std::iota

 

C++

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Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
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copycopy_if (C++11) copy_n (C++11) copy_backward move (C++11) move_backward (C++11) shift_leftshift_right (C++20)(C++20) 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)
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is_partitioned (C++11) partition_point (C++11)	partition partition_copy (C++11)	stable_partition
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is_sorted (C++11) is_sorted_until (C++11)	sort stable_sort	partial_sort partial_sort_copy	nth_element
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lower_bound	upper_bound	binary_search	equal_range
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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)	transform_inclusive_scan (C++17) transform_exclusive_scan (C++17)	partial_sum inclusive_scan (C++17) exclusive_scan (C++17)
Operations on uninitialized storage
uninitialized_copy uninitialized_move (C++17) uninitialized_fill	uninitialized_copy_n (C++11) uninitialized_move_n (C++17) uninitialized_fill_n	uninitialized_default_construct (C++17) uninitialized_value_construct (C++17) uninitialized_default_construct_n (C++17) uninitialized_value_construct_n (C++17)	destroy (C++17) destroy_n (C++17) destroy_at (C++17) construct_at (C++20)
C library
qsort	bsearch

 

Defined in header <numeric>
template< class ForwardIt, class T > void iota( ForwardIt first, ForwardIt last, T value );		(since C++11) (until C++20)
template< class ForwardIt, class T > constexpr void iota( ForwardIt first, ForwardIt last, T value );		(since C++20)

Fills the range [first, last) with sequentially increasing values, starting with value and repetitively evaluating ++value.

Equivalent operation:

*(first)   = value;
*(first+1) = ++value;
*(first+2) = ++value;
*(first+3) = ++value;
...

Parameters

first, last	-	the range of elements to fill with sequentially increasing values starting with value
value	-	initial value to store; the expression ++value must be well-formed

Return value

(none)

Complexity

Exactly last - first increments and assignments.

Possible implementation

template<class ForwardIt, class T> constexpr // since C++20 void iota(ForwardIt first, ForwardIt last, T value) { while(first != last) { *first++ = value; ++value; } }

Notes

The function is named after the integer function ⍳ from the programming language APL. It was one of the STL components that were not included in C++98, but made it into the standard library in C++11.

Example

The following example applies std::shuffle to a vector of std::lists' iterators. std::iota is used to populate containers.

Run this code

#include <algorithm>
#include <iomanip>
#include <iostream>
#include <list>
#include <numeric>
#include <random>
#include <vector>
 
class BigData // inefficient to copy
{
    int data[1024]; /* some raw data */
public:
    explicit BigData(int i = 0) { data[0] = i; /* ... */ }
    operator int () const { return data[0]; }
    BigData& operator=(int i) { data[0] = i; return *this; }
    /* ... */
};
 
int main()
{
    std::list<BigData> l(10);
    std::iota(l.begin(), l.end(), -4);
 
    std::vector<std::list<BigData>::iterator> v(l.size());
    std::iota(v.begin(), v.end(), l.begin());
    // Vector of iterators (to original data) is used to avoid expensive copying,
    // and because std::shuffle (below) cannot be applied to a std::list directly.
 
    std::shuffle(v.begin(), v.end(), std::mt19937{std::random_device{}()});
 
    std::cout << "Original contents of the list l:\t";
    for(auto const& n: l) std::cout << std::setw(2) << n << ' ';
    std::cout << '\n';
 
    std::cout << "Contents of l, viewed via shuffled v:\t";
    for(auto const i: v) std::cout << std::setw(2) << *i << ' ';
    std::cout << '\n';
}

Possible output:

Original contents of the list l:        -4 -3 -2 -1  0  1  2  3  4  5
Contents of l, viewed via shuffled v:   -1  5 -4  0  2  1  4 -2  3 -3

See also

ranges::iota_viewviews::iota (C++20)	a view consisting of a sequence generated by repeatedly incrementing an initial value (class template) (customization point object)
fill	copy-assigns the given value to every element in a range (function template)
ranges::fill (C++20)	assigns a range of elements a certain value (niebloid)
generate	assigns the results of successive function calls to every element in a range (function template)
ranges::generate (C++20)	saves the result of a function in a range (niebloid)
ranges::iota (C++23)	fills a range with successive increments of the starting value (niebloid)