Print the missing number in a unique sequential list with an arbitrary starting range or starting from 1 - perl

Question

This question is similar to How can I find the missing integers in a unique and sequential list (one per line) in a unix terminal?.
The difference being is that I want to know if it is possible to specify a starting range to the list
I have noted the following provided solutions:
awk '{for(i=p+1; i<$1; i++) print i} {p=$1}' file1
and
perl -nE 'say for $a+1 .. $_-1; $a=$_'
file1 is as below:
5
6
7
8
15
16
17
20
Running both solutions, it gives the following output:
1
2
3
4
9
10
11
12
13
14
18
19
Note that the output start printing from 1.
Question is how to pass an arbitrary starting/minimum to start with and if nothing is provided, assume the number 1 as the starting/minimum number?
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Yes, sometimes you will want the starting number to be 1 but sometimes you will want the starting number as the least number from the list.

Answer 1

You can use your awk script, slightly modified, and pass it an initial p value with the -v option:
$ awk 'BEGIN{p=p<1?1:p} {for(i=p; i<$1; i++) print i} {p=p<=$1?$1+1:p}' file1
1
2
3
4
9
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19
$ awk -v p=10 'BEGIN{p=p<1?1:p} {for(i=p; i<$1; i++) print i} {p=p<=$1?$1+1:p}' file1
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The BEGIN block initializes p to 1 if it is not specified or set to 0 or a negative value. The loop starts at p instead of p+1, and the last block assigns $1+1 to p (instead of $1), if and only if p is less or equal $1.
This assumes that the default (1) is the minimum starting number you would want. If you would like to start from 0 or even from a negative number just replace BEGIN{p=p<1?1:p} by BEGIN{p=(p==""?1:p)}:
$ awk -v p=-2 'BEGIN{p=(p==""?1:p)} {for(i=p; i<$1; i++) print i} {p=p<=$1?$1+1:p}' file1
-2
-1
0
1
...

Answer 2

Slight variations of those one-liners to include a start point:
awk
# Optionally include start=NN before the first filename
$ awk 'BEGIN { start= 1 }
$1 < start { next }
$1 == start { p = start }
{ for (i = p + 1; i < $1; i++) print i; p = $1}' start=5 file1
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$ awk 'BEGIN { start= 1 }
$1 < start { next }
$1 == start { p = start }
{ for (i = p + 1; i < $1; i++) print i; p = $1}' file1
1
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perl
# Optionally include -start=NN before the first file and after the --
$ perl -snE 'BEGIN { $start //= 1 }
if ($_ < $start) { next }
if ($_ == $start) { $a = $start }
say for $a+1 .. $_-1; $a=$_' -- -start=5 file1
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$ perl -snE 'BEGIN { $start //= 1 }
if ($_ < $start) { next }
if ($_ == $start) { $a = $start }
say for $a+1 .. $_-1; $a=$_' -- file1
1
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Answer 3

Using Raku (formerly known as Perl_6)
raku -e 'my #a=lines.map: *.Int; .put for (#a.Set (^) #a.minmax.Set).sort.map: *.key;'
Sample Input:
5
6
7
8
15
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20
Sample Output:
9
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Here's an answer coded in Raku, a member of the Perl-family of programming languages. No, it doesn't address the OP's request for a user-definable starting point. Instead the code above is a general solution that computes the input's minimum Int and counts up from there, returning any missing Ints found up--to the input's maximum Int.
Really need a user-defined lower limit? Try the following code, which allows you to set a $init variable:
~$ raku -e 'my #a=lines.map: *.Int; my $init = 1; .put for (#a.Set (^) ($init..#a.max).Set).sort.map: *.key;'
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For explanation and shorter code (including single-line return and/or return without sort), see the link below.
https://stackoverflow.com/a/72221301/7270649
https://raku.org

Answer 4

not as elegant as i hoped :
< file | mawk '
BEGIN { _= int(_)^(\
( ORS = "")<_)
} { ___[ __= $0 ] }
END {
do {
print _ in ___ \
? "" : _ "\n"
} while(++_ < __) }' \_=10
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