I am trying to write a function in racket that takes in a list of numbers and outputs a list containing the square of those numbers. I am trying to not use the map implementation and instead solve the problem recursively. However, the code I have now is outputting the same list that was the input. The code I have is as follows:
(define (my-square lst)
(cond (cons? lst)
(append (* (first lst) (first lst)) (my-square (rest lst)))))
I appreciate any help!
The base case is wrong (it should ask for the empty list), and cons should be used for recursively building a list (instead of append). Also, you have a couple of errors regarding parentheses, and the last condition in a cond should be specified using else. This is the standard template for building an output list:
(define (my-square lst)
(cond ((empty? lst)
empty)
(else
(cons (* (first lst) (first lst))
(my-square (rest lst))))))
That's fine for a solution written from scratch, but the idiomatic way to solve this problem in Racket would be to use a higher-order function, such as map - so we don't reinvent the wheel:
(define (my-square lst)
(map (lambda (x) (* x x)) lst))
Either way, it works as expected:
(my-square '(1 2 3 4 5))
=> '(1 4 9 16 25)
append should be cons, and you should handle the case when lst is empty. Finally, cond conditions should be wrapped in parentheses:
(define (my-square lst)
(cond ((cons? lst) (cons (* (first lst) (first lst)) (my-square (rest lst))))
(#t '())))
