Outputting Table Contents

In Lua, you will commonly need to store items or references in a table, and sometimes it's necessary to know which items/members exist inside that table. This is simple enough to do with a key-value loop using the Lua pairs() function:

local myTable = {
    firstName = "Fred",
    lastName = "Bob",
    phoneNumber = "(555) 555-1212",
    age = 30
}

for k,v in pairs(myTable) do
    print( k,v )
end

This will output something like the following to the console:

firstName   Fred
age 30
lastName    Bob
phoneNumber (555) 555-1212

Note that the order may differ since the pairs() function does not necessarily iterate through the table in the same order in which the key-values were declared. However, if you only need to check for a key's existence and its value, this loop method is perfectly acceptable.

What About Sub-Tables?

What if your table has sub-tables as in the following example?

local myTable = {
    firstName = "Fred",
    lastName = "Bob",
    phoneNumber = "(555) 555-1212",
    age = 30,
    favoriteSports = { "Baseball", "Hockey", "Soccer" },
    favoriteTeams  = { "Cowboys", "Panthers", "Lightning" }
}

In this case, the pairs() loop method will output something like this:

firstName   Fred
favoriteSports  table: 0x7fb1bbd00750
phoneNumber (555) 555-1212
favoriteTeams   table: 0x7fb1bbd00340
lastName    Bob
age 30

As you can see, the loop will display the internal references to those tables, for example 0x7fb1bbd00750, but it won't display the actual content (keys or values) of the sub-tables.

Convenience Function

Fortunately, we can use a powerful function that will intelligently step through tables and sub-tables, revealing the contents for your reference. Here is the function in its entirety:

local function printTable( t )

    local printTable_cache = {}

    local function sub_printTable( t, indent )

        if ( printTable_cache[tostring(t)] ) then
            print( indent .. "*" .. tostring(t) )
        else
            printTable_cache[tostring(t)] = true
            if ( type( t ) == "table" ) then
                for pos,val in pairs( t ) do
                    if ( type(val) == "table" ) then
                        print( indent .. "[" .. pos .. "] => " .. tostring( t ).. " {" )
                        sub_printTable( val, indent .. string.rep( " ", string.len(pos)+8 ) )
                        print( indent .. string.rep( " ", string.len(pos)+6 ) .. "}" )
                    elseif ( type(val) == "string" ) then
                        print( indent .. "[" .. pos .. '] => "' .. val .. '"' )
                    else
                        print( indent .. "[" .. pos .. "] => " .. tostring(val) )
                    end
                end
            else
                print( indent..tostring(t) )
            end
        end
    end

    if ( type(t) == "table" ) then
        print( tostring(t) .. " {" )
        sub_printTable( t, "  " )
        print( "}" )
    else
        sub_printTable( t, "  " )
    end
end

This function will navigate through the tables/sub-tables and output the contents. To use it, simply call the function and pass in the table reference you want to output, for example:

printTable( myTable )

In this case, the output becomes:

table: 0x600001260c00 {
  [firstName] => "Fred"
  [favoriteSports] => table: 0x600001260c00 {
                        [1] => "Baseball"
                        [2] => "Hockey"
                        [3] => "Soccer"
                      }
  [phoneNumber] => "(555) 555-1212"
  [favoriteTeams] => table: 0x600001260c00 {
                       [1] => "Cowboys"
                       [2] => "Panthers"
                       [3] => "Lightning"
                     }
  [lastName] => "Bob"
  [age] => 30
}

As you can see, this function makes the table contents significantly easier to read! Also, when applicable, it shows the index value of non-key items within a table, for instance [2] => "Hockey", which can be extremely useful when accessing table contents in associated code within your app.

Alternative Reference

Since this function is for outputting table contents, it may not be intuitive to use it directly as in:

printTable( myTable )

Thus, an alternative is to add the printTable() function to the Lua table library object as so:

table.print = printTable

Then, calling it via the reference becomes more sensible:

table.print( myTable )

Conclusion

Using tables to store items and references is focal to any serious development in Lua and Corona, and knowing the content of those tables is often essential to debugging. Hopefully this convenience function will help you throughout the course of your development!