Question

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Write a scheme function named up-to-first-number that takes a list as its input and returns a list containing all the elements up to the first numeric element in the input list. You can use the number? predicate function to determine whether an element is a number or not.

Sample runs:

(up-to-first-number '(a b c d 1 2 3 )) returns (a b c d)

(up-to-first-number '(d e f 7)) returns (d e f)

(up-to-first-number '(g h i)) returns (g h i)

(up-to-first-number '(1 2 3)) returns NIL

Answer 1

The simple scheme function is described below for obtaining the required output

(define (up-to-first-number lst)
  (if (or <list[]=null>   ; if either the list is empty
          <list[0] = no. >)  ; or the first element is a number
      <list= Null>       ; then return the empty list
      (cons <Print the first element> ; otherwise cons the first element
            (up-to-first-number <list[length]>)))) ; and advance the recursion till the number is detected.

If your interpreter implements pre existing procedure takef , we can write the scheme function as follows :-

This will obtain the same output as required .

(define (up-to-first-number lst)
  (takef lst (negate number?)))