How to deal with nil values in variables and concatenation them

Forum|Forum|1 year ago
February 3, 2024
2 replies
453 views

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.

SDK

This topic has been closed for replies.

Correct answer johnrellis

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).

K

kimaldis

Inspiring

.. StrOption1 or "" ..

johnrellisCorrect answer

Legend

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).