Thread Subject: Memory pre-allocation issue

Hi group:

I'm in the process of optimising code that someone else
wrote.

Can anyone tell me how to pre-allocate memory for a <1xd
cell> array.

So far I've failed miserably.

Cheers,

Alex

"Alex " <aroussel@firthrixson.com> wrote in message
news:f9a0ke$7ar$1@fred.mathworks.com...
> Hi group:
>
> I'm in the process of optimising code that someone else
> wrote.
>
> Can anyone tell me how to pre-allocate memory for a <1xd
> cell> array.
>
> So far I've failed miserably.
>
> Cheers,
>
> Alex

C = cell(1,d) will look like it, but there isn't anything really there.

Penny.

"Alex " <aroussel@firthrixson.com> wrote in message
<f9a0ke$7ar$1@fred.mathworks.com>...
> Hi group:
>
> I'm in the process of optimising code that someone else
> wrote.
>
> Can anyone tell me how to pre-allocate memory for a <1xd
> cell> array.
>
> So far I've failed miserably.
>
> Cheers,
>
> Alex

since a 'cell' can contain just about anything, how could
you preallocate memory for it? it makes sense to allocate
an array of numbers because you know how big they are and
how they are accessed. but a cell array could have
elements of different sizes with no way to know in advance.

"David " <dave@bigcompany.com> wrote in message <f9a1c2$mv8
$1@fred.mathworks.com>...
> "Alex " <aroussel@firthrixson.com> wrote in message
> <f9a0ke$7ar$1@fred.mathworks.com>...
> > Hi group:
> >
> > I'm in the process of optimising code that someone else
> > wrote.
> >
> > Can anyone tell me how to pre-allocate memory for a
<1xd
> > cell> array.
> >
> > So far I've failed miserably.
> >
> > Cheers,
> >
> > Alex
>
> since a 'cell' can contain just about anything, how could
> you preallocate memory for it? it makes sense to
allocate
> an array of numbers because you know how big they are and
> how they are accessed. but a cell array could have
> elements of different sizes with no way to know in
advance.

True, and I have to admit I hadn't thought about this.

So, taking the worst case scenario in my case, the <1xd
cell> array would contain <1xd double> vectors.
How can I preallocate that ?

NB: i'm onl;y familiar with 'a=zeros(1,d)'-type pre-
allocation routines.

Alex

Alex wrote:

> So, taking the worst case scenario in my case, the <1xd
> cell> array would contain <1xd double> vectors.
> How can I preallocate that ?
>
> NB: i'm onl;y familiar with 'a=zeros(1,d)'-type pre-
> allocation routines.

If I know in advance that all the elements of a cell array
are going to hold equally-sized arrays, I usually use deal
to preallocate, e.g.

ncell = 5;
nelement = 10;
newcell = cell(ncell,1);
[newcell{:}] = deal(zeros(1,nelement))


newcell =

    [1x10 double]
    [1x10 double]
    [1x10 double]
    [1x10 double]
    [1x10 double]

-Kelly

"Alex " <aroussel@firthrixson.com> schrieb im Newsbeitrag
news:f9a1t7$5ft$1@fred.mathworks.com...
> "David " <dave@bigcompany.com> wrote in message <f9a1c2$mv8
> $1@fred.mathworks.com>...
>> "Alex " <aroussel@firthrixson.com> wrote in message
>> <f9a0ke$7ar$1@fred.mathworks.com>...
>> > Hi group:
>> >
>> > I'm in the process of optimising code that someone else
>> > wrote.
>> >
>> > Can anyone tell me how to pre-allocate memory for a
> <1xd
>> > cell> array.
>> >
>> > So far I've failed miserably.
>> >
>> > Cheers,
>> >
>> > Alex
>>
>> since a 'cell' can contain just about anything, how could
>> you preallocate memory for it? it makes sense to
> allocate
>> an array of numbers because you know how big they are and
>> how they are accessed. but a cell array could have
>> elements of different sizes with no way to know in
> advance.
>
> True, and I have to admit I hadn't thought about this.
>
> So, taking the worst case scenario in my case, the <1xd
> cell> array would contain <1xd double> vectors.
> How can I preallocate that ?
>
> NB: i'm onl;y familiar with 'a=zeros(1,d)'-type pre-
> allocation routines.
>
> Alex

Hi Alex,

one way could be:

x = cell(1,d);
[x{:}] = deal(zeros(1,d))

but in this case: why not switch do a dxd matrix instead?

Titus

In article <f9a36q$4s8$1@fred.mathworks.com>,
titus.edelhofer@mathworks.de says...
>
> "Alex " <aroussel@firthrixson.com> schrieb im Newsbeitrag
> news:f9a1t7$5ft$1@fred.mathworks.com...
> > "David " <dave@bigcompany.com> wrote in message <f9a1c2$mv8
> > $1@fred.mathworks.com>...
> >> "Alex " <aroussel@firthrixson.com> wrote in message
> >> <f9a0ke$7ar$1@fred.mathworks.com>...
> >> > Hi group:
> >> >
> >> > I'm in the process of optimising code that someone else
> >> > wrote.
> >> >
> >> > Can anyone tell me how to pre-allocate memory for a
> > <1xd
> >> > cell> array.
> >> >
> >> > So far I've failed miserably.
> >> >
> >> > Cheers,
> >> >
> >> > Alex
> >>
> >> since a 'cell' can contain just about anything, how could
> >> you preallocate memory for it? it makes sense to
> > allocate
> >> an array of numbers because you know how big they are and
> >> how they are accessed. but a cell array could have
> >> elements of different sizes with no way to know in
> > advance.
> >
> > True, and I have to admit I hadn't thought about this.
> >
> > So, taking the worst case scenario in my case, the <1xd
> > cell> array would contain <1xd double> vectors.
> > How can I preallocate that ?
> >
> > NB: i'm onl;y familiar with 'a=zeros(1,d)'-type pre-
> > allocation routines.
> >
> > Alex
>
> Hi Alex,
>
> one way could be:
>
> x = cell(1,d);
> [x{:}] = deal(zeros(1,d))
>
> but in this case: why not switch do a dxd matrix instead?
>
> Titus
>
>
>

Actually that won't full initialize everything because MATLAB will be
smart and have all the zero arrays point to the same array, until they
each start getting filled with something different.

--
Loren
http://blogs.mathworks.com/loren/

Loren Shure <loren@mathworks.com> wrote in message
>
> Actually that won't full initialize everything because
MATLAB will be
> smart and have all the zero arrays point to the same
array, until they
> each start getting filled with something different.

This should work, right?

%Preallocate a cell array containing vectors of random lengths
numVectors = 10;
vectorLengths = round(9*rand(1,numVectors))+1;
ncell = arrayfun(@(x) (zeros(1,x)), vectorLengths, 'uni',
false);

Alex:
<SNIP initializing an array of ML constructs...

this has been discussed extensively here at

http://www.mathworks.com/matlabcentral/newsreader/view_threa
d/146789

us