From 4d382f82b38e508dc7734fb5d7ac41af4fa6ada6 Mon Sep 17 00:00:00 2001
From: Vidar Holen <vidar@vidarholen.net>
Date: Tue, 17 Aug 2021 13:39:25 -0700
Subject: [PATCH] Created SC2302 (markdown)

---
 SC2302.md | 67 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 67 insertions(+)
 create mode 100644 SC2302.md

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+## This loops over values. To loop over keys, use "${!array[@]}".
+
+Plus companion warning [[SC2303]]: `i is an array value, not a key. Use directly or loop over keys instead.`
+
+### Problematic code:
+
+```sh
+array=(foo bar)
+for i in "${array[@]}"
+do
+  echo "Value is ${array[$i]}"
+done
+```
+
+### Correct code:
+
+Either loop over values 
+
+```sh
+for i in "${array[@]}"
+do
+  echo "Value is $i"
+done
+```
+
+or loop over keys:
+
+```sh
+for i in "${!array[@]}"
+do
+  echo "Key is $i"
+  echo "Value is ${array[$i]}"
+done
+```
+
+### Rationale:
+
+ShellCheck found a `for` loop over array *values*, where the variable is used as an array *key*. 
+
+In the problematic example, the loop will print `Value is foo` twice. On the second iteration, `i=bar`, and `bar` is unset and considered zero, so `${array[$i]}` becomes `${array[bar]}` becomes `${array[0]}` becomes `foo`.
+
+If you don't care about the key, simply loop over array values and use `$i` to refer to the array value, like in the first correct example.
+
+If you do want the key, loop over array keys with `"${!array[@]}"`, use `$i` to refer to the array key, and `${array[$i]}` to refer to the array value.
+
+### Exceptions:
+
+If you do want to use values from the arrays as keys in the same array, you can [[ignore]] these messages with a directive:
+
+```sh
+declare -A fatherOf=(
+  ["Eric Bloodaxe"]="Harald Fairhair"
+  ["Harald Fairhair"]="Halfdan the Black"
+  ["Halfdan the Black"]="Gudrød the Hunter"
+  ["Gudrød the Hunter"]="Halfdan the Mild"
+)
+
+# shellcheck disable=SC2302,SC23203
+for i in "${fatherOf[@]}" 
+do
+  echo "${fatherOf[$i]:-(missing)} begat $i"
+done
+```
+
+### Related resources:
+
+* Help by adding links to BashFAQ, StackOverflow, man pages, POSIX, etc!
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