Fundamentals of CS I (CS151 2001S) : Reference

list-ref: extract an element of a list

Parametersl, a list
i, an integer
PurposeExtracts the ith element of the list. Elements are numbered starting at 0.
ProducesAn element of the list.
PreconditionsTwo parameters (verified).
l is a list (verified). i >= 0 (verified). i < (length l) (verified).
PostconditionsDoes not affect l.
> (list-ref (list 1 2 3) 0)
> (list-ref (list 'a 'b 'c) 0)
> (list-ref (list 'a 'b 'c) 2)
> (list-ref (list 'a 'b 'c) 3)
list-ref: index 3 too large for list: (a b c)
> (define lists (list (list 'a 'b 'c) (list 1) (list 4 2 1 0)))
> lists
((a b c) (1) (4 2 1 0))
> (list-ref lists 0)
(a b c)
> (list-ref (list-ref lists 0) 1)
> (list-ref (list 'a 'b 'c) -1)
list-ref: expects type <non-negative exact integer> as 
  2nd argument, given: -1; other arguments were: (a b c)

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