Names of keys in tables

Defining a keyed table or "dictionary" is quite straightforward in lua and not really diffferent from other programming languages. It is for instance done like this:

functions         = {}

functions["j"]    = printLower

functions["J"] = printUpper

functions["C-j"]  = printControl

* printUpper, -Lower and -Control are names of functions

The same result can be achieved with this syntax.

functions  = {

  j  = printLower,

  J  = printUpper,

  C-j  = printControl

}

This alternative can come in quite handy when a table is part of a larger table/object structure, like this one.

menu  = {

  state  = nil,

  display  = {

  keys  = {"M"},

  functions  = {

  M  = displayMenu

  }

  },

  choice  = {

  keys  = {"j", "J", "C-j"},

  functions  = {

  j  = printLower,

  J  = printUpper,

  C-j  = printControl

  }

  }

}

Still there is a problem with this perticular example and it wil produce a syntax error. The reason is in the third key/value pair:

• C-j = printControl

where the "-" character is not interpreted as part of the key value but as a minus sign. All key values that are not syntactic valid Lua identifiers will result in this same problem.

But we can't be daunted by such a small problem and will simply put the offending key value in qoutes right? Wrong! A syntax like this:

functions  = {

  "j"  = printLower,

  "J"  = printUpper,

  "C-j"  = printControl

}

unfortunately is not allowed in lua and will yield an error. So in situations with problematic key values you will have to revert to a syntax like in the first example. That cóuld however get you into trouble with the way lua resolves field and/or function adresses; see also Function names in table fields.

I ran into problems with this phenomenon when I wanted to change the menu script in such a way that it would also correctly process [Control]+[Key] combinations. But the Celestia designers decided to specify [Control] combinations as a "C-x" string where the x represents the character pressed. A very unfortunate choice; the underscore character "_" would have been a better choice, since it is a valid part of a lua identifier.