Scheme language: merge two numbers

Question

How can I merge two integers from a list into one? (in Scheme) Example:
'(11 223) -> 11223

Answer 1

Assuming that the list has exactly two elements, and that both are numbers:

(define (merge-numbers lst)
  (let ((1st (number->string (first  lst)))
        (2nd (number->string (second lst))))
    (string->number (string-append 1st 2nd))))

It works as expected:

(merge-numbers '(11 223))
> 11223

Alternatively, without using a let:

(define (merge-numbers lst)
  (string->number
   (string-append
    (number->string (first  lst))
    (number->string (second lst)))))

Answer 2

This is my original answer from Jan 25 '12 at 3:05. It only handles the given test. It was a mindless answer, sorry.

(define (merge ls) 11223)    
(merge '(11 223))

This is my new answer that handles all cases when passed a list of numbers. Thank you blubberdiblub for the pointer! ; list of zero or more strings -> string (define (merge-numbers ns) (if (null? ns) "" (let ((first-n (number->string (car ns))) (rest-ns (cdr ns))) (string-append first-n (merge-numbers rest-ns)))))

(merge-numbers '(11 223))
(merge-numbers '())
(merge-numbers '(1))
(merge-numbers '(1 2))
(merge-numbers '(1 2 3))
(merge-numbers '(1 2 3 4))
(merge-numbers '(1 2 3 4 5))

"11223"
""
"1"
"12"
"123"
"1234"
"12345"

Since the original question probably meant that the result should be a number, and this results in a string, then string->number needs to be used to get the final answer. Thank you again blubberdiblub for an explanation of what the poster probably meant, I had missed it.

Answer 3

There are many ways to write this procedure, depending on what you plan for it. For example, if the list might contain more than two numbers (in the future?) then you can write it as follows:

(define merge-numbers
  (lambda (s)
    (string->number
      (apply string-append
        (map number->string s)))))

So now you can type:

> (merge-numbers '(4 9 66 33 555 1))
4966335551

If there's a real reason why you want the number two, then I think the use of the procedure format would be more readable:

(define merge-two-numbers
  (lambda (s)
    (string->number
      (format "~a~a"
        (car s)
        (cadr s)))))

etc.