I am learning Scheme, and I’ve read the basics but I still can’t figure how to “map” a Java class to Scheme code. Could any of you guys help me out here? I just need someone to show me how this looks in Scheme to grasp the final details and get things going in my head:

public class sumFibonacciValues { public static void main(String [] args) { int n = 4000000; long i2 = 1, i1 = 1, Fibo = 0, temp = 1; while(i2 < n) { temp = i1 + i2; i1 = i2; i2 = temp; if(i2 % 2 == 0) Fibo += i2; } System.out.println(Fibo); } }

## Answer

I wouldn’t have answered something that looks so much like homework, but the “idiomatic” comment just begged for a demonstration that it’s really not that far. First, a direct translation:

(define (sum-fibonacci-values) (define n 4000000) (define i2 1) (define i1 1) (define fibo 0) (define temp 1) (let loop () (when (< i2 n) (set! temp (+ i1 i2)) (set! i1 i2) (set! i2 temp) (when (zero? (modulo i2 2)) (set! fibo (+ fibo i2))) (loop))) (write fibo))

Second, make it “idiomatic”, by removing the redundant mutations, and instead just bind new values, and using a tail-recursive loop. Note that this code is still in direct correlation with the original:

(define (sum-fibonacci-values) (define n 4000000) (let loop ([i2 1] [i1 1] [fibo 0] [temp 1]) (if (< i2 n) (let* ([temp (+ i1 i2)] [i1 i2] [i2 temp] [fibo (if (zero? (modulo i2 2)) (+ fibo i2) fibo)]) (loop i2 i1 fibo temp)) fibo)))

Finally, now that the code is clearer, you can see that there are some redundancies. Here’s a cleaned up version:

(define (sum-fibonacci-values) (define n 4000000) (let loop ([i2 1] [i1 1] [fibo 0]) (if (< i2 n) (let ([i3 (+ i1 i2)]) (loop i3 i2 (if (zero? (modulo i3 2)) (+ fibo i3) fibo))) fibo)))

Note that the same cleanup can be done on the Java code. (But that’s really left as an exercise to the reader…)