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Looping holding old copy of array

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  Looping holding old copy of array jonas.thornvall@gmail.com - 2013-06-23 22:50 -0700
    Re: Looping holding old copy of array "Evertjan." <exxjxw.hannivoort@inter.nl.net> - 2013-06-24 09:44 +0200
      Re: Looping holding old copy of array "Michael Haufe (TNO)" <tno@thenewobjective.com> - 2013-06-24 16:45 -0700
        Re: Looping holding old copy of array Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2013-06-25 03:06 +0200
          Re: Looping holding old copy of array Cezary Tomczyk <cezary.tomczyk@gmail.com> - 2013-06-25 21:58 +0200
            Re: Looping holding old copy of array Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2013-06-25 23:03 +0200
    Re: Looping holding old copy of array JJ <duh@nah.meh> - 2013-06-24 15:19 +0700
      Re: Looping holding old copy of array jonas.thornvall@gmail.com - 2013-06-24 03:38 -0700
        Re: Looping holding old copy of array JJ <duh@nah.meh> - 2013-06-25 10:52 +0700

#20408 — Looping holding old copy of array

I have a looping algorithm that must use least amount of work. 
I am working on an array and that use a counter for loop behaviour (loop in a loop). 

However when loops finished, i need a copy of the array containing not the last item but the item before the last.  

Basicly i have two while loops
while x>y
    while  z>t
    change Aarray

I want the content saved of the pre Aarray before loopend 


Here is my thought either i use       

while x>y
newArray = Aarray.slice();
    while  z>t
    change Aarray

or i use


while x>y
for(i=0;i<Aarray.length;i++){
 newCopy[i] = Aarray[i];
 }
    while  z>t
    change Aarray

I do not like to make a copy everytime, but is there away around it?
Which of the two suggested methods is faster?

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#20409

jonas.thornvall wrote on 24 jun 2013 in comp.lang.javascript:

> I have a looping algorithm that must use least amount of work. 
> I am working on an array and that use a counter for loop behaviour
> (loop in a loop). 
> 
> However when loops finished, i need a copy of the array containing not
> the last item but the item before the last.  
> 
> Basicly i have two while loops
> while x>y
>     while  z>t
>     change Aarray
> 
> I want the content saved of the pre Aarray before loopend 
> 
> 
> Here is my thought either i use       
> 
> while x>y
> newArray = Aarray.slice();
>     while  z>t
>     change Aarray
> 
> or i use
> 
> 
> while x>y
> for(i=0;i<Aarray.length;i++){
>  newCopy[i] = Aarray[i];
> }
>     while  z>t
>     change Aarray
> 
> I do not like to make a copy everytime, but is there away around it?
> Which of the two suggested methods is faster?

Could you tell what you what to accomplish,
in stead of giving crooked scripting examples 
of how you want to do something,
we [I] cannot understand.

And please, jonas.thornvall, use a signature.

-- 
Evertjan.
The Netherlands.
(Please change the x'es to dots in my emailaddress)

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#20423

On Monday, June 24, 2013 2:44:44 AM UTC-5, Evertjan. wrote:
> jonas.thornvall wrote on 24 jun 2013 in comp.lang.javascript:
> 
> 
> 
> > I have a looping algorithm that must use least amount of work. 
> 
> > I am working on an array and that use a counter for loop behaviour
> 
> > (loop in a loop). 
> 
> > 
> 
> > However when loops finished, i need a copy of the array containing not
> 
> > the last item but the item before the last.  
> 
> > 
> 
> > Basicly i have two while loops
> 
> > while x>y
> 
> >     while  z>t
> 
> >     change Aarray
> 
> > 
> 
> > I want the content saved of the pre Aarray before loopend 
> 
> > 
> 
> > 
> 
> > Here is my thought either i use       
> 
> > 
> 
> > while x>y
> 
> > newArray = Aarray.slice();
> 
> >     while  z>t
> 
> >     change Aarray
> 
> > 
> 
> > or i use
> 
> > 
> 
> > 
> 
> > while x>y
> 
> > for(i=0;i<Aarray.length;i++){
> 
> >  newCopy[i] = Aarray[i];
> 
> > }
> 
> >     while  z>t
> 
> >     change Aarray
> 
> > 
> 
> > I do not like to make a copy everytime, but is there away around it?
> 
> > Which of the two suggested methods is faster?

[...]

I can't see the original message for some reason...

But at any rate, I'd suggest using myArray.map(...)

<https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/map>

Why the concern over general speed? How big is the array?

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#20424

Michael Haufe (TNO) wrote:

> On Monday, June 24, 2013 2:44:44 AM UTC-5, Evertjan. wrote:
>> jonas.thornvall wrote on 24 jun 2013 in comp.lang.javascript:
>> 
>> 
>> 
>> > I have a looping algorithm that must use least amount of work.
>> 
>> > I am working on an array and that use a counter for loop behaviour
>> 
>> > (loop in a loop).
>> 
>> > 
>> 
>> > However when loops finished, i need a copy of the array containing not
>> 
>> > the last item but the item before the last.
>> 
>> > 
>> 
>> > Basicly i have two while loops
>> 
>> > while x>y
>> 
>> >     while  z>t
>> 
>> >     change Aarray
>> 
>> > 
>> 
>> > I want the content saved of the pre Aarray before loopend
>> 
>> > 
>> 
>> > 
>> 
>> > Here is my thought either i use
>> 
>> > 
>> 
>> > while x>y
>> 
>> > newArray = Aarray.slice();
>> 
>> >     while  z>t
>> 
>> >     change Aarray
>> 
>> > 
>> 
>> > or i use
>> 
>> > 
>> 
>> > 
>> 
>> > while x>y
>> 
>> > for(i=0;i<Aarray.length;i++){

Safer and more efficient:

  for (var i = 0, len = Aarray.length; i < len; ++i)
  {

Using identifiers starting with capital letters for variables that do not 
hold references to constructors is unwise.
 
>> >  newCopy[i] = Aarray[i];
>> 
>> > }
>> 
>> >     while  z>t
>> 
>> >     change Aarray
>> 
>> > 
>> 
>> > I do not like to make a copy everytime, but is there away around it?
>> 
>> > Which of the two suggested methods is faster?
> 
> [...]
> 
> I can't see the original message for some reason...
> 
> But at any rate, I'd suggest using myArray.map(...)
> 
> <https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/map>

Unwise here for several reasons.  Array.prototype.slice() is the most 
efficient method to make a shallow copy of an encapsulated array structure.  
It can also be used to make a shallow copy of only a part of the array 
structure, hence its name.

I find the original question to be too unclearly worded to suggest a correct 
alternative.  On top of that it is hardly readable thanks to lack of Pretty 
Printing, and to borken Google Groups; obviously it gets worse with any 
followup to it posted via Google Groups (full quote intended here to show 
that).
 
Do not use Google Groups for posting (unless you know how to work around its 
bugs, assuming that possibility still exists).

-- 
PointedEars

Twitter: @PointedEars2
Please do not Cc: me. / Bitte keine Kopien per E-Mail.

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#20438

On 2013-06-25 03:06, Thomas 'PointedEars' Lahn wrote:
[...]
> Safer and more efficient:
>
>    for (var i = 0, len = Aarray.length; i < len; ++i)
>    {

By the way of above. According to discussion here 
http://stackoverflow.com/questions/6261953/do-modern-javascript-jiters-need-array-length-caching-in-loops 
in your opinion property "length" still needs to be stored in variable 
before loop? Or this can be omitted because modern browsers caching (?) 
"length" property?

-- 
Cezary Tomczyk
http://www.ctomczyk.pl/

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#20441

Cezary Tomczyk wrote:

> On 2013-06-25 03:06, Thomas 'PointedEars' Lahn wrote:
> [...]
>> Safer and more efficient:
>>
>>    for (var i = 0, len = Aarray.length; i < len; ++i)
>>    {
> 
> By the way of above. According to discussion here
> http://stackoverflow.com/questions/6261953/do-modern-javascript-jiters-need-array-length-caching-in-loops
> in your opinion property "length" still needs to be stored in variable
> before loop? Or this can be omitted because modern browsers caching (?)
> "length" property?

One should never rely on what one cannot know.  You cannot know the runtime 
environments code once written will be exposed to.  There is no good reason 
to assume the value of the ”length” property should be automatically cached 
for a loop in the first place as it could change by statements in the loop.

One *can* know instead that avoiding to access a property repeatedly whose 
value does *not* change, in favor of a local variable that holds this value, 
reduces code complexity.  That has a chance – and it has been showed – to 
increase runtime efficiency, IOW to be faster.  So it is wise to do that.

-- 
PointedEars

Twitter: @PointedEars2
Please do not Cc: me. / Bitte keine Kopien per E-Mail.

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#20411

On Sun, 23 Jun 2013 22:50:49 -0700 (PDT), jonas.thornvall@gmail.com wrote:
> I have a looping algorithm that must use least amount of work. 
> I am working on an array and that use a counter for loop behaviour (loop in a loop). 
> 
> However when loops finished, i need a copy of the array containing not the last item but the item before the last.  
> 
> Basicly i have two while loops
> while x>y
>     while  z>t
>     change Aarray
> 
> I want the content saved of the pre Aarray before loopend 
> 
> Here is my thought either i use       
> 
> while x>y
> newArray = Aarray.slice();
>     while  z>t
>     change Aarray
> 
> or i use
> 
> while x>y
> for(i=0;i<Aarray.length;i++){
>  newCopy[i] = Aarray[i];
>  }
>     while  z>t
>     change Aarray
> 
> I do not like to make a copy everytime, but is there away around it?
> Which of the two suggested methods is faster?

Save the array only when needed.

Assuming the original code is like this:

while (x > y) {
  newArray = Aarray.slice();
  while (z > t) {
    change(Aarray);
    z++;
  }
  x++;
}

Change it to:

newArray = Aarray.slice();
while (x > y) {
  if ((x+1) > y) {
    newArray = Aarray.slice();
  }
  while (z > t) {
    change(Aarray);
    z++;
  }
  x++;
}

The first array save will make sure that it'll be saved if the execution
never enters the "outer" loop.

The second array save will be executed only at the last "outer" loop
iteration.

If you use break anywhere in the "outer" loop, then you'll need to add a
conditional array save for that too.

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#20415

Den måndagen den 24:e juni 2013 kl. 10:19:47 UTC+2 skrev JJ:
> On Sun, 23 Jun 2013 22:50:49 -0700 (PDT), jonas.thornvall@gmail.com wrote:
> 
> > I have a looping algorithm that must use least amount of work. 
> 
> > I am working on an array and that use a counter for loop behaviour (loop in a loop). 
> 
> > 
> 
> > However when loops finished, i need a copy of the array containing not the last item but the item before the last.  
> 
> > 
> 
> > Basicly i have two while loops
> 
> > while x>y
> 
> >     while  z>t
> 
> >     change Aarray
> 
> > 
> 
> > I want the content saved of the pre Aarray before loopend 
> 
> > 
> 
> > Here is my thought either i use       
> 
> > 
> 
> > while x>y
> 
> > newArray = Aarray.slice();
> 
> >     while  z>t
> 
> >     change Aarray
> 
> > 
> 
> > or i use
> 
> > 
> 
> > while x>y
> 
> > for(i=0;i<Aarray.length;i++){
> 
> >  newCopy[i] = Aarray[i];
> 
> >  }
> 
> >     while  z>t
> 
> >     change Aarray
> 
> > 
> 
> > I do not like to make a copy everytime, but is there away around it?
> 
> > Which of the two suggested methods is faster?
> 
> 
> 
> Save the array only when needed.
> 
> 
> 
> Assuming the original code is like this:
> 
> 
> 
> while (x > y) {
> 
>   newArray = Aarray.slice();
> 
>   while (z > t) {
> 
>     change(Aarray);
> 
>     z++;
> 
>   }
> 
>   x++;
> 
> }
> 
> 
> 
> Change it to:
> 
> 
> 
> newArray = Aarray.slice();
> 
> while (x > y) {
> 
>   if ((x+1) > y) {
> 
>     newArray = Aarray.slice();
> 
>   }
> 
>   while (z > t) {
> 
>     change(Aarray);
> 
>     z++;
> 
>   }
> 
>   x++;
> 
> }
> 
> 
> 
> The first array save will make sure that it'll be saved if the execution
> 
> never enters the "outer" loop.
> 
> 
> 
> The second array save will be executed only at the last "outer" loop
> 
> iteration.
> 
> 
> 
> If you use break anywhere in the "outer" loop, then you'll need to add a
> 
> conditional array save for that too.

Thank you for your answer, of course it must save computation time doing a check in the outer loop, rather then create the array every time.

I have another related question i just read there is break in javascript but does it work with while, using your idea could i do something like

newArray = Aarray.slice();

loop1:
while (x > y) {
  if ((x+1) > y) {
      break loop1; 
  }

  while (z > t) {
    change(Aarray);
    z++;
  }
  x++;
} 

The whole package will run many times in a global loop, so there will be many if checks depending on input size of array

Is there no other way then if clause to specify that the inner loop will break earlier last time it is run?

JT
Thanks again.

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#20425

On Mon, 24 Jun 2013 03:38:20 -0700 (PDT), jonas.thornvall@gmail.com wrote:
> I have another related question i just read there is break in javascript but does it work with while, using your idea could i do something like
> 
> newArray = Aarray.slice();
> 
> loop1:
> while (x > y) {
>   if ((x+1) > y) {
>       break loop1; 
>   }
> 
>   while (z > t) {
>     change(Aarray);
>     z++;
>   }
>   x++;
> } 

"break" works for any loops as well as for "switch". But your labelled
"break" usage there, will cause infinite loop when ((x+1) > y), because
"break" will simply go back to the start of outer loop without allowing x to
be incremented first.

The labelled "break" is best used in the nested loop.

newArray = Aarray.slice();

loop1:
while (x > y) {
  while (z > t) {
    if (allStop()) {
      newArray = Aarray.slice();
      x = y; //update x to fail the outer loop condition
      break loop1;
    }
    change(Aarray);
    z--;
  }
  x--;
} 

> Is there no other way then if clause to specify that the inner loop will break earlier last time it is run?

All loops only has one condition check. What you need is a different
condition, so you'll need to add one manually.

A loop will break only if you put a "break" in it. Otherwise, the loop will
simply ends once the loop condition fails.

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