Missing numbers

We are given an array of unique numbers and asked to write a script to find out all missing numbers in the range 0..$n where $n is the array size.

Examples

Example 1
Input:  = (0,1,3)
Output: 2
The array size i.e. total element count is 3, so the
   range is 0..3.
The missing number is 2 in the given array.

Example 2
Input:  = (0,1)
Output: 2
The array size is 2, therefore the range is 0..2.
The missing number is 2.

Analysis

The first thing to note is that there will always be at least one missing number, as there are $n + 1 numbers in the range 0 .. $n and only $n entries in the array.

The easy way to do this task is to convert the array to a string:

$string = ',' . join(',', @$array) . ',';

The string will look something like this:

,1,2,3,4,5,6,

Now all we have to do is to loop $j over 0 to $n - 1 and:

$result .= qq[$j, ] unless $string =~ m|,$j,|;

and with a little tidying up of the format, we have the result requested.

Some smart tricks used in the solutions. Thanks for sharing.

Try it 

Try running the script with any input:

Example: 1, 2, 4, 5, 6

⤡

#!/usr/bin/perl

# Peter Campbell Smith - 2023-01-23
# PWC 201 task 1

use v5.28;
use utf8;
use warnings;

# Task: You are given an array of unique numbers. Write a script to find out all missing numbers in the range 
# 0..$n where $n is the array size.

# Blog: http://ccgi.campbellsmiths.force9.co.uk/challenge

my (@tests, $test, $string, $j, $result);

@tests = ([0, 1, 3], [0, 1], [4, 5, 6], [6, 5, 4, 2, 1, 0], [0], []);

# loop over tests
for $test (@tests) {
    
    # make a string from the array
    $result = '';
    $string = ',' . join(',', @$test) . ',';   # $string = ,1,2,3,
    
    # check for missing numbers
    for $j (0 .. scalar @$test) {
        $result .= qq[$j, ] unless $string =~ m|,$j,|;
    }
    
    # report result
    $string =~ s|,|, |g;
    say qq[\nInput:  \$array = (] . substr($string, 2, -2) . ')';
    say qq[Output: ] . substr($result, 0, -2);
}   

13 lines of code

Output from script

Input:  $array = (0, 1, 3)
Output: 2

Input:  $array = (0, 1)
Output: 2

Input:  $array = (4, 5, 6)
Output: 0, 1, 2, 3

Input:  $array = (6, 5, 4, 2, 1, 0)
Output: 3

Input:  $array = (0)
Output: 1

Input:  $array = ()
Output: 0