Defaulting Values (on missing object Key_ Topic is solved

[RichardPlester]

Background:
I'm working on code to extract attributes from XML then store them in an assoc. array as key/value pairs for use elsewhere in my code.
The XML is written by others, so may not always contain attributes I expect, therefore I will end up with "missing" keys in my array object,
I'm ok with this as I will just use default values where the XML failed to specify them.

Questions:
1. Is there a preferred "pattern" for defaulting values when a key can't be found in an object? *see code below for my thinking
2. Why does AHK fail so silently in the case of missing keys? Is there anything that can be done about this?

*Is there a clean and readable way of doing the following or is this optimal?

Code: Select all

o := {carmen:"San Diego"}

;Extract some values from object, Default to "not found if key if missing from object
c := o.carmen ? o.carmen : "not found" 
w := o.wally ? o.wally : "not found"

msgbox % "Carmen:"c "`nWally:"w

I'm using AHK 1.1.30.03 (no particular reason) do I have better options in other versions?

[Hellbent]

Can't speak much on this, but you might want to do it this way over the way you have it.

Code: Select all

o := {carmen: 0 }
c := ( o.carmen ) ? ( o.carmen ) : ( "not found" )  ; Will miss the actual value 0

w := {wally: 0 }
d := ( w.wally != "" ) ? ( w.wally ) : ( "not found" ) ;Will catch the 0

msgbox, % "c: " c "`nd: " d

[RichardPlester]

Thanks HB, that's an improvement as treating 0 as a "missing" key/value would have been a bug.

[RichardPlester]

Further thinking has led me to the following as "" could be an acceptable value too (just like 0 is) - it doesn't mean the key is missing!..

This actually does what I intended, is it optimal though? (definite a DRY issue here at least)

Code: Select all

o := {carmen: 0 } ;..or o := {carmen: ""}
c := o.HasKey("carmen") ?  o.carmen : "not found"  ; Will catch the actual value 0 or "", but use "not found" if key is missing
c := o.HasKey("wally") ?  o.wally : "not found"  ; Will catch the actual value 0 or "", but use "not found" if key is missing

[Mod edit: [code][/code] tags added.]

[just me]

If you cannot use "" to determine a missing key, o.HasKey() is the only option.

[Chunjee]

I prefer to set a default object then use a merge to overwrite the keys I do have:

Code: Select all

A := new biga() ; requires https://www.npmjs.com/package/biga.ahk

defaultObj := {"carmen": "not found", "wally": "not found"}
XML := {"wally": ["123", "456"]}
XML := A.merge(defaultObj, XML)
; => {"carmen": "not found", "wally": [123, 456]}

You can also use https://biga-ahk.github.io/biga.ahk/#/?id=defaults but once a property is set; it won't be overwritten in that call, so kindof reverse order:

Code: Select all

A := new biga() ; requires https://www.npmjs.com/package/biga.ahk

defaultObj := {"carmen": "not found", "wally": "not found"}
XML := {"wally": ["123", "456"]}
XML := A.defaults(XML, defaultObj)
; => {"carmen": "not found", "wally": [123, 456]}

[RichardPlester]

I'll check that library out, I'm parsing SVG elements within the XML file so I can render the SVG using GDI+, the parsing feels hackish in places, hopefully it will become more uniform with biga - thanks.

[swagfag]

1. Is there a preferred "pattern" for defaulting values when a key can't be found in an object?

u could create ur own custom associative array class implementing meta-__Get() and meta-__Set() and make it return some default values. the problem is this is hard to implement fully/correctly in v1 due to how the object model discerns between keys and properties not

2. Why does AHK fail so silently in the case of missing keys? Is there anything that can be done about this?
...
I'm using AHK 1.1.30.03 (no particular reason) do I have better options in other versions?

1. has been resolved in v2, so has 2.
u can extend a built-in Map class (the evolution of the associative array) and override its getter/setter to include additional functionality(such as returning default values upon querying nonexistent keys)
the problem is if ure already using v1 specific libraries in ur project, there might not be a v2 equivalent(yet)