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Lua Tables

Lua's Tables are its "collection" variables - in other words, they hold a whole series of other variables, which can be looked up by a key of some sort.

I can set up a table like this:
  occupancy = {2,5,5,3,2}
which will set up a Lua variable called "occupancy" with five members, numbered 1 to 5, with values 2, 5, 5, 3 and 2.

I can then reference the values by their position, using square brackets:
  print (occupancy[4])
will give me the result 3 since the fourth element - element number 4 - contains the value 3.

I can change values, for example:
  occupancy[4] = occupancy[4] + 2
and even extend the table by adding another element onto the end
  occupancy[6] = 4

I can use a variable to refer to a the position number in a table too - thus:
  print (occupancy[k])

will give me the result 5 as that's the value in the second position (position number 2) of the table.

By using a # sign in front of a table, I can refer to the number of elements in a table - thus I can write a generic piece of code to extend a table:
  occupancy[#occupancy + 1] = 2
and I can loop through every member of a table such as the one in the example I have used so far using a loop:
  roomnights = 0
  for k = 1,#occupancy do
    roomnights = roomnights + occupancy[k]
  print ("You have a total occupancy of "..roomnights.." room nights")

And I can set up an empty table like this:
  another = {}

In the example above, I've only stored numbers in my table (in Lua, all numbers are actually double precision floats, so there's no "integer v real" question to address). But I'm not limited to that - I can store anything that I can store into a regular variable into a table, including:
• a number
• a string of text
• a function (a callable piece of code)
• another table
and I can even store different types of data into different elements of the same table.

There's an example from the course notes - showing what I've talked about above - [here].

So far, I have used position numbers starting from 1 within my example tables - and so it's been rather like a list if you're familiar with Perl, Python or Tcl, like an array if you're familiar with C or PHP, or like a vector if you're familiar with C++ or Java.

But in Lua I can go further and use named elements in my table. That will mean I can compare with a hash in Perl or Java, an associative array in PHP, a dict in Python, an array or a dict in Tcl, etc. Let's see how that works:

Setting up a table with named members:
  roles = {chair = "Sion", treasurer = "Chris", secretary = "Phil"}
adding on more:
  roles["vicechair"] = "Peter"
  roles["press and publicity"] = "Graham"

I can then access individual members using the key name:
  print (roles["secretary"])
or indeed using a variable which contains the role name:
  contact = "press and publicity"
  print (roles[contact])

If I re-use a key as I assign a value, I'm replacing a member (cell) of my table. If I give a new key, then that will add a new member to my table:
  roles["supporter"] = "Dominic" -- adds new then ...
  roles["supporter"] = "Roger" -- replaces that value

When I had numbered elements I could easily loop through my table, but now that the elements are named, I can't simply write a loop in quite the same way as the names are unpredictable. However, I'm provided with a function in the Lua standard implementation called pairs which lets me loop through all the key / value pairs of a table so that I can perform an action on each of them in turn:
  for k,v in pairs(roles) do
    print (v .. " is in the role " .. k)


  Sion is in the role chair
  Graham is in the role press and publicity
  Phil is in the role secretary
  Roger is in the role supporter
  Peter is in the role vicechair
  Chris is in the role treasurer

You'll notice that the order is unpredicatable (and certainly not useful) and this is because a "hashing algorithm" is used, which makes it very fast to access individual elements, but impossible to sort them directly into any form or order.

I can delete individual members by setting them to nil, and I can delete an entire table by setting in to nil.

An alternative notation for table members allows me to replace the pair of square brackets for a named member with a dot - you'll see this commonly used, and forming an object-like approach in the process if you're familiar with object oriented programming. Thus
  print (roles["vicechair"])
  print (roles.vicechair)

use different syntax to mean exactly the same thing.

There's an example of a table with named members from our course material [here].

On yesterday's Lua Programming Course, this was the point we got to in tables. There's an extra colon notation to cover yet, and the whole big subject of metatables ... something for me to come back to later today - or you can find some examples already from the course notes and previous courses via [here].

The final example from yesterday is [here]. We put together string handling, file handling and tables to produce a useful piece of code which reads the records in a file, etracts some key / value pairs from selected columns, and stores all of the pairs which meet certain criteria. Once the table has been set up, our demonstration simply prints out the whole table, but we could go on and extend the application to do much more. In fact, I'm sure we'll start today by sorting the results - it's not directly possible, but indirectly it's very easy indeed when you know how ;-)
(written 2012-05-10, updated 2012-05-12)

Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
U105 - Lua - Tables and the table library.
  [4248] Metatables, Metamethods, classes and objects in Lua - (2014-03-18)
  [3694] Special __ methods you can use in Lua metatables - (2012-04-12)
  [2703] Lua Metatables - (2010-04-02)
  [2699] Lua tables - they are everything - (2010-03-30)
  [2499] ourdog is Greyhound, Staffie and Ginger Cake - (2009-11-09)
  [2346] The indexed and hashed parts of a Lua table - (2009-08-10)
  [1742] Lua - Table elements v table as a whole - (2008-08-07)
  [1697] Sorting in lua - specifying your own sort routine - (2008-07-05)

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