CODEWITHSUNDEEP
variablesinitializationrebolred-lang
rnso
rnso

Reputation: 24623

Initialization in Rebol/Red language

In example given on http://business-programming.com/business_programming.html#section-2.6 : 

REBOL []
items: copy []               ; WHY NOT JUST "items: []"
prices: copy []              ; WHY NOT JUST "prices: []"
append items "Screwdriver"
append prices "1.99"
append items "Hammer"
append prices "4.99"
append items "Wrench"
append prices "5.99"

Why should one do items: copy [] and not items: [] ? Also should this be done for all variable initializations or are there some selective types for which this is needed?

Edit: I find that following code works all right: 

REBOL []
items: []   
prices: []  
append items "Screwdriver"
append prices "1.99"
append items "Hammer"
append prices "4.99"
append items "Wrench"
append prices "5.99"

probe items
probe prices

items: []   
prices: []  
append items "Screwdriver"
append prices "1.99"
append items "Hammer"
append prices "4.99"
append items "Wrench"
append prices "5.99"

probe items
probe prices

Output is ok: 

["Screwdriver" "Hammer" "Wrench"]
["1.99" "4.99" "5.99"]
["Screwdriver" "Hammer" "Wrench"]
["1.99" "4.99" "5.99"]

But not following: 

REBOL []

myfn: func [][
    items: []   
    prices: []  
    append items "Screwdriver"
    append prices "1.99"
    append items "Hammer"
    append prices "4.99"
    append items "Wrench"
    append prices "5.99" ]

do myfn
probe items
probe prices

do myfn
probe items
probe prices

Output is duplicated here: 

["Screwdriver" "Hammer" "Wrench"]
["1.99" "4.99" "5.99"]
["Screwdriver" "Hammer" "Wrench" "Screwdriver" "Hammer" "Wrench"]
["1.99" "4.99" "5.99" "1.99" "4.99" "5.99"]

Is the problem only when initialization is in a function?

Apparently, all variables in a function are taken as global variables by default and created only once at start. It seems that the language is converting my function to: 

items: []   
prices: []  
myfn: func [][
    append items "Screwdriver"
    append prices "1.99"
    append items "Hammer"
    append prices "4.99"
    append items "Wrench"
    append prices "5.99" ]

Now the response of multiple calls to myfn is understandable. Global functions created in a loop are also created only once.

Upvotes: 1

Views: 122

Answers (1)

Graham Chiu
Graham Chiu

Reputation: 4886

The copy [] is not needed in this script because when it's run again, all prior references to the series items and prices will be created anew.

But if there's a possibility that the items: [] is going to run more than once inside the same script, then you need to copy to make sure you create a new series each time, and not just reference the existing series.

Upvotes: 2

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