From 4f65953da9fadcb3ad3b6b9e3b7baeea75bdb330 Mon Sep 17 00:00:00 2001
From: Saq Imtiaz <saq.imtiaz@gmail.com>
Date: Sat, 30 Oct 2021 10:52:38 +0200
Subject: [PATCH] Added zero based zth[] operator and documentation (#6150)

---
 core/modules/filters/listops.js                | 12 ++++++++++++
 .../examples/zth Operator (Examples).tid       | 10 ++++++++++
 editions/tw5.com/tiddlers/filters/nth.tid      | 16 +++++++++-------
 .../tw5.com/tiddlers/filters/zth Operator.tid  | 18 ++++++++++++++++++
 4 files changed, 49 insertions(+), 7 deletions(-)
 create mode 100644 editions/tw5.com/tiddlers/filters/examples/zth Operator (Examples).tid
 create mode 100644 editions/tw5.com/tiddlers/filters/zth Operator.tid

diff --git a/core/modules/filters/listops.js b/core/modules/filters/listops.js
index 0c30c22e7..ab194339a 100644
--- a/core/modules/filters/listops.js
+++ b/core/modules/filters/listops.js
@@ -103,4 +103,16 @@ exports.nth = function(source,operator,options) {
 	return results.slice(count - 1,count);
 };
 
+/*
+The zero based nth member of the list
+*/
+exports.zth = function(source,operator,options) {
+	var count = $tw.utils.getInt(operator.operand,0),
+		results = [];
+	source(function(tiddler,title) {
+		results.push(title);
+	});
+	return results.slice(count,count + 1);
+};
+
 })();
diff --git a/editions/tw5.com/tiddlers/filters/examples/zth Operator (Examples).tid b/editions/tw5.com/tiddlers/filters/examples/zth Operator (Examples).tid
new file mode 100644
index 000000000..32f610213
--- /dev/null
+++ b/editions/tw5.com/tiddlers/filters/examples/zth Operator (Examples).tid	
@@ -0,0 +1,10 @@
+created: 20211029023246203
+modified: 20211029023559616
+tags: [[Operator Examples]] [[zth Operator]]
+title: zth Operator (Examples)
+type: text/vnd.tiddlywiki
+
+<<.using-days-of-week>>
+
+<<.operator-example 1 "[list[Days of the Week]zth[]]">>
+<<.operator-example 2 "[list[Days of the Week]zth[5]]">>
diff --git a/editions/tw5.com/tiddlers/filters/nth.tid b/editions/tw5.com/tiddlers/filters/nth.tid
index 032ac53ca..fc3315d72 100644
--- a/editions/tw5.com/tiddlers/filters/nth.tid
+++ b/editions/tw5.com/tiddlers/filters/nth.tid
@@ -1,15 +1,17 @@
+caption: nth
 created: 20150122204111000
-modified: 20150203192048000
+modified: 20211029023739450
+op-input: a [[selection of titles|Title Selection]]
+op-output: the <<.place N>>th input title
+op-parameter: an integer, defaulting to 1
+op-parameter-name: N
+op-purpose: select the <<.place N>>th input title
 tags: [[Filter Operators]] [[Order Operators]]
 title: nth Operator
 type: text/vnd.tiddlywiki
-caption: nth
-op-purpose: select the <<.place N>>th input title
-op-input: a [[selection of titles|Title Selection]]
-op-parameter: an integer, defaulting to 1
-op-parameter-name: N
-op-output: the <<.place N>>th input title
 
 <<.place N>> is one-based. In other words, `nth[1]` has the same effect as the <<.olink first>> operator.
 
+<<.tip "See <<.olink zth>> for an equivalent operator with a 0 based parameter">>
+
 <<.operator-examples "nth">>
diff --git a/editions/tw5.com/tiddlers/filters/zth Operator.tid b/editions/tw5.com/tiddlers/filters/zth Operator.tid
new file mode 100644
index 000000000..4c8323c7e
--- /dev/null
+++ b/editions/tw5.com/tiddlers/filters/zth Operator.tid	
@@ -0,0 +1,18 @@
+caption: zth
+created: 20211029023242012
+modified: 20211029023711450
+op-input: a [[selection of titles|Title Selection]]
+op-output: the <<.place Z>>th input title
+op-parameter: an integer, defaulting to 0
+op-parameter-name: Z
+op-purpose: select the <<.place Z>>th input title
+tags: [[Filter Operators]] [[Order Operators]]
+title: zth Operator
+type: text/vnd.tiddlywiki
+
+<<.from-version 5.2.1>>
+<<.place Z>> is zero-based. In other words, `zth[0]` has the same effect as `nth[1]` and the <<.olink first>> operator.
+
+<<.tip "See <<.olink nth>> for an equivalent operator with a 1 based parameter">>
+
+<<.operator-examples "zth">>