A question for C/C++ coders

[pythagoras_156]

Hi,

I'm converting some C++ code to PB and, even though i have a little knowledge of C/C++, i have a doubt on the following lines :

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Stack[StackTop++]=PrevPixel;

Does it mean that the variable StackTop is actually incremented by 1 or does it only refer to the value StackTop + 1 ?
If the variable is really incremented, i would translate it like this in PB :

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StackTop+1 : Stack(StackTop) = PrevPixel 

if it's not incremented, like this :

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Stack(StackTop+1) = PrevPixel 

Is this correct ?

I also got the same problem with another line of code :

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RetVal = Stack[--StackTop]

Does the variable StackTop is decremented or is it only a reference to the value StackTop - 1 ?

If any C/C++ gurus could give me hint, i would be most grateful

Pythagoras

[gkaizer]

the

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Stack[StackTop++]=PrevPixel;

, means
take the current index of array/stack referred to by StackTop, assign the PrevPixel value and after that increment StackTop variable (increment the index)

AND

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RetVal = Stack[--StackTop]

decrements the StackTop variable by one and assigns a value contained in the Stack[StackTop with the new value] to RetVal

in PB i would translate that code like this

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Stack(StackTop)=PrevPixel
StackTop= StackTop+1

AND

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RetVal=Stack(StackTop-1)

[pythagoras_156]

Thanks for your help.

Pythagoras

[dontmailme]

Just to clarify in plain english......

If the ++ or -- is before the var i.e. ++var then it's incremented before

If it's after..... i.e. var++ then it's incremented after !

Quite simple once you know [/b]

[HASO]

Pythagoras,

There is an error in the -- example: RetVal=Stack(StackTop-1)

Correct code:

StackTop = StackTop - 1
RetVal = Stack(StackTop)

HTH, Hans.

[Dare2]

Sort of on topic question:

With the automatic adding, eg

  var+10 ; var now incremented by ten

This would never happen if var was part of an expression, would it?

  varX = varA + 17 * (varB - 3 )

Would leave varA and varB unchanged, always. Surely?

[HASO]

Dare2,

varX = varA + 17 * (varB - 3 )
Would leave varA and varB unchanged, always. Surely?

Yes, it would. But remember the sequence for operators: multiplication before addition!

In C the operator ++ means: increment by 1 and -- means: decrement by 1.

If you put this operator before a variable it means: FIRST change the variable, then use it.
After variable means: FIRST use variable, then change it.

This rule is used for the current expression only. When you start a new expression the variable has the changed value.

Hope this helps,
Hans.

[Dare2]

Thanks.