{"id":68372,"date":"2022-11-04T09:01:51","date_gmt":"2022-11-04T09:01:51","guid":{"rendered":"https:\/\/www.cryptocabaret.com\/?p=68372"},"modified":"2022-11-04T09:01:51","modified_gmt":"2022-11-04T09:01:51","slug":"how-to-iterate-over-tables-in-lua","status":"publish","type":"post","link":"https:\/\/www.cryptocabaret.com\/?p=68372","title":{"rendered":"How to iterate over tables in Lua"},"content":{"rendered":"

How to iterate over tables in Lua<\/span>
\nSeth Kenlon<\/a><\/span>
\nFri, 11\/04\/2022 – 03:00<\/span><\/p>\n

\n

In the Lua<\/a> programming language, an array is called a table. A table is used in Lua to store data. If you’re storing a lot of data in a structured way, it’s useful to know your options for retrieving that data when you need it.<\/p>\n<\/p>\n

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Programming and development<\/div>\n
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eBook: Modernizing Enterprise Java<\/a><\/div>\n
An open source developer’s guide to building applications<\/a><\/div>\n<\/p><\/div>\n<\/p><\/div>\n

Creating a table in Lua<\/h2>\n

To create a table in Lua, you instantiate the table with an arbitrary name:<\/p>\n

\nmytable =<\/span> {<\/span>}<\/span><\/code><\/span><\/pre>\n
There are different ways you can structure your data in a table. You could fill it with values, essentially creating a list (called a list<\/em> in some languages):<\/p>\n
\nmytable =<\/span> {<\/span> 'zombie'<\/span>,<\/span> 'apocalypse'<\/span> }<\/span><\/code><\/span><\/pre>\n
Or you could create an associated array (called a map<\/em> or dictionary<\/em> in some languages). You can add arbitrary keys to the table using dot notation. You can also add a value to that key the same way you add a value to a variable:<\/p>\n
\n
myarray =<\/span> {<\/span>}<\/span>
\nmyarray.baz<\/span> =<\/span> 'happy'<\/span>
\nmyarray.qux<\/span> =<\/span> 'halloween'<\/span><\/div><\/div><\/pre>\n
You can add verification with the assert()<\/code> function:<\/p>\n
\n
assert<\/span><\/a>(<\/span>myarray.baz<\/span> ==<\/span> 'happy'<\/span>,<\/span> 'unexpected value in myarray.baz'<\/span>)<\/span>
assert<\/span><\/a>(<\/span>myarray.qux<\/span> ==<\/span> 'halloween'<\/span>,<\/span> 'unexpected value in myarray.qux'<\/span>)<\/span><\/div><\/div><\/pre>\n
You now have two tables: a list-style mytable<\/code> and an associative array-style myarray<\/code>.<\/p>\n
Iterating over a table with pairs<\/h2>\n
Lua’s pairs()<\/code> function extracts key and value pairs from a table.<\/p>\n
\n
print(<\/span>'pairs of myarray:'<\/span>)<\/span>

for<\/span> k,<\/span>v in pairs(<\/span>myarray)<\/span> do<\/span>
\n\u00a0 print(<\/span>k,<\/span>v)<\/span>
\nend<\/div><\/div><\/pre>\n
Here’s the output:<\/p>\n
\n
pairs of myarray:<\/span>
\nbaz \u00a0 \u00a0 happy
\nqux \u00a0 \u00a0 halloween<\/div><\/div><\/pre>\n
If there are no keys in a table, Lua uses an index. For instance, the mytable<\/code> table contains the values zombie<\/code> and apocalypse<\/code>. It contains no keys, but Lua can improvise:<\/p>\n
\n
print(<\/span>'pairs of mytable:'<\/span>)<\/span>

for<\/span> k,<\/span>v in pairs(<\/span>mytable)<\/span> do<\/span>
\n\u00a0 print(<\/span>k,<\/span>v)<\/span>
\nend<\/div><\/div><\/pre>\n
Here’s the output:<\/p>\n
\n
1<\/span> \u00a0 zombie
2<\/span> \u00a0 apocalypse<\/div><\/div><\/pre>\n
Iterating over a table with ipairs<\/h2>\n
To account for the fact that tables without keys are common, Lua also provides the ipairs<\/code> function. This function extracts the index and the value:<\/p>\n
\n
print(<\/span>'ipairs of mytable:'<\/span>)<\/span>

for<\/span> i,<\/span>v in ipairs(<\/span>mytable)<\/span> do<\/span>
\n\u00a0 print(<\/span>i,<\/span>v)<\/span>
\nend<\/div><\/div><\/pre>\n
The output is, in this case, the same as the output of pairs<\/code>:<\/p>\n
\n
1<\/span> \u00a0 zombie
2<\/span> \u00a0 apocalypse<\/div><\/div><\/pre>\n
However, watch what happens when you add a key and value pair to mytable<\/code>:<\/p>\n
\n
mytable.surprise<\/span> =<\/span> 'this value has a key'<\/span>

\nprint(<\/span>'ipairs of mytable:'<\/span>)<\/span>

for<\/span> i,<\/span>v in ipairs(<\/span>mytable)<\/span> do<\/span>
\n\u00a0 print(<\/span>i,<\/span>v)<\/span>
\nend<\/div><\/div><\/pre>\n
Lua ignores the key and value because ipairs<\/code> retrieves only<\/em> indexed entries:<\/p>\n
\n
1<\/span> \u00a0 zombie
2<\/span> \u00a0 apocalypse<\/div><\/div><\/pre>\n
The key and value pair, however, have been stored in the table:<\/p>\n
\n
print(<\/span>'pairs of mytable:'<\/span>)<\/span>

for<\/span> k,<\/span>v in ipairs(<\/span>mytable)<\/span> do<\/span>
\n\u00a0 print(<\/span>k,<\/span>v)<\/span>
\nend<\/div><\/div><\/pre>\n
The output:<\/p>\n
\n
1<\/span> \u00a0 \u00a0 \u00a0 \u00a0 \u00a0zombie
2<\/span> \u00a0 \u00a0 \u00a0 \u00a0 \u00a0apocalypse
\nsurprise \u00a0 this value has a key<\/div><\/div><\/pre>\n
Retrieving arbitrary values<\/h2>\n
You don’t have to iterate over a table to get data out of it. You can call arbitrary data by either index or key:<\/p>\n
\n
print(<\/span>'call by index:'<\/span>)<\/span>
\nprint(<\/span>mytable[<\/span>2<\/span>]<\/span>)<\/span>
\nprint(<\/span>mytable[<\/span>1<\/span>]<\/span>)<\/span>
\nprint(<\/span>myarray[<\/span>2<\/span>]<\/span>)<\/span>
\nprint(<\/span>myarray[<\/span>1<\/span>]<\/span>)<\/span>

\nprint(<\/span>'call by key:'<\/span>)<\/span>
\nprint(<\/span>myarray[<\/span>'qux'<\/span>]<\/span>)<\/span>
\nprint(<\/span>myarray[<\/span>'baz'<\/span>]<\/span>)<\/span>
\nprint(<\/span>mytable[<\/span>'surprise'<\/span>]<\/span>)<\/span><\/div><\/div><\/pre>\n
The output:<\/p>\n
\n
call by index:<\/span>
\napocalypse
\nzombie
\nnil
\nnil

\ncall by key:<\/span>
\nhalloween
\nhappy
\nthis value has a key<\/div><\/div><\/pre>\n
Data structures<\/h2>\n
Sometimes using a Lua table makes a lot more sense than trying to keep track of dozens of individual variables. Once you understand how to structure and retrieve data in a language, you’re empowered to generate complex data in an organized and safe way.<\/p>\n<\/div>\n
\n
Create structure that makes it easier to find stored data.<\/p>\n<\/div>\n
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How to iterate over tables in Lua Seth Kenlon Fri, 11\/04\/2022 – 03:00 In the Lua programming language, an array is called a table. A table is used in Lua to store data. If you’re storing a lot of data in a structured way, it’s useful to know your options for retrieving that data when […]<\/p>\n","protected":false},"author":1,"featured_media":68373,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[307],"tags":[],"class_list":["post-68372","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-open-source"],"_links":{"self":[{"href":"https:\/\/www.cryptocabaret.com\/index.php?rest_route=\/wp\/v2\/posts\/68372","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.cryptocabaret.com\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.cryptocabaret.com\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.cryptocabaret.com\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.cryptocabaret.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=68372"}],"version-history":[{"count":0,"href":"https:\/\/www.cryptocabaret.com\/index.php?rest_route=\/wp\/v2\/posts\/68372\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.cryptocabaret.com\/index.php?rest_route=\/wp\/v2\/media\/68373"}],"wp:attachment":[{"href":"https:\/\/www.cryptocabaret.com\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=68372"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.cryptocabaret.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=68372"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.cryptocabaret.com\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=68372"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}