Any idea of printing in Racket?

Question

I've been trying hard to do this and it's just frustrating that I can't actually do it. I'm even embarrassed of posting the code I wrote. I'm new to recursion so I need some help over here. What I'm trying to do is to define a function star like this:

> (star 5)
*
**
***
****
*****

It should print * and then ** in the next line and so on up to the number you put. A classic beginner exercise but much more difficult in Racket with recursion. Thank you.

Answer 1

To iterate from 1 to 5 using nothing but recursion:

(let loop ((n 1))
    (cond ((> n 5)
           (void))
          (else
            (displayln ...)
            (loop (+ 1 n)))))

The above is equivalent to defining a function called loop and then calling it, like this:

(define (loop n)
    (cond ((> n 5)
           (void))
          (else
            (displayln ...)
            (loop (+ 1 n)))))
(loop 1)

This can appear inside the body of another function:

(define (stars num-stars)
    (define (loop ...

To print n stars:

 (displayln (make-string n #\*))

Racket isn't raw Scheme, and it has a looping construct of its own, which you could also use:

 (for ((n (in-range 1 6)))
    (displayln ...))

There's also a library that implements the loop construct from Common Lisp/Maclisp (which also has implementations for Chicken Scheme and EMACS Lisp):

(require (planet jphelps/loop)) ;; Download, install, and
                                ;; use the library.
(loop for n from 1 to 5
    do (displayln ...))