How to create a row vector with elements from 0 up to and including N in C++?

Question

I would like to create a row vector in C++ with integer elements from and including 0 to N (an integer variable I assign in my C++ program). I have seen the Armadillo C++ library and tried using its span function but it does not create a vector (rather creates an object with type arma::span) so writing:

vec n = span(0,N);

does not create the desired vector. If it helps (like if my explanation of what I want is unclear) I know that in MATLAB this creates the vector I want:

n=0:N;

I do not really care which library (if any) is used, provided the library is available on most major Linux distributions (like my present one, Fedora 25).

Can you give an example of your required output. Is it just a vector like {0,1,2,3,4,5,6,7,8,9}, where N=9? If so maybe [`std::iota`](http://en.cppreference.com/w/cpp/algorithm/iota)? — Paul Rooney, Dec 08 '16 at 02:02
Yeah, just a vector nothing special, just like you described in your example. — Josh Pinto, Dec 08 '16 at 02:04
OK, well if you can show me how I can use `std::iota` to create a vector `n` with elements of (and including) `0` to `N` I'll accept your answer. — Josh Pinto, Dec 08 '16 at 02:09
[Create fast a vector from in sequential values](http://stackoverflow.com/q/18625223/995714) — phuclv, Dec 08 '16 at 04:07

Paul Rooney · Accepted Answer · 2016-12-08T02:20:18.493

2

You could use std::iota like so.

#include <numeric>
#include <vector>
#include <iostream>

int main()
{
    int N = 9;
    std::vector<int> n(N + 1);
    std::iota(begin(n), end(n), 0);

    for(auto i: n)
    {
        std::cout << i << '\n';
    }
}

Theres probably also a cool way to do it at compile time using std::integer_sequence and some metaprogramming.

edited Dec 08 '16 at 02:20

answered Dec 08 '16 at 02:09

Paul Rooney

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I'm somewhat confused by this answer because you seemed to have switched my vector variable `n` with `x`. Have you? – Josh Pinto Dec 08 '16 at 02:10
yes. I didnt consider the name of the variable as important. I can rename it to n in the answer no problem. Although thats very similar to `N`. – Paul Rooney Dec 08 '16 at 02:11
1

Although TBH neither `n` nor `x` are very descriptive names. – Jack Deeth Dec 08 '16 at 02:12
n is one I've used for many years in MATLAB, so it has a very specific meaning to me. – Josh Pinto Dec 08 '16 at 02:12
1

Although in the context of such a short piece of educational code, its not really critical to have descriptive names. – Paul Rooney Dec 08 '16 at 02:13

score 1 · Answer 2 · answered Dec 08 '16 at 02:09

You can use the function std::iota like this:

std::vector<int> v(n + 1);
std::iota(v.begin(), v.end(), 0);

Or you could wrap that up into a function like this:

inline std::vector<int> zero_to_n_vector(std::size_t n)
{
    std::vector<int> v(n + 1);
    std::iota(v.begin(), v.end(), 0);
    return v;
}

auto v = zero_to_n_vector(20);

How to create a row vector with elements from 0 up to and including N in C++?

2 Answers2

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