first_of	adjacent_find search search_n
Modifying sequence operations
copycopy_if (C++11) copy_n (C++11) copy_backward move (C++11) move_backward (C++11) transform	fill fill_n generate generate_n swap iter_swap swap_ranges sample (C++17)	removeremove_if replacereplace_if reverse rotate unique random_shuffle (until C++17)	remove_copyremove_copy_if replace_copyreplace_copy_if reverse_copy rotate_copy unique_copy shuffle (C++11)
Operations on uninitialized storage
uninitialized_copy uninitialized_move (C++17)	uninitialized_copy_n (C++11) uninitialized_move_n (C++17)	uninitialized_fill	uninitialized_fill_n
Partitioning operations
is_partitioned (C++11) partition_point (C++11)	partition partition_copy (C++11)	stable_partition
Sorting operations
is_sorted (C++11) is_sorted_until (C++11)	sort stable_sort	partial_sort partial_sort_copy	nth_element
Binary search operations
lower_bound	upper_bound	binary_search	equal_range
Set operations (on sorted ranges)
merge inplace_merge	set_difference set_intersection	set_symmetric_difference set_union	includes
Heap operations
is_heap (C++11) is_heap_until (C++11)	make_heap sort_heap	push_heap pop_heap
Minimum/maximum operations
max max_element	min min_element	minmax (C++11) minmax_element (C++11)	clamp (C++17)
Permutations
is_permutation (C++11)	next_permutation	prev_permutation
Numeric operations
iota (C++11) inner_product adjacent_difference	accumulate reduce (C++17) transform_reduce (C++17)	partial_sum inclusive_scan (C++17) exclusive_scan (C++17)	transform_inclusive_scan (C++17) transform_exclusive_scan (C++17)
C library
qsort	bsearch

 

Defined in header <algorithm>
template<class InputIt, class OutputIt, class UnaryOperation > OutputIt transform( InputIt first1, InputIt last1, OutputIt d_first,                     UnaryOperation unary_op );	(1)
template<class ExecutionPolicy, class InputIt, class OutputIt, class UnaryOperation > OutputIt transform( ExecutionPolicy&& policy, InputIt first1, InputIt last1, OutputIt d_first,                     UnaryOperation unary_op );	(2)	(since C++17)
template<class InputIt1, class InputIt2, class OutputIt, class BinaryOperation > OutputIt transform( InputIt1 first1, InputIt1 last1, InputIt2 first2,                     OutputIt d_first, BinaryOperation binary_op );	(3)
template<class ExecutionPolicy, class InputIt1, class InputIt2, class OutputIt, class BinaryOperation > OutputIt transform( ExecutionPolicy&& policy, InputIt1 first1, InputIt1 last1, InputIt2 first2,                     OutputIt d_first, BinaryOperation binary_op );	(4)	(since C++17)

std::transform applies the given function to a range and stores the result in another range, beginning at d_first.

1) The unary operation unary_op is applied to the range defined by [first1, last1).

3) The binary operation binary_op is applied to pairs of elements from two ranges: one defined by [first1, last1) and the other beginning at first2.

2,4) Same as (1,3), but executed according to policy. These overloads do not participate in overload resolution unless std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true

unary_op and binary_op must not have side effects.	(until C++11)
unary_op and binary_op must not invalidate any iterators, including the end iterators, or modify any elements of the ranges involved.	(since C++11)

Parameters

first1, last1	-	the first range of elements to transform
first2	-	the beginning of the second range of elements to transform
d_first	-	the beginning of the destination range, may be equal to first1 or first2
policy	-	the execution policy to use. See execution policy for details.
unary_op	-	unary operation function object that will be applied. The signature of the function should be equivalent to the following:  Ret fun(const Type &a); The signature does not need to have const&. The type Type must be such that an object of type InputIt can be dereferenced and then implicitly converted to Type. The type Ret must be such that an object of type OutputIt can be dereferenced and assigned a value of type Ret. ​
binary_op	-	binary operation function object that will be applied. The signature of the function should be equivalent to the following:  Ret fun(const Type1 &a, const Type2 &b); The signature does not need to have const&. The types Type1 and Type2 must be such that objects of types InputIt1 and InputIt2 can be dereferenced and then implicitly converted to Type1 and Type2 respectively. The type Ret must be such that an object of type OutputIt can be dereferenced and assigned a value of type Ret. ​
Type requirements
- InputIt must meet the requirements of InputIterator.
- InputIt1 must meet the requirements of InputIterator.
- InputIt2 must meet the requirements of InputIterator.
- OutputIt must meet the requirements of OutputIterator.

Return value

Output iterator to the element past the last element transformed.

Complexity

1-2) Exactly std::distance(first1, last1) applications of unary_op

3-4) Exactly std::distance(first1, last1) applications of binary_op

Exceptions

The overloads with a template parameter named ExecutionPolicy report errors as follows:

• If execution of a function invoked as part of the algorithm throws an exception and ExecutionPolicy is one of the three 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.

Possible implementation

First version
template<class InputIt, class OutputIt, class UnaryOperation> OutputIt transform(InputIt first1, InputIt last1, OutputIt d_first, UnaryOperation unary_op){while(first1 != last1){*d_first++= unary_op(*first1++);}return d_first;}
Second version
template<class InputIt1, class InputIt2, class OutputIt, class BinaryOperation> OutputIt transform(InputIt1 first1, InputIt1 last1, InputIt2 first2, OutputIt d_first, BinaryOperation binary_op){while(first1 != last1){*d_first++= binary_op(*first1++, *first2++);}return d_first;}

Notes

std::transform does not guarantee in-order application of unary_op or binary_op. To apply a function to a sequence in-order or to apply a function that modifies the elements of a sequence, use std::for_each

Example

The following code uses transform to convert a string to uppercase using the toupper function:

Run this code

#include <string>#include <cctype>#include <algorithm>#include <iostream>   int main(){std::string s("hello"); std::transform(s.begin(), s.end(), s.begin(), [](unsignedchar c){return std::toupper(c);});std::cout<< s;}

Output:

HELLO

See also

for_each	applies a function to a range of elements (function template)
std::experimental::parallel::transform (parallelism TS)	parallelized version of std::transform (function template)