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Creating variables on the stack

4 replies on 1 page. Most recent reply: Aug 22, 2003 3:44 AM by Dennis Benzinger

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Ryan Slander

Posts: 3
Nickname: nirvana
Registered: Aug, 2003

Creating variables on the stack Posted: Aug 21, 2003 4:07 AM
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Hi,

Would there be a difference between the following 2 blocks of code at runtime

Block 1
-------

boolean check;
do
{
   check = true;
   ...
   ...
   ...
   if(check)
   {
      more = false;
   }
}
while(more);


Block 2
-------

do
{
   boolean check = true;
   ...
   ...
   ...
   if (check)
   {
      more = false;
   }
}
while(more);


I prefer Block 2 because it looks more readable.

regards
Ryan


David

Posts: 150
Nickname: archangel
Registered: Jul, 2003

Re: Creating variables on the stack Posted: Aug 21, 2003 6:10 AM
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Yeah, there is a difference, be it a small one. The second version will create a *new* boolean variable and put it on the stack frame on *each* iteration of the do...while loop. The first version only creates one.

Ryan Slander

Posts: 3
Nickname: nirvana
Registered: Aug, 2003

Re: Creating variables on the stack Posted: Aug 21, 2003 10:10 PM
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The reason I asked this question is because it seemed quite locgial that the compiler would perform some kind of optimization by not creating multiple variables on the stack?

David

Posts: 150
Nickname: archangel
Registered: Jul, 2003

Re: Creating variables on the stack Posted: Aug 22, 2003 1:53 AM
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Some compilers *might*, but:

1. Most of them won't.
2. You shouldn't rely on such optimisations.

I used to code very much in the style of (2), but I've now graduated to (1), factoring out the declarations to a higher level - it's more efficient and I think it's easier to read.

Dennis Benzinger

Posts: 1
Nickname: dcb
Registered: Aug, 2003

Re: Creating variables on the stack Posted: Aug 22, 2003 3:44 AM
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There's no difference at runtime.

The second block is better programming practice as the scope of the variable check is more limited.

For more evidence that there's no difference read the following discussion on Sun's Java Programming Forum:
http://forum.java.sun.com/thread.jsp?forum=31&thread=359397

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