Thread Subject:

Is it Ok to shrink with [ ]?

Dear all,

I know that growing with indexing,

clear x
for i=1:10
    x(i)=1;
end,

is not recommended.

My question: does shrinking with indexing,

x=ones(1,10);
for i=1:10
    x(1)=[];
end,

cause memory problems? I.e. does Matlab reallocate memory for variable x every time x(1)=[]; is executed?

Thanks.

It would appear so:

n = 10000;
x=rand(1,n); % data
y=x; % copies
z=x;

tic
for ix=n:-1:500
    x(ix) = []; % deleting from the end
end
toc

tic
for ix=1:n-499
    y(1) = []; % deleting from the beginning
end
toc

tic
z(500:end) = []; % deleting all at once
toc

% displays:

Elapsed time is 0.517248 seconds.
Elapsed time is 0.516376 seconds.
Elapsed time is 0.001210 seconds.

"Andy " <myfakeemailaddress@gmail.com> wrote in message <i43df8$ba6$1@fred.mathworks.com>...
> It would appear so:
>
> n = 10000;
> x=rand(1,n); % data
> y=x; % copies
> z=x;
>
> tic
> for ix=n:-1:500
> x(ix) = []; % deleting from the end
> end
> toc
>
> tic
> for ix=1:n-499
> y(1) = []; % deleting from the beginning
> end
> toc
>
> tic
> z(500:end) = []; % deleting all at once
> toc
>
> % displays:
>
> Elapsed time is 0.517248 seconds.
> Elapsed time is 0.516376 seconds.
> Elapsed time is 0.001210 seconds.

You also have to be careful with a for loop since the total length may not be as long at the end (after removing spots) when it was when you initialized the loop.

> You also have to be careful with a for loop since the total length may not be as long at the end (after removing spots) when it was when you initialized the loop.

I don't follow you. The total length at the end will NECESSARILY be less than the length when you initialized the loop, since you're using the loop to remove elements from the array. Are you saying to be careful not to do the following?:

n = 1000;
x = rand(1,n);
for ix = 1:800
    x(ix) = []; % will seem to work for a while, then will break
end

"Andy " <myfakeemailaddress@gmail.com> wrote in message <i43hvm$rc0$1@fred.mathworks.com>...
> > You also have to be careful with a for loop since the total length may not be as long at the end (after removing spots) when it was when you initialized the loop.
>
> I don't follow you. The total length at the end will NECESSARILY be less than the length when you initialized the loop, since you're using the loop to remove elements from the array. Are you saying to be careful not to do the following?:
>
> n = 1000;
> x = rand(1,n);
> for ix = 1:800
> x(ix) = []; % will seem to work for a while, then will break
> end

I meant more like this:
x = rand(1,1000);
for ix = 1:length(x)
    if(mod(100,ix)==0)
        x(ix) = []; %This will cause an error at some point
    end
end

You would want a while loop
ix = 1;
while ix < length(x)
    %would be stable
    if(mod(100,ix)==0)
        x(ix) = []; %This will cause an error at some point
    else
        ix = ix+1;
    end
end

"Sean " <sean.dewolski@nospamplease.umit.maine.edu> wrote in message <i43itl$r5n$1@fred.mathworks.com>...

> You would want a while loop
> ix = 1;
> while ix < length(x)
> %would be stable
> if(mod(100,ix)==0)
> x(ix) = []; %This will cause an error at some point
> else
> ix = ix+1;
> end
> end

That was a bad example since it's using an unchanging ix for its criterion to delete. Thus it would delete the whole thing.
I meant something like this:
if(mod(100,x(ix))==0)

"Akim " <aaa@bbb.ccc> wrote in message <i43bor$n7h$1@fred.mathworks.com>...
> Dear all,
>
> I know that growing with indexing,
>
> clear x
> for i=1:10
> x(i)=1;
> end,
>
> is not recommended.
>
> My question: does shrinking with indexing,
>
> x=ones(1,10);
> for i=1:10
> x(1)=[];
> end,
>
> cause memory problems? I.e. does Matlab reallocate memory for variable x every time x(1)=[]; is executed?

Yes. Shrinking an array with [] inside a loop is exactly the same problem as growing an array inside a loop. In both cases MATLAB will be forced to copy the array at each iteration. There *is* a way to delete the *last* column of an array without a data copy involved using a mex routine, but that is likely beyond the scope of this thread.


James Tursa

Sean, I see your point. Since the while loop recalculates length(x) on each loop iteration, you won't accidentally index outside the bounds of x. I would argue, however, that both of these loops are quite dangerous in that they present a silent error: it seems the intent is to eliminate every 100th element of x. But of course this is not what either loop does.

It should be pointed out that shrinking an array in a loop did not give a warning in m-lint:

for ix = 1:100
    x(ix) = rand; % warning
end

for ix = 100:-1:1
    x(ix) = []; % no warning
end

Thanks all for your help.