Check consecutive numbers recursively using Lisp

Question

I'm trying to write a recursive function to check if the elements of a list are increasing consecutively.

(defun test (lst)   
 (if (null lst)
   1
   (if (= (car lst) (1- (test (cdr lst))))
    1     
    0)))

(setq consecutive '(1 2 3 4))
(setq non-consecutive '(2 5 3 6))

The results are:

CL-USER> (test non-consecutive)
0
CL-USER> (test consecutive)
0

(test consecutive) should return 1. How can I write this function correctly?

your recursion tests (car lst) vs. (1- "return of test call"), that is always (1- 0) or (1- 1) — Ondrej
– Ondrej, Commented Dec 17, 2018 at 18:21
@sds consecutive odd numbers need not be checked at all. Aynway, if I can check the '(1 2 3 4) sequence, I can check for it too I guess. By the way is the '(1 2 3 4) sequence called also the "successive numbers" in English? What is the most correct name for such a sequence, i.e. wehre the numbers increase one by one? — Terry
– Terry, Commented Dec 17, 2018 at 19:09

sds · Accepted Answer · 2019-02-11 02:36:32Z

2

To check that the numbers in the sequence are consecutive, i.e., increasing with step 1, you need this:

(defun list-consecutive-p (list)
  (or (null (cdr list))
      (and (= 1 (- (second list) (first list)))
           (list-consecutive-p (rest list)))))

Then

(list-consecutive-p '(1 2 3 4))
==> T
(list-consecutive-p '(1 4))
==> NIL
(list-consecutive-p '(4 1))
==> NIL

NB. Numbers are a poor substitute for booleans.

PS. I wonder if this is related to How to check if all numbers in a list are steadily increasing?...

edited Feb 11, 2019 at 2:36

answered Dec 17, 2018 at 19:16

sds

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4 Comments

Terry Over a year ago

That looks fine. The 'or' & 'and' construct is what I thought of initially but which I couldn't do.

sds Over a year ago

That's because you are trying to return numbers instead of booleans.

Terry Over a year ago

Yes. At first I 've done it using booleans (i.e. nil & t) but the Lisp compiler threw errors like "nil is not an integer" and the CLISP compiler in particular exited whenever I tried to run it. So I tried in vain doing it with the numbers :)

sds Over a year ago

That's because you tried to conflate the step with the return value.

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