On 5/14/2013 2:32 AM, virat rehani wrote: > dpb <firstname.lastname@example.org> wrote in message <email@example.com>... ...
>> >> 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
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
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
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
If all you need is the locations and order isn't important and then
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.