Code takes too long to run

[mmorga9]

I am fairly new to matlab, and I have written some code to process some image data. My code works fine, except one of the early steps takes an extremely long time to compute. I have a 3 column matrix with approximately 1.4 million rows and I am performing the following code on that matrix:

k=1;
SIZE0=size(Scan);
N=SIZE0(1,1);
for i=1:N
if (Scan(i,1)>=minx) && (Scan(i,1)<=maxx) && (Scan(i,2)>=miny) && (Scan(i,2)<=maxy)
Scanx(k,1)=Scan(i,1);
Scanx(k,2)=Scan(i,2);
Scanx(k,3)=Scan(i,3);
k=k+1;
end
end

I couldn't think of another way to write this for loop, but its too slow. Is there a faster way? Thank you for your help.

-Michael

 

[GregLocock]

Scanx(k,1)=Scan(i,1);
Scanx(k,2)=Scan(i,2);
Scanx(k,3)=Scan(i,3);

could be written more concisely, see the help intro. I doubt that will have much bearing on the speed of the thing though.

breaking the IF up into a cascade of 4 IFs MAY be faster, depending on how Matlab evaluates IF. I expect the Help explains that as well.



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[SomptingGuy]

Some other comments:

- It seems to me that i and k will always be the same.

- Scanx() is growing each time you add a new row. If you pre-allocate before you start the loop, you'll see a big speedup

Scanx=zeros(size(Scan);

% rest of code.

-----------

However, since you are using Matlab, why not use it in anger:

- Look up "logical indexing". You can use it to eliminate the if() test.

- Think about vectorizing this. If can operate on the entire Scan matrix in one go, it should speed things up.


- Steve

 

[asixth]

-Mike,

I would try to write a simply logic function, say:

function [output]=logic(x,y,minx,maxx,miny,maxy)
if(x>=minx) && (x<=maxx) && (y>=miny) && <=maxy)
output=1
else
output=0
end

Then you can run your m-file:

for i=1:size(Scan,1)
temp(i,1)=logic(Scan(i,1),Scan(i,2),minx,maxx,miny,maxy)
end

Scanx=Scan(find(temp),

You can use the following to determine how long your analysis is taking:

% At the first command line

tic;fprintf('Analysis time...');

% At the final command line

t=toc;fprintf('%5.2f sec\n',t);

 

[SomptingGuy]

I wasn't going to give the whole answer away, but since asixth has tried, here's my approach. It may not be exactly what you want, but it does demonstrate some Matlab fundamentals.

% Just in case
clear Scanx

% Locate which rows to copy
idx = Scan,1)>=minx & Scan,1)<=maxx & Scan,2)>=miny & Scan,2)<=maxy;

% Copy them
Scanx(idx,=Scan(idx,

- Steve

 

[SomptingGuy]

... the above takes about 0.5 seconds on my dog of a PC, using rand(14000000,3) for Scan.

- Steve