Data update

Task/Search-a-list/Emacs-Lisp/search-a-list.l

@@ -0,0 +1,26 @@
+(defun get-index (item list)
+  "Return index of first ITEM in LIST.
+Display error message if ITEM is not
+part of LIST."
+  (if (seq-position list item) ; if it's in LIST
+      (seq-position list item) ; return index
+    ;; else, provide an error message
+    (message "%s is not an element in %s." item list)))
+
+;; My thanks to Protesileos Stavrou (aka "Prot") for
+;; key ideas for the section below, including reversing
+;; the list in order to easily find the last item in it.
+(defun last-position (item list)
+  "Return last ITEM index in LIST."
+  ;; test if ITEM is in LIST
+  (if (seq-position list item) ; if it's in LIST
+      (progn
+        ;; subtract the adjusted length of the list from
+        ;; the position of ITEM in the reverse of LIST
+        (-
+         ;; 1- corrects for length being 1 based, but seq-position is 0 based
+         (1- (length list))
+         ;; reverse list because
+         ;; seq-position searches from beginning of list
+         (seq-position (nreverse list) item)))
+    (message "%s is not an element in %s." item list)))