C++ How to insert a consecutive inter range into std::vector?

Question

Say I want all numbers from 23 to 57 be in a vector. I can do this:

vector<int> result;
for (int i = 23; i <= 57; ++i)
{
    result.push_back(i);
}

But this is a 5 line solution for a simple job. Can't I do that more elegantly? The best syntax would be vector<int> result{23 .. 57}; for example or such a trivial one line code. Any options with C++17?

score 17 · Accepted Answer · edited Jun 20 '20 at 09:12

17

You can use std::iota (since C++11).

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

The function is named after the integer function ⍳ from the programming language APL.

e.g.

std::vector<int> result(57 - 23 + 1);
std::iota(result.begin(), result.end(), 23);

edited Jun 20 '20 at 09:12

Community

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answered May 16 '18 at 07:12

songyuanyao

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And what `iota` means? Any acronym? – Narek May 16 '18 at 07:17
8

@Narek check this post: https://stackoverflow.com/questions/9244879/what-does-iota-of-stdiota-stand-for – Ja_cpp May 16 '18 at 07:19

Jarod42 · Answer 2 · 2020-05-19T07:46:18.023

6

With range-v3, it would be:

const std::vector<int> result = ranges::view::ints(23, 58); // upper bound is exclusive

With C++20, std::ranges::iota_view:

const auto result1 = std::ranges::views::iota(23, 58); // upper bound is exclusive
const auto result2 = std::ranges::iota_view(23, 58); // upper bound is exclusive

edited May 19 '20 at 07:46

answered May 16 '18 at 07:48

Jarod42

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This code doesn't compile with gcc-10. Do you know current C++20 equivalent? – Paul Jurczak May 17 '20 at 01:34
@PaulJurczak: C++20 iota view added. – Jarod42 May 18 '20 at 14:57
I'm getting `error: conversion from ‘std::ranges::iota_view’ to non-scalar type ‘const std::vector’ requested` with gcc 10.0.1 – Paul Jurczak May 19 '20 at 02:14
@PaulJurczak: Indeed, C++20 ranges seems not convertible directly to `std::vector`, `std::vector v{range.begin(), range.end()};` should to the job. – Jarod42 May 19 '20 at 07:52

score 0 · Answer 3 · answered May 16 '18 at 14:26

Yet another possibility is to use boost::counting_iterator[1]. This also has the advantage of working with C++98, should you be unfortunate enough to be stuck with it.

#include <boost/iterator/counting_iterator.hpp>

…

result.insert(result.end(), boost::counting_iterator<int>(23), boost::counting_iterator<int>(58));

or, even simpler:

vector<int> result(boost::counting_iterator<int>(23), boost::counting_iterator<int>(58));

Note that, as a normal half-open range is expected in either case, you'll have to use lastNum+1, and you will not be able to insert numeric_limits<int>::max() (aka INT_MAX) for this reason.

[1] https://www.boost.org/doc/libs/1_67_0/libs/iterator/doc/counting_iterator.html

C++ How to insert a consecutive inter range into std::vector?

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