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Topic: correct syntax for matlab..
Replies: 5   Last Post: May 14, 2013 10:11 AM

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dpb

Posts: 8,181
Registered: 6/7/07
Re: correct syntax for matlab..
Posted: May 14, 2013 10:11 AM
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On 5/14/2013 2:32 AM, virat rehani wrote:
> dpb <none@non.net> wrote in message <kmo69j$7nn$1@speranza.aioe.org>...
...

>> >> x=rand(5);
>> >> x(x>.6)=1;x(x<1)=0; % make up a set of data

...

>> Your solution...
>> >> k=1;for i=1:5,for j=1:5,if x(i,j)==1,e(k,:)=[i j];k=k+1;end,end,end
>>
>> The "Matlab way"...(or one of the ways, anyway)...

>> >> [e1(:,1) e1(:,2)]=find(x==1);
>> >> e1=sortrows(e1);

>>
>> >> all(all(e==e1))
>> ans =
>> 1

>> >>
>>
>> Shows one-for-one consonance of each element in e and e1...


...

> Dear sir I did it this way counter=1;
> eligible=[10,2];
> for i=1:M
> for j=1:M
> if (dis2(i,j)==1) row=i;
> col=j;
> eligible(counter,1)=row;
> eligible(counter,2)=col;
> counter=counter+1;
> end
> end
> end


I understand you did it that way...but the point is twofold--first the line

eligible=[10,2];

at the beginning serves absolutely no useful purpose. It does _NOT_
allocate an array of 10x2 and the values that are stored there are wiped
out if there even a single value found that satisfies the if()
condition. Also, if there weren't any found, then your function would
return the indication that there was a single value find at row 10,
column 2 which is simply wrong in that instance. Try your
implementation on an array

dis2=rand(10);

as input to it and see what it returns.

Also, even if it _did_ preallocate a 10x2 array, if there were more than
a total of one '1' per line in the dis2 array, there would be a bounds
error when the eleventh element was found.

What you really need instead at that point to help would be

eligible=zeros(M*M,2);

to preallocate the maximum size of the returned array. Of course, this
is bigger than needed for every case except the one of all elements of
dis2 == 1, but it's the only way to ensure not going outside the bounds
of the array if you preallocate a fixed size w/o adding the logic to
test and dynamically reallocate during the loop.

The problem w/ _not_ preallocating is that while for such a small M the
overhead won't be particularly noticeable, in general "growing" such
memory in a loop in Matlab will become a big overhead as problem sizes
grow and therefore it's best practice to preallocate when at all possible.

In this case you could preallocate to M*M rows then truncate the
trailing unused when the loop finishes.

But, the second point is that Matlab provides vectorized functional
forms and a major if not the whole point of there _being_ a Matlab is to
utilize that builtin functionality. It shortens up source code and as
problem sizes grow it will in general help performance immeasurably.

So, I still submit the "Matlab way" is

[e(:,1) e(:,2)]=find(x==1);

If all you need is the locations and order isn't important and then

e=sortrows(e);

if the order by row _is_ significant. Or, rearrange x first, of course.

I've shown the coding above the demonstrates the two results are the
same, identically.

--







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