std::iota
From cppreference.com
- This page is about the algorithms for generating increasing integer ranges, known as iota sequences after APL's use of the Greek letter . This is unrelated to the C-style ASCII-to-integer conversion, atoi.
| Defined in header <numeric>
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template< class ForwardIt, class T >
void iota( ForwardIt first, ForwardIt last, T value );
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(since C++11) (constexpr since C++20) |
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Fills the target range [first, last) with sequentially increasing values, starting with value and repetitively evaluating ++value.
Equivalent operation (assuming ++value returns the incremented value):
*first = value;
*++first = ++value;
*++first = ++value;
*++first = ++value;
// repeats until last is reached
If any of the following conditions is satisfied, the program is ill-formed:
Tis not convertible to the value type ofForwardIt.- The expression
++valis ill-formed, wherevalis a variable of typeT.
Parameters
| first, last | - | the pair of iterators defining the target range |
| value | - | initial value to fill |
Complexity
Exactly std::distance(first, last) increments and assignments.
Possible implementation
template<class ForwardIt, class T>
constexpr // since C++20
void iota(ForwardIt first, ForwardIt last, T value)
{
for (; first != last; ++first, ++value)
*first = value;
}
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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 <list>
#include <numeric>
#include <print>
#include <random>
#include <ranges>
#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{}()});
auto view_l = l | std::views::transform([](const auto& x){ return (int)x; });
auto view_v = v | std::views::transform([](auto x){ return (int)*x; });
std::print("Original contents of the list l:\t\t{::2}\n", view_l);
std::print("Contents of l, viewed via shuffled v:\t{::2}\n", view_v);
}
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: [ 0, 5, -1, 4, -2, -4, 1, 2, -3, 3]
See also
(C++23) |
fills a range with successive increments of the starting value (algorithm function object) |
| copy-assigns the given value to every element in a range (function template & algorithm function object) | |
(C++20) |
|
| assigns the results of successive function calls to every element in a range (function template & algorithm function object) | |
(C++20) |
|
(C++20) |
a view consisting of a sequence generated by repeatedly incrementing an initial value (class template) (customization point object) |
(C++26) |
a view that generates a sequence of increasing values from 0 up to n(customization point object) |
| a view that maps each element of adapted sequence to a tuple of both the element's position and its value (class template) (range adaptor object) |