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thwill

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Joined: 16/09/2019
Location: United Kingdom
Posts: 4301

Posted: 11:21pm 15 Dec 2022

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Grogster said I thought I was using the global D$ to pass the text to the string, when in efect, I guess the sub was defining a totally seperate 2nd D$ for itself during the call, then it vanished when the sub exited, and the global D$ was not touched at all.

Don't try this at home then:

OPTION DEFAULT NONE
OPTION EXPLICIT ON

DIM D$ = "HELLO"
FOO(D$)
PRINT D$
END

SUB FOO(D$)
� D$ = "WORLD"
END SUB

Should (I hope) print "WORLD", but why if the global and parameter D$ are "separate" ?

Best wishes,

Tom
Edited 2022-12-16 09:26 by thwill

MMBasic for Linux, Game*Mite, CMM2 Welcome Tape, Creaky old text adventures

Turbo46

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Joined: 24/12/2017
Location: Australia
Posts: 1638

Posted: 11:44pm 15 Dec 2022

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Quote A parameter is a bit like a LOCAL

A parameter is a LOT like a LOCAL.

As I have pointed out before:

SUB Swap a, b
LOCAL t
t = a
a = b
b = t
END SUB

SUB Xch a, b, t
t = a
a = b
b = t
END SUB

The two subs are interchangeable, the variables a, b and t are created when the SUB is run and disappear when it is finished. Of course in this example the type of variable is not specified.

Bill

Keep safe. Live long and prosper.

CaptainBoing

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Joined: 07/09/2016
Location: United Kingdom
Posts: 2170

Posted: 08:51am 16 Dec 2022

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thwill said

OPTION DEFAULT NONE
OPTION EXPLICIT ON

DIM D$ = "HELLO"
FOO(D$)
PRINT D$
END

SUB FOO(D$)
� D$ = "WORLD"
END SUB

Should (I hope) print "WORLD", but why if the global and parameter D$ are "separate" ?

down to priorities it seems. I feel a delve into the source coming on. <smugMode>I have to say I had never thought of or do, use global names as aliases in definition. probably more luck than judgement</smugMode>

you can of course defeat this by using the "ByVal" trick : replace FOO(D$) with FOO((D$)) in line 5

@Turbo46 using a global variable is a nice cheat to get more than one value from a Function(). In the example you give (with global t) I imagine it will execute faster because there is no house-keeping you get associated with LOCALs. Possible a nice speed tweak on low-power systems...

just tried that... a *very* healthy speed increase avoiding the LOCAL option:

> cpu 48
> list
Dim integer n,a,b,t

Timer=0
For n=1 To 10000
Swap
Next
Print Timer

Timer=0
For n=1 To 10000
Xch
Next
Print Timer

Sub Swap
Local t
t = a
a = b
b = t
End Sub

Sub Xch
t = a
a = b
b = t
End Sub
> run
3393
2532
>

~25% speed increase with defaulted floats and

> RUN
3186
2532
>

~20% with defaulted integers

that is not inconsiderable... of course this only pays when you are calling a Sub over and over... if there are "human time-frame" times between calls you won't see the 300uS involved each time.
Edited 2022-12-16 19:17 by CaptainBoing

JohnS
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Joined: 18/11/2011
Location: United Kingdom
Posts: 4034

Posted: 09:11am 16 Dec 2022

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thwill said

Don't try this at home then:

OPTION DEFAULT NONE
OPTION EXPLICIT ON

DIM D$ = "HELLO"
FOO(D$)
PRINT D$
END

SUB FOO(D$)
� D$ = "WORLD"
END SUB

Should (I hope) print "WORLD", but why if the global and parameter D$ are "separate" ?

Best wishes,

Tom

It's a sort of quirk.

MMBasic (unlike many, maybe most, languages) passes arguments by "reference" rather than by "value".

The "value" case is where a sort of hidden temporary holder is created and passed in, so if the SUB/FUNCTION alters it there's no change in the calling environment.

You can simulate that in MMBasic by passing in an expression, such as extra parentheses or a calculation (even if you just add 0 or an empty string "").

The "reference" situation means that enough is passed in by MMBasic that the SUB/FUNCTION can alter the argument i.e. changes are seen by the caller.

Other languages typically are value by default and need something extra to pass in by reference (e.g. in C you explicitly pass in a pointer and the function also has to state it is to receive a pointer).

The MMBasic case is somewhat dangerous and I suspect will catch some people out, but it is what it is.

John

thwill

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Joined: 16/09/2019
Location: United Kingdom
Posts: 4301

Posted: 10:33am 16 Dec 2022

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JohnS said The MMBasic case is somewhat dangerous and I suspect will catch some people out, but it is what it is.

It's a case of getting used to it. I find it very useful when I need to return multiple values from a "function" or get a CSUB to return a value otherwise it would need global variables - and then woe betide if you need recursive function call. You just need to be wary of manipulating "in" parameters just because it is convenient to do so.

Best wishes,

Tom

MMBasic for Linux, Game*Mite, CMM2 Welcome Tape, Creaky old text adventures

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