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[#]: subject: "How to iterate over tables in Lua"
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[#]: via: "https://opensource.com/article/22/11/iterate-over-tables-lua"
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[#]: author: "Seth Kenlon https://opensource.com/users/seth"
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[#]: collector: "lkxed"
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[#]: translator: " "
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[#]: reviewer: " "
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[#]: publisher: " "
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[#]: url: " "
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How to iterate over tables in Lua
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======
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Create structure that makes it easier to find stored data.
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In the [Lua][1] 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.
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### Creating a table in Lua
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To create a table in Lua, you instantiate the table with an arbitrary name:
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```
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mytable = {}
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```
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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 in some languages):
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```
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mytable = {'zombie','apocalypse'}
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```
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Or you could create an associated array (called a map or dictionary 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:
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```
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myarray = {}
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myarray.baz = 'happy'
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myarray.qux = 'halloween'
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```
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You can add verification with the `assert()` function:
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```
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[assert][2](myarray.baz == 'happy', 'unexpected value in myarray.baz')
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[assert][2](myarray.qux == 'halloween', 'unexpected value in myarray.qux')
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```
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You now have two tables: a list-style `mytable` and an associative array-style `myarray`.
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### Iterating over a table with pairs
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Lua's `pairs()` function extracts key and value pairs from a table.
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```
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print('pairs of myarray:')
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for k, v in pairs(myarray) do
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print(k, v)
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end
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```
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Here's the output:
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```
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pairs of myarray:
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baz happy
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qux halloween
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```
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If there are no keys in a table, Lua uses an index. For instance, the `mytable` table contains the values `zombie` and `apocalypse`. It contains no keys, but Lua can improvise:
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```
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print('pairs of mytable:')
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for k, v in pairs(mytable) do
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print(k, v)
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end
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```
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Here's the output:
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```
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1 zombie
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2 apocalypse
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```
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### Iterating over a table with ipairs
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To account for the fact that tables without keys are common, Lua also provides the `ipairs` function. This function extracts the index and the value:
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```
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print('ipairs of mytable:')
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for i, v in ipairs(mytable) do
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print(i, v)
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end
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```
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The output is, in this case, the same as the output of `pairs`:
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```
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1 zombie
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2 apocalypse
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```
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However, watch what happens when you add a key and value pair to `mytable`:
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```
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mytable.surprise = 'this value has a key'
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print('ipairs of mytable:')
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for i, v in ipairs(mytable) do
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print(i, v)
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end
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```
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Lua ignores the key and value because `ipairs` retrieves only indexed entries:
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```
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1 zombie
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2 apocalypse
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```
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The key and value pair, however, have been stored in the table:
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```
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print('pairs of mytable:')
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for k, v in ipairs(mytable) do
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print(k, v)
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end
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```
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The output:
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```
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1 zombie
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2 apocalypse
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surprise this value has a key
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```
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### Retrieving arbitrary values
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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:
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```
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print('call by index:')
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print(mytable[2])
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print(mytable[1])
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print(myarray[2])
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print(myarray[1])
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print('call by key:')
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print(myarray['qux'])
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print(myarray['baz'])
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print(mytable['surprise'])
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```
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The output:
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```
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call by index:
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apocalypse
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zombie
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nil
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nil
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call by key:
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halloween
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happy
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this value has a key
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```
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### Data structures
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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.
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--------------------------------------------------------------------------------
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via: https://opensource.com/article/22/11/iterate-over-tables-lua
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作者：[Seth Kenlon][a]
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选题：[lkxed][b]
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译者：[译者ID](https://github.com/译者ID)
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校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
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[a]: https://opensource.com/users/seth
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[b]: https://github.com/lkxed
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[1]: https://opensource.com/article/22/11/lua-worth-learning
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[2]: http://www.opengroup.org/onlinepubs/009695399/functions/assert.html
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