Question

(cons 2 (cons ( cons 2 3 ) (cons 4 5 )))

This gives me a list that looks like this : (2 (2 . 3) 4 . 5) when I try to count the number of elements in this list the output is 3 as exepected.

How do I calculate the number of individual elements of a pair ? The output in this case should be 5 for example.

Answer 1

Here’s a possible solution, the question is essentially asking for the number of atoms in a list structure (not necessarily null-terminated proper lists):

(define (count-all seq)
  (cond ((null? seq) 0)
        ((not (pair? seq)) 1)
        (else (+ (count-all (car seq))
                 (count-all (cdr seq))))))

It works on sequences of elements like this:

• If the sequence is empty, it has zero elements
• If the sequence is not a cons cell (a pair of elements), it’s because it’s a single element – an atom
• Otherwise add the elements of both the car and the cdr of the sequence

It works as expected for arbitrarily nested list structures:

(count-all '(2 (2 . 3) 4 . 5))
=> 5

(count-all '(1 (2 . (3 (4 . 5) 6)) 7 . 8))
=> 8