Implementation and tests for "combinations" function

1 files changed, 19 insertions, 0 deletions

diff --git a/combinations/combinations.scm b/combinations/combinations.scm
new file mode 100644
index 0000000..8817176
--- /dev/null
+++ b/combinations/combinations.scm
@@ -0,0 +1,19 @@
+(define-module (combinations)
+  #:export (combinations))
+
+
+(define (combinations lst n)
+  "Return N length subsequences of elements from
+LST."
+  (if (or (null? lst) (zero? n))
+      (if (zero? n)'(()) '())
+      (let loop ((lst lst) (result '()))
+        (if (null? lst)
+            result
+            (let ((next-lst (cdr lst)))
+	      (loop next-lst
+                    (append result
+                            (map (lambda (combo)
+                                   (cons (car lst) combo))
+                                 (combinations next-lst
+					       (1- n))))))))))