LowestAvailableNaturalNumber

Naming Conventions
	Scalar	Vector	Matrix	Any rank	Non- scalar
Boolean	`BS`	`BV`	`BM`	`BA`	`BN`
Integer	`IS`	`IV`	`IM`	`IA`	`IN`
Float	`FS`	`FV`	`FM`	`FA`	`FN`
Numeric	`NS`	`NV`	`NM`	`NA`	`NN`
Character	`CS`	`CV`	`CM`	`CA`	`CN`
Enclosed vector	`ES`	`EV`	`EM`	`EA`	`EN`
enclosed Text vector	`TS`	`TV`	`TM`	`TA`	`TN`
Simple	`SS`	`SV`	`SM`	`SA`	`SN`
Any	`AS`	`AV`	`AM`	`AA`	`AN`
phrase e.g.: `.X...Y. ⍝ X←BS; Y←IV`
combinations: use `ISV` for Integer scalar`∨`vector
`B3`, `I4` &c. to specify higher ranks.

Open full technical reference

Find the lowest available natural number not in a list of numbers already in use.

This ⎕IO dependent phrase emerged after a brainstorming session involving half a dozen experienced APLers and one relative newbie. We wanted a new algorithm to generate a new file tie-number. Some processes used prescribed "magic" numbers. The common method of 1+0⌈⌈/⎕FNUMS was therefore suspect. Predictably the newbie was the one who hit the mark.

      ''⍴(⍳1+⍴IV)~IV

Naturally the direct route for the experts was to optimise the search among 1+0⌈⌈/iv possibilities. What could the number of numbers currently in use (⍴iv) possibly have to do with it?

Examples

      ⎕FX ⍕2 1⍴'r←Next iv' 'r←''''⍴(⍳1+⍴iv)~iv'
      Next 0 1 3 4
2
      Next⍳0
0
      Next⍳9
9
      Next 0 1 100 1000 10000 100000 1000000
2

It should be noted that an origin of zero does not preclude zero's candidature:

      ⎕IO←0 ⋄ Next 1 2 3
0
      ⎕IO←1 ⋄ Next 1 2 3
4

How not to do it!

      ''⍴(⍳2+0⌈⌈/iv)~iv

A cursory look at the problem leads to examining all the numbers up to 1 greater than the maximum presently in use. Not a good idea if they can be chosen by another process such as a user's date-of-birth.

Conforming Variants

The search for a tie number must exclude zero so we include it in the "in use" list:

      ''⍴(⍳1+⍴iv)~iv←0,⎕FNUMS

A very similar variant deals with negative ⎕NNUMS:

      -''⍴(⍳1+⍴iv)~iv←0,|⎕NNUMS

Another IO dependent (will provide numbers starting at []IO):

      ((⍳⍴,iv)∊iv)⍳0

Another always providing positive integers:

      ((⍳1+⍴,iv)∊0,iv)⍳0

Specialities

Both APLX and Dyalog APL offer an alternative method of selecting a free tie number. If you specify a tie number of 0, an operation like ⎕NTIE will select the first available tie number, for example:

      tie←'C:\somedir\filename.txt' ⎕NTIE 0

Compatibility

Checked with: APL2, APLX, Dyalog, NARS2000

Test Cases

Replace "PhraseName" accordingly Show Hide test cases test cases

 Test;Next;⎕IO
 ⎕IO←0
 ⎕FX⍕2 1⍴'r←Next iv' 'r←''''⍴(⍳1+⍴iv)~iv'
⍝ ---- Start Test cases (do not delete this!)
 2≡Next 0 1 3 4
 0≡Next⍳0
 9≡Next⍳9
 2≡Next 0 1 100 1000 10000 100000 1000000

For details see the PhraseBook/TestCasesGuidelines.

Test my code, both Examples and Test Cases.

PhraseBook/LowestAvailableNaturalNumber