Lua/Corona "OR" operator

Solar2D Lua Language
#1

I am working on an app the returns a json table from a network request. When the user enters a 3 letter airport code and it gets returned in a newText object. If the type in the airport LAX the stateCode from the json file gets returned as CA but if the type in an airport code from a different country its returns an error because what has been returned doesnt have a stateCode. So i tried to use the “or” operator in the following code.

Local display.newText(myData.airports[1].stateCode or myData.airports[1].countryCode)

How can I return the stateCode and if it doenst have a stateCode, return the countryCode?

#2

Are you saying the example line of code you showed did not work? 

If so, just break it apart:

local myData = myData.airports[1].stateCode or myData.airports[1].countryCode local display.newText(myData)

Give that a try and see what happens.

 Jay

#3

It may depend on what ‘doesn’t have a state code’ means. What you actually are testing for is this stateCode being ‘nil’ - not (say) an empty string. I would put 

print(myData.airports,myData.airports[1],myData.airports[1].stateCode,myData.airports[1],countryCode) 

to see exactly what is coming out. It may well be an empty string.

#4

@MohawkMan thanks. That worked perfectly. @paulscottrobson oh. Ok so I checked the data and it does return a string. I do want it to return nil or ‘empty’. Should my code read: local stateCodeText = myData.airports[1].stateCode or “” local myText = display.newText(stateCodeText, 0, 0, font, 16) ??

#5

TBH that’s probably over using or. I’d be inclined to write :

local text = myData.airports[1].stateCode if text == nil or text == "" then text = myData.airports[1].whateverItwas or "" end local myText = display.newText(.....)

Things like that can introduce subtle bugs where this sort of answer is cleaner and more reliable. I tend to use or only for default parameter values (excepting logical or obviously :wink: )

#6

One-liner:

[lua]

display.newText((myData.airports[1].stateCode ~= “” and myData.airports[1].stateCode) or myData.airports[1].countryCode)

[/lua]

The Lua “(a and b) or c” is in most cases equivalent to most other languages’ “a ? b : c” operator.

  • C
#7

Indeed. Though as with the a?b:c operator the minimal speed gains in code (if any) and code size probably don’t make its regular use worth the risk.

#8

Are you saying the example line of code you showed did not work? 

If so, just break it apart:

local myData = myData.airports[1].stateCode or myData.airports[1].countryCode local display.newText(myData)

Give that a try and see what happens.

 Jay

#9

It may depend on what ‘doesn’t have a state code’ means. What you actually are testing for is this stateCode being ‘nil’ - not (say) an empty string. I would put 

print(myData.airports,myData.airports[1],myData.airports[1].stateCode,myData.airports[1],countryCode) 

to see exactly what is coming out. It may well be an empty string.

#10

@MohawkMan thanks. That worked perfectly. @paulscottrobson oh. Ok so I checked the data and it does return a string. I do want it to return nil or ‘empty’. Should my code read: local stateCodeText = myData.airports[1].stateCode or “” local myText = display.newText(stateCodeText, 0, 0, font, 16) ??

#11

TBH that’s probably over using or. I’d be inclined to write :

local text = myData.airports[1].stateCode if text == nil or text == "" then text = myData.airports[1].whateverItwas or "" end local myText = display.newText(.....)

Things like that can introduce subtle bugs where this sort of answer is cleaner and more reliable. I tend to use or only for default parameter values (excepting logical or obviously :wink: )

#12

One-liner:

[lua]

display.newText((myData.airports[1].stateCode ~= “” and myData.airports[1].stateCode) or myData.airports[1].countryCode)

[/lua]

The Lua “(a and b) or c” is in most cases equivalent to most other languages’ “a ? b : c” operator.

  • C
#13

Indeed. Though as with the a?b:c operator the minimal speed gains in code (if any) and code size probably don’t make its regular use worth the risk.