Vectorize nested for-loop and if statement

Question

i'm trying to vectorize the following nested loop, so I don't have to plot the values in a loop:

for i=1:size(validMaskX,1)
   for j=1:size(validMaskX,2)
      if( validMaskX(i,j) )
         plot(ah, [dataX(i,j) dataX(i,j+1)], [dataY(i,j) dataY(i,j+1)], 'g-')               
      end
    end
 end

size(validMaskX) = 45x44
size(dataX)=size(dataY)=45x45

Any suggestions on how to do this?

Not tested, but does vind=find(validMaskX); vindn = vind + size(validMaskX, 1); plot(ah, [dataX(vind), dataX(vindn)], [dataY(vind), dataY(vindn)] ); work? That's assuming validMaskX and dataX and dataY have the same number of rows. — zeeMonkeez
– zeeMonkeez, Commented Dec 2, 2015 at 12:32
Do you want all the lines on the same axis? Because your code plots one line at a time, you'll need a loop for the time term. — Adiel
– Adiel, Commented Dec 2, 2015 at 13:01

zeeMonkeez · Accepted Answer · 2015-12-02 13:45:58Z

2

With

vind=find(validMaskX);
vindn = vind + size(validMaskX, 1);

you can find the valid points and the second indices. Then, you can plot with

plot(ah, [dataX(vind), dataX(vindn)], [dataY(vind), dataY(vindn)], 'g-');

If you want only one plot object (which would make rendering much faster), consider

dx = [dataX(vind), dataX(vindn), nan(numel(vind), 1)]';
dy = [dataY(vind), dataY(vindn), nan(numel(vind), 1)]';
plot(ah, dx(:), dy(:), 'g-');

answered Dec 2, 2015 at 13:45

zeeMonkeez

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8 Comments

K. Grouleff Over a year ago

This is EXACTLY what I needed! Thanks!

Adiel Over a year ago

At least you changed the variables names... :)

zeeMonkeez Over a year ago

Hey, I posted this as a comment way before you!

zeeMonkeez Over a year ago

But then again, this is pretty standard stuff :)

K. Grouleff Over a year ago

Hi zeeMokeez, I thought I understood your solution yesterday, but apparently I don't, because when I try to modify the following code, which is more or less the same, as the question I posted yesterday, it dosen't work:

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Adiel · Accepted Answer · 2015-12-02 13:17:21Z

1

If you want all the lines together on the figure, you can do that:

ind=find(validMask);
X=[dataX(ind) dataX(ind+45) nan(length(ind),1)];
Y=[dataY(ind) dataY(ind+45) nan(length(ind),1)];
plot(ah,X',Y','g-')

edited Dec 2, 2015 at 13:17

answered Dec 2, 2015 at 13:08

Adiel

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Comments

K. Grouleff · Accepted Answer · 2015-12-03 08:24:38Z

0

I thought I understood your solution yesterday, but apparently I don't, because when I try to modify the following code according to your answer, it dosen't work: How would you modify this according to your former answer?

for i=1:size(validMaskY,1)
   for j=1:size(validMaskY,2)
      if( validMaskY(i,j) )
      plot(ah, [dataX(i,j) dataX(i+1,j)], [dataY(i,j) dataY(i+1,j)], 'r-')
      end
   end
end

size(dataX)=size(dataY)

answered Dec 3, 2015 at 8:24

K. Grouleff

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8 Comments

Adiel Over a year ago

Try to change this line: vindn = vind + 1;

Adiel Over a year ago

A. size(validMaskY,1) is 45, so when i=45 you can not get dataX(i+1,j), it will exceed matrix dimensions. B. It will be much helpful if you can explain what exactly the problem. We can't guess what "your problem" is, and why it didn't solve.

K. Grouleff Over a year ago

I'll try to explain a bit more: When I try to vectorize the double for loop, I don't get the same figure, as when I run the double for-loop (which is the correct one!). The first question I posted, draw's connects lines between dots in one direction, and the latest post from me draw lines perpendicular to the other lines. The result is a grid where all dots that satisfy validMaskX, validMaskY is connected. I can draw the lines in one direction, but not in the other direction.

K. Grouleff Over a year ago

I'm looping through a lot of images, so the size of dataX and dataY will vary, but will always equal each other. Size(validMaskX)=[m,n] where size(validMaskY)=[n,m], hope this helps

zeeMonkeez Over a year ago

The original solution hinged on the fact that matrices are indexed in column-major order. @Adiel's comment regarding vindn = vind + 1; should work. Are the differences in your sizes for validMaskX and validMaskY correct?

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