Interpolating many matrices without for loop

2011 11 17
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Hi everybody, well I want to interpolate a 3D matrix composed of 700x2x60 values. I want to interpolate with respect to an array of 700 values. But i want an interpolation only 1 dimension, for example
if i had:
3 1
6 2
9 3
60 times, with different values, i want to interpolate with respect to 1.5, 2.5 and 3 I would get 4.5 7.5 and 10.5, 60 times and with the values relative to each one of the 60 matrices.
So in essence its like interp1 but 60 times. And I can't manage to do it without a for loop which is time consuming. and i need to do it over 15000 times.
thank you for your answers

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Fangjun Jiang
Fangjun Jiang 2011-11-17
Without proper {}Code format, it's actually very hard to understand your question.
j_solar
j_solar 2011-11-17
the format has been changed a bit. The 3 1 6 2 9 3 is a 3x2 matrix where 3 1 6 is the first column and 2 9 3 the second one.
I have also tried to repmat the array 60 times and use interp2, however it doesn't seem to work right, or I am doing wrong. But I don't think that interp2 is what i need.
thanks again
j_solar
j_solar 2011-11-17
how can I put the question in proper format?
Fangjun Jiang
Fangjun Jiang 2011-11-17
You can edit your question and apply {}Code format to make it look right. I did only the {}Code formatting part but didn't change any of your text.
Fangjun Jiang
Fangjun Jiang 2011-11-17
Look at all the buttons above your question text box when you post or edit your question.

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Fangjun Jiang
Fangjun Jiang 2011-11-17

1 个投票

You can use reshape().
x=1:3;
y=3:3:9;
X=magic(3);
[M,N]=size(X);
Y=interp1(x,y,X(:),'linear','extrap');
Y=reshape(Y,M,N);

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j_solar
j_solar 2011-11-17
Ty for the format info, Fangjun.
I'm not sure if my question is clear. Right now I have this:
initial = matrix(700x2x60)
I separate into
a = matrix(700x60);
b = matrix(700x60);
and build an array with values similar to b but not exactly
c = array(700x1);
c = repmat(c,700,1,60)
final = zeros(700x2x60)
final(:,2,:) = c;
for i = 1:60
final(:,1,i) = interp1(a(:,i),b(:,i),c(i));
end
and this works, however I don't like the for loop.
Maybe your answer is right and if so I don't seem to understand
ty
Fangjun Jiang
Fangjun Jiang 2011-11-17
I understand your algorithm now. I don't see an obvious way to avoid the for-loop. arrayfun() might work but in my opinion, that just hides the for-loop.
Why so obsessed with no loops? Numerous examples have shown that no loop doesn't necessarily mean faster.
j_solar
j_solar 2011-11-17
I am building a very large simulation program which needs to do many iterations > 15000, (i can't avoid that for loop), with a lot of processing in each iteration. I had bad software(many for loops) and each iteration was taking many seconds(even a minute) which means a veeeery long time. I have managed to reduce time of each iteration to a few seconds, aprox, and i would like to bring it down even more, so I am trying to get rid of all for loops.
But it is true that I have taken some away which have barely reduced my time, and probably it is not worth it eliminating all, but any I can, I try.
thanks once again for everything......... see u in another for loop
Jan
Jan 2011-11-17
Matlab's INTERP1 is lame and there is no reason to limit the algorithm to vectors. But a linear interpolation is not complicated - roughly:
Si = Ti - floor(Ti);
Ti = floor(Ti);
Yi = Y(Ti, :) .* (1 - Si) + Y(Ti + 1, :) .* Si;
I do not understand the value of c. It is a [700x1x60] array any you use c(i) for the interpolation.
j_solar
j_solar 2011-11-18
Really c could be a 700x1 array, but since it is going to be the second column of final I replicate it 60 times so I can insert it in final.
Then I use only one(really it could be c(1) instead of c(i)) for the interpolation

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