Write a function nextinteger(v) which takes as input a vector v and as output returns the smallest positive integer which does not appear in v. For example, if the input is [1, 2, 3], the output would be 4. If the input is [4, 0, 1, -10], output =2

This funtion worked for the vector [1,2,3,4] but not for [4, 1, 0, -10]

function [ ] = nextinteger(v)
    n = max(v);
    if n<0
        n=0
    else
        n=n+1
    end
end

[Walter Roberson]

max([4, 1, 0, -10]) is 4, and you would transform that to 5. However, the question is not about one more than the largest value: it is about the first positive value that is not in the list. 1 is in the list, 2 is not, 3 is not, 4 is, 5 is not... smallest that is not in the list is 2.

[Kristina Swais]

so would my function here be correct?

Answer 1

[Andrei Bobrov]

function out = nextinteger(v)
    z = sort(v(v > 0));
    s = setdiff(1:floor(z(end))+1,z);
    out = s(1);
end

use

>> v = [6*rand(8,1) - 4;1;3];
>> v = v(randperm(10))
v =
      -2.4935
      0.88571
      -2.4743
      -1.9001
       1.5756
       1.0443
            3
            1
      -2.5389
      -2.8204
>> nextinteger(v)
ans =
     2
>>

[Seth Spatacean]

could you solve this using a while loop?

Answer 2

[Stephen23]

Currently your code only calculates the value 1+max(v), whereas you need to identify the first integer that is not in v, something like this:

function out = nextinteger(v)
tmp = 1:1+max(v); % positive integers
tmp = tmp(~ismember(tmp,v)); % not in v
out = tmp(1); % the first one
end

and tested:

>> nextinteger([1,2,3])
ans =  4
>> nextinteger([4,1,0,-10])
ans =  2
>>

[Hussein Moussaoui]

is it possible to do this with only for, if and while loops?

[Walter Roberson]

You will also need some indexing, some assignments, and some comparison operations.