topic Re: $ sign expansion in QlikView

$ sign expansion

Fri, 09 May 2014 14:51:23 GMT

Hi,

In wonder why when I use $sign expansion in the condition of a do while instruction it doesn't work :

The following code :

LET vCompteur = 1;

DO while $(vCompteur) <= 10

TRACE $(vCompteur);

LET vCompteur = $(vCompteur) + 1;

LOOP

results in an infinite loop

while the following code :

LET vCompteur = 1;

DO while vCompteur <= 10

TRACE $(vCompteur);

LET vCompteur = $(vCompteur) + 1;

LOOP

prints in the script execution console (as expected)

I have to say i'm kind of lost here, I tought $ sign expansion will return the value of vCompteur.

thanks

Re: $ sign expansion

alexandros17 — Fri, 09 May 2014 15:00:07 GMT

It works in this way because $ expansion computer again the expression and assign the value of 1 (as it was a constant) while the new variable value is stored in vCompteur

Re: $ sign expansion

Mon, 12 May 2014 08:17:44 GMT

Hi,

I just don't seem to understand.

At the first loop, $ expansion will compute the expression and will find that vCompteur is equal 1. But in the loop the value of the variable vCompteur will change to 2 and then when $ expansion will compute the expression normally it should find that vCompteur is equal 2.

Something doesn't make sense, or maybe I'm missing something.

Thanks

Re: $ sign expansion

vadimtsushko — Mon, 12 May 2014 11:08:44 GMT

Welcome to fun world of QlikView scripting

I believe not all assumptions from common stock programming languages are working here.

To illustrate my point I slightly modified your script. It is still perfectly valid, and give correct output, though syntax highlighter not happy with lines 6-7 now.

///$tab Main
LET vCompteur = 1;
LET whileVar = 'vCompteur';
LET doWhile = 'DO while';
LET whileCondition = '<= 10';
$(doWhile) $(whileVar) $(whileCondition)
    TRACE $(vCompteur);
    LET vCompteur = $(vCompteur) + 1;
    LET whileVar = 'ABRAKADABRA';
LOOP
LET vCompteur=;
LET whileVar=;
LET doWhile=;
LET whileCondition=;

Best way to look at inner working of qlikview script engine is to step through script debugger.

Lets step through up to 6 line.

Before debugger step into command line each occurence of $(something) in that line is textually replaced by its value in key/value map in right-bottom box (dictionary/map for variables).

If we step through futher we can see that debugger never return to line 6. Loop doing its job between lines 7-10.

So our intentional distortion of variable whileVar do not break script either.

Firstly I intuitively think about $(someThing) as of evaluation of variable someThing. It's actually purely a string substitution (regex replace) on a stage before any evaluation.

I hope it would clarify this vague behaviour.

Re: $ sign expansion

jonathandienst — Mon, 12 May 2014 11:26:13 GMT

Actually your second script is actually the correct way to code this loop. Used this way vCompteur behaves like a variable in a normal language, and so does the do while loop

To explain why the first one does not work - which is that the variable expansion happens before the script line is interpreted, this creates a compare between two number literals, not a variable and a number. The while condition is set up from this (once), hence an infinite loop.

HTH

Jonathan

Re: $ sign expansion

Mon, 12 May 2014 11:30:02 GMT

Thank you very much for taking the time to explain it.

It is indeed a very strange behavior quite unlike what I'm used to see in Java or PL/SQL.

Thank you again

Re: $ sign expansion

Mon, 12 May 2014 13:23:12 GMT

Yes,

This makes sense now, I guess I was a bit lost as I'm just discovering Qlikview these days.

Thank you.