Question

I am trying to remove members which exists in a string from a list consisting of those members.

Example:

String: "ABC"

List: ('A 'B 'C 'D)

To do: Remove first element of string from the list

To remove the the element from the string:

I am converting it to a list using:

(car (string->list"ABC") 

This gives me character list and first element: #\A

But I do not get how I can remove it from the list as both values do not compare: the character and list values.

Tried this weird approach which did not work:

((eq? (make-string 1 (car (string->list"ABC"))) (car (list 'A 'B 'C 'D))))

Does not work as string value does not compare with the first list value.

How I can compare & remove the first string alphabet from the original list??

Answer 1

Error as list cannot have string element values:

This is not true. A list can have strings as well as characters as its elements, no problem. You’re getting the error because you’re using car on a string, not a list.

(car (list->string(list(car (string->list "ABC")))))

Ok, here you’re calling (string->list "ABC") which gives you a list containing the character A, B and C. Now you’re calling car on that and get the character A back. Up to here there’s no problem.

But then you’re calling list on that character getting a list which only contains the character A and then use list-string to turn this into the string "A". This is still perfectly legal. But then you’re calling car on the string "A" and that’s the error because you’re calling car on a string, but car only accepts a list (well, any pair actually) as its argument.

Since you’re trying to compare A against that first element of ('A 'B 'C 'D), i.e. 'A, which is a symbol, you probably want to convert the string "A" into the symbol 'A (or the symbol 'A into the string "A"). To do this, you can use the function string->symbol instead of car.

Once you have the symbol A, you can easily remove it from the list using the filter function:

(let ((a (string->symbol (list->string (list (car (string->list "ABC")))))))
  (filter (lambda (x) (not (eq? x a))) '(A B C)))