Merge pull request #27831 from lkxed/20221104-7-How-to-iterate-over-tables-in-Lua

[手动选题][tech]: 20221104.7 ⭐️ How to iterate over tables in Lua.md

sources/tech/20221104.7 ⭐️ How to iterate over tables in Lua.md

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+[#]: subject: "How to iterate over tables in Lua"
+[#]: via: "https://opensource.com/article/22/11/iterate-over-tables-lua"
+[#]: author: "Seth Kenlon https://opensource.com/users/seth"
+[#]: collector: "lkxed"
+[#]: translator: " "
+[#]: reviewer: " "
+[#]: publisher: " "
+[#]: url: " "
+
+How to iterate over tables in Lua
+======
+
+Create structure that makes it easier to find stored data.
+
+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.
+
+### Creating a table in Lua
+
+To create a table in Lua, you instantiate the table with an arbitrary name:
+
+```
+mytable = {}
+```
+
+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):
+
+```
+mytable = {'zombie','apocalypse'}
+```
+
+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:
+
+```
+myarray = {}
+myarray.baz = 'happy'
+myarray.qux = 'halloween'
+```
+
+You can add verification with the `assert()` function:
+
+```
+[assert][2](myarray.baz == 'happy', 'unexpected value in myarray.baz')
+[assert][2](myarray.qux == 'halloween', 'unexpected value in myarray.qux')
+```
+
+You now have two tables: a list-style `mytable` and an associative array-style `myarray`.
+
+### Iterating over a table with pairs
+
+Lua's `pairs()` function extracts key and value pairs from a table.
+
+```
+print('pairs of myarray:')
+for k, v in pairs(myarray) do
+  print(k, v)
+end
+```
+
+Here's the output:
+
+```
+pairs of myarray:
+baz     happy
+qux     halloween
+```
+
+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:
+
+```
+print('pairs of mytable:')
+for k, v in pairs(mytable) do
+  print(k, v)
+end
+```
+
+Here's the output:
+
+```
+1   zombie
+2   apocalypse
+```
+
+### Iterating over a table with ipairs
+
+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:
+
+```
+print('ipairs of mytable:')
+for i, v in ipairs(mytable) do
+  print(i, v)
+end
+```
+
+The output is, in this case, the same as the output of `pairs`:
+
+```
+1   zombie
+2   apocalypse
+```
+
+However, watch what happens when you add a key and value pair to `mytable`:
+
+```
+mytable.surprise = 'this value has a key'
+print('ipairs of mytable:')
+for i, v in ipairs(mytable) do
+  print(i, v)
+end
+```
+
+Lua ignores the key and value because `ipairs` retrieves only indexed entries:
+
+```
+1   zombie
+2   apocalypse
+```
+
+The key and value pair, however, have been stored in the table:
+
+```
+print('pairs of mytable:')
+for k, v in ipairs(mytable) do
+  print(k, v)
+end
+```
+
+The output:
+
+```
+1          zombie
+2          apocalypse
+surprise   this value has a key
+```
+
+### Retrieving arbitrary values
+
+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:
+
+```
+print('call by index:')
+print(mytable[2])
+print(mytable[1])
+print(myarray[2])
+print(myarray[1])
+print('call by key:')
+print(myarray['qux'])
+print(myarray['baz'])
+print(mytable['surprise'])
+```
+
+The output:
+
+```
+call by index:
+apocalypse
+zombie
+nil
+nil
+
+call by key:
+halloween
+happy
+this value has a key
+```
+
+### Data structures
+
+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.
+
+--------------------------------------------------------------------------------
+
+via: https://opensource.com/article/22/11/iterate-over-tables-lua
+
+作者：[Seth Kenlon][a]
+选题：[lkxed][b]
+译者：[译者ID](https://github.com/译者ID)
+校对：[校对者ID](https://github.com/校对者ID)
+
+本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
+
+[a]: https://opensource.com/users/seth
+[b]: https://github.com/lkxed
+[1]: https://opensource.com/article/22/11/lua-worth-learning
+[2]: http://www.opengroup.org/onlinepubs/009695399/functions/assert.html