How to deal with nil values in variables and concatenation them

Forum|Forum|2 years ago
February 3, 2024
2 Antworten
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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

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Beste Antwort von 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 "" ..

johnrellisAntwort

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).