Question

So basically this code’s purpose is to simply print out the first n even numbers.

for (i = 0; i <=n; i+= 2)
{
    print i;
}

Thing is though, I don’t understand Scheme at all. So, help please.

Answer 1

There are several ways to convert the code in the question to Scheme. The first one I can think of:

(define (print-even n)
  (let loop ((i 0))
    (if (<= i n)
        (begin
          (print i)
          (newline)
          (loop (+ i 2))))))

Notice this:

• The solution is written as a recursive procedure
• Instead of a for loop, I’m using a construct called a named let, which permits the initialization of some iteration variables (i in this case, initialized to 0) and the repeated execution of a recursive procedure (loop in this case), producing an effect similar to a for, even in performance
• The stop condition in the “loop” is handled with essentially the same expression: repeat the body of the iteration as long as (<= i n), when that condition becomes false, the iteration ends
• The begin surrounds the body of the “loop”, just as the curly braces {} do in the original code
• The print procedure performs the expected operation; for readability I added a new line after printing each number
• The increment part of the original loop i += 2 is handled by the expression (+ i 2), inside the recursive call

So you see, the process being executed is essentially the same, only the way to write it (the syntax!) is different. Give it a try, type this:

(print-even 6)

… And the following will get printed on the screen:

0
2
4
6

Another possible way to implement the procedure, more similar to the original code, although (this is completely subjective) less idiomatic than the previous one:

(define (print-even n)
  (do ((i 0 (+ i 2))) ((> i n))
    (print i)
    (newline)))

Finally, if you’re using Racket this will seem even more familiar to you:

#lang racket

(define (print-even n)
  (for ((i (in-range 0 (+ n 1) 2)))
    (print i)
    (newline)))