[Issue #83] Add `step` to `-slice`

README.md

@@ -47,7 +47,7 @@ Include this in your emacs settings to get syntax highlighting:
 * [-flatten-n](#-flatten-n-num-list) `(num list)`
 * [-concat](#-concat-rest-lists) `(&rest lists)`
 * [-mapcat](#-mapcat-fn-list) `(fn list)`
-* [-slice](#-slice-list-from-optional-to) `(list from &optional to)`
+* [-slice](#-slice-list-from-optional-to-step) `(list from &optional to step)`
 * [-take](#-take-n-list) `(n list)`
 * [-drop](#-drop-n-list) `(n list)`
 * [-take-while](#-take-while-pred-list) `(pred list)`
@@ -335,15 +335,20 @@ Thus function `fn` should return a list.
 (--mapcat (list 0 it) '(1 2 3)) ;; => '(0 1 0 2 0 3)
 ```
 
-#### -slice `(list from &optional to)`
+#### -slice `(list from &optional to step)`
 
 Return copy of `list`, starting from index `from` to index `to`.
-`from` or `to` may be negative.
+
+`from` or `to` may be negative.  These values are then interpreted
+modulo the length of the list.
+
+If `step` is a number, only each STEPth item in the resulting
+section is returned.  Defaults to 1.
 
 ```cl
 (-slice '(1 2 3 4 5) 1) ;; => '(2 3 4 5)
 (-slice '(1 2 3 4 5) 0 3) ;; => '(1 2 3)
-(-slice '(1 2 3 4 5) 1 -1) ;; => '(2 3 4)
+(-slice '(1 2 3 4 5 6 7 8 9) 1 -1 2) ;; => '(2 4 6 8)
 ```
 
 #### -take `(n list)`

dash.el

@@ -432,13 +432,19 @@ Returns `nil` both if all items match the predicate, and if none of the items ma
 (defalias '-only-some-p '-only-some?)
 (defalias '--only-some-p '--only-some?)
 
-(defun -slice (list from &optional to)
+(defun -slice (list from &optional to step)
   "Return copy of LIST, starting from index FROM to index TO.
-FROM or TO may be negative."
+
+FROM or TO may be negative.  These values are then interpreted
+modulo the length of the list.
+
+If STEP is a number, only each STEPth item in the resulting
+section is returned.  Defaults to 1."
   (let ((length (length list))
         (new-list nil))
     ;; to defaults to the end of the list
     (setq to (or to length))
+    (setq step (or step 1))
     ;; handle negative indices
     (when (< from 0)
       (setq from (mod from length)))
@@ -447,7 +453,8 @@ FROM or TO may be negative."
 
     ;; iterate through the list, keeping the elements we want
     (--each-while list (< it-index to)
-      (when (>= it-index from)
+      (when (and (>= it-index from)
+                 (= (mod (- from it-index) step) 0))
         (push it new-list)))
     (nreverse new-list)))
 

dev/examples.el

@@ -70,10 +70,19 @@
   (defexamples -slice
     (-slice '(1 2 3 4 5) 1) => '(2 3 4 5)
     (-slice '(1 2 3 4 5) 0 3) => '(1 2 3)
-    (-slice '(1 2 3 4 5) 1 -1) => '(2 3 4)
+    (-slice '(1 2 3 4 5 6 7 8 9) 1 -1 2) => '(2 4 6 8)
     (-slice '(1 2 3 4 5) 0 10) => '(1 2 3 4 5) ;; "to > length" should not fill in nils!
     (-slice '(1 2 3 4 5) -3) => '(3 4 5)
-    (-slice '(1 2 3 4 5) -3 -1) => '(3 4))
+    (-slice '(1 2 3 4 5) -3 -1) => '(3 4)
+    (-slice '(1 2 3 4 5 6) 0 nil 1) => '(1 2 3 4 5 6)
+    (-slice '(1 2 3 4 5 6) 0 nil 2) => '(1 3 5)
+    (-slice '(1 2 3 4 5 6) 0 nil 3) => '(1 4)
+    (-slice '(1 2 3 4 5 6) 0 nil 10) => '(1)
+    (-slice '(1 2 3 4 5 6) 1 4 2) => '(2 4)
+    (-slice '(1 2 3 4 5 6) 2 6 3) => '(3 6)
+    (-slice '(1 2 3 4 5 6) 2 -1 2) => '(3 5)
+    (-slice '(1 2 3 4 5 6) -4 -1 2) => '(3 5)
+    (-slice '(1 2 3 4 5 6) 1 2 10) => '(2))
 
   (defexamples -take
     (-take 3 '(1 2 3 4 5)) => '(1 2 3)