Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!cwjcc!gatech!gitpyr!gus
From: gus@pyr.gatech.EDU (gus Baird)
Newsgroups: comp.lang.pascal
Subject: funny evaluation of functions
Message-ID: <9033@pyr.gatech.EDU>
Date: 23 Aug 89 18:46:08 GMT
Distribution: usa
Organization: Georgia Institute of Technology
Lines: 48

>Article 2362 of comp.lang.pascal:
>>From: edkay@lehi3b15.csee.Lehigh.EDU (Ed Kay)
>Newsgroups: comp.lang.pascal
>Subject: Funny evaluation of functions
>Date: 23 Aug 89 15:50:10 GMT
>Organization: CSEE Dept. Lehigh University, Bethlehem, PA
>
>   The following code produces 25 as output in Turbo Pascal 4.0.  Can
>anyone explain what is going on?
> 
>  var y:integer;
>
>  Function one(var x:integer):integer;
>  begin
>   x:=x+1;
>   one:=x;
>  end;
> begin
>  y:=4;
>  writeln(output,y*one(y));
> end.
                                                                   
                                                                   
                                                                   
                                                                   

Huh?  Why shouldn't it give 25?  It looks to me like you TOLD
it to...
                                                                   
In "one", x is a variable parameter.  starts at (4).  
gets incremented to (5) as the first op in "one".  
Thenceforth y of main has value (5).
"one" returns the current value of (x of "one") == (y of main) = 5.
                                                                   
Functions get evaluated first, in expression precedence rules.
you get (5) back from "one(y)", multiply by (current value of y) = 5,
giving 25.
                                                                   
BTW, standard practice in Pascal shops is to not use variable
parameters in functions.  Usual pejorative phrase is "side effect".
                                                                   
                                                                   
                                                                   

-- 
gus Baird
School of ICS, Georgia Institute of Technology, Atlanta, Georgia, 30332
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