How to deal with nil values in variables and concatenation them

[patrick_marq]

I have some checkboxes that represents options for executing an external program.

some pseudo code:

if box == checked

 strOption1 = aa

if box 2 == checked

  strOption2 == bb

strCommand = "software " .. strOption1 .. " " ..  strOption2

------

But if the strOption(s) are empty (nil) I get an error.
I could the write some if else but that would make it more complicated.

[Sean McCormack]

Would initialising the strings prevent this? Obviously you'd need an option for that intialising, but it could be the default choice. 

[kimaldis]

.. StrOption1 or "" ..

[John R Ellis · Accepted solution]

Building on kimaldis' reply, in your example you'll need parentheses around the "or" expression:

strCommand = "software " .. (strOption1 or "") .. " " ..  strOption2

This will evaluate to "" if "strOption1" is nil or false, to "strOption1" otherwise.

More generally, the common way to approximate a conditional expression in Lua (which lacks them) is to write:

cond and x or y

which usually is "y" if "cond" is nil or false,  "x" otherwise. But you need to be very careful: if "x" is nil or false, then the value of the expression is always "y" (a different result than conditional expressions in other languages).