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pythonlispschemeappendracket
Hanfei Sun
Hanfei Sun

Reputation: 47101

how to do append like python in scheme?

Like this:

> (my-append (list 1 2) 3)
'(1 2 3)

I know append in racket is actually to concatenate two list. And cons just add an element to the head of a list instead of tail

Does anyone have ideas about this?

Upvotes: 2

Views: 395

Answers (3)

dyoo
dyoo

Reputation: 12033

If you're in Racket, you probably want to look into the growable vector library (data/gvector). This provides a container type that supports many of the features you're used to with Python's growable lists.

Example:

#lang racket
(require data/gvector)
(define lst (gvector 1 2))
(gvector-add! lst 3)
(for ([elt lst]) (printf "I see: ~s\n" elt))

Otherwise, your question ends up reducing to: how do I make immutable linked lists work like mutable sequentially-allocated arrays? And that's not going to work well: data types are different for different reasons.

Upvotes: 3

Óscar López
Óscar López

Reputation: 236170

In Pyton, the append() method modifies the list in-place:

lst = [1, 2]
lst.append(3)

lst
=> [1, 2, 3]

Racket's lists are immutable by default, the closest thing to an in-place append() requires you to use mutable lists, and you have to pack the element to be appended in a list of its own:

(require scheme/mpair)

(define lst (mlist 1 2))
(mappend! lst (mlist 3))

lst
=> (mlist 1 2 3)

Notice that using immutable lists with the append procedure will produce a new list, leaving the original list untouched - so it wouldn't be "like Python":

(define lst (list 1 2))
(append lst (list 3))   ; this returns the new list '(1 2 3)

lst
=> (list 1 2)

In fact, Scheme's append procedure behaves just like Python's + operation between lists:

lst = [1, 2]
lst + [3]    # this returns the new list [1, 2, 3]

lst
=> [1, 2]

Come to think of it, Python's append() might have a misleading name - in most functional programming languages, the append operation is always defined between two lists, whereas in Python it's an operation between a list and an element. Maybe add() would have been a better name, like Java's add() operation of the List interface.

Upvotes: 5

acelent
acelent

Reputation: 8135

I don't know what you mean with "append like python", but if all you want is to insert a new element at the end of the list, you may use this:

(define (my-append lst el)
  (append lst (list el)))

Upvotes: 0

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