Fetch elements from List in scheme

Question

Hi, How to go through a list or fetch element from a list in scheme?

How can I name each element (like we do for variables in java) in a list?

Thanks in advance.

I want to compare every point in a list to another point. So, as we do in java or python-

for(int i;i<list.size();i++){
    if (list[i]> k){ 
        //do something
    }

}

How can I do similar thing in scheme?

Answer 1

Example

`(define func
  (lambda (a b)
    (if (> a b)
        (dosomething a))))`

In this case, "dosomething" would be some other definition, either predefined or not. Definition being analogous to a "function" let's say, such as square() or mult(), as examples.

I think a recursive one like this should work for the list needs:

(define (func list) (if (> (car list) k) (dosomething)) (func (cdr list)))

You can also write two definitions, have one contain a list and send it to the first example and return the result, compare, do something or not, and continue.

Answer 2

(map (lambda (x) (if (< x k) (dosomething x) x)) list)

Answer 3

You can use for-each:

    (let ((a-list (list 9 2 7 6 1 4 3 8 1))
          (k 4)
          (something display))
      (for-each (lambda (i) ; The 'name' of the list element is i
                  (if (> i k)
                      (something i) ; Do something with i
                      #f)) ; Do nothing
                a-list))

for-each is similar to map but the return value is unspecified. It is called for its side-effects only.

On a side note, It appears to me that you are learning scheme coming from a java background (which is ok). Scheme programs are often written in different style than in Java. Scheme is a functional language and looping constructs are not used quite as often. Pick up a book on Scheme, for example The Scheme Programming Language or How to Design Programs to learn how to write programs "the Scheme way"

Answer 4

leppie and Jonas give the right answer for iterating over a list in Scheme. However, if you need to get a single value in a list, use list-ref.

(let ((l '(1 2 3 4)))
  (list-ref l 2))

=> 3

Is mostly equivalent to the Java code

int[] l = new int[] { 1, 2, 3, 4 };
return l[2];

Answer 5

You can use list-ref to pull out a value from a list by its index. But note that in Scheme lists are actually linked lists -- so (list-ref l 100) will need to trace through a 100 references. If you truly want random-access values, use a vector.

Answer 6

You should think differently when writing in a functional language. Actually you should forget the programming thinking (for loops, etc.), just define what you want. For example "I want to apply an-action to every odd number in my list." So you would write :

(map an-action
     (filter odd? my-list))

Or the condition can be element > 4

(define (higher-than-four? n) (> n 4))
(map an-action
     (filter higher-than-four?
             my-list))

For an action, you give any function :

> (map number->string
       (filter higher-than-four? my-list))
("5" "6" "7" "8" "9" "10")

Answer 7

I am not sure if this is what you want, but this goes to the whole list (linked) and compare each element with the element that you pass as parameter:

(define (compare aList element)
      (if (> element (car aList)) 
         (display (car aList)) ;here do something great
         ;here the else if you have
        )
      (if (not (null? (cdr aList)))
               (compare (cdr aList) element)) 'ok)

The issue that this is a procedure, maybe it is helpful for someone.

greets