1D Interpolation on array without using loops.

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Angela
Angela 2012-7-23
Hello.
I have been pondering this all day.
So, I have an 3D-array, X1, of size 48 x 232 x 61. Y1 is a row vector of size 1 x 232. Each row of X1 corresponds with Y1. I want to interpolate between each row of X1 and Y1 without using any loops. The output is a 48 x 21 x 61 array, Y2_array. With loops, my code is as follows:
% make Y1 array for interpolation
logz = log(10e-8):.10:log(1080);
Y1 = d + exp(logz); % This is the 1 x 232 vector.
Y2 = ones(48,21); % Preallocating Y2 2D-array (size = 48 x 21).
Y2_array = ones(48,21,61); % Preallocating Y2 3D-array (size = 48 x 21 x 61)
% Interpolation
for S = 1:61
for T = 1:48
X1_row = X1(T,:,S);
Y2 = interp1(X1_row, Y1, X2(T,:,S));
% X2 is a 3D-array, size (48 x 21 x 61)
Y2_array(T,:,S) = Y2;
end
end
If I make the inputs of interp1 arrays, Matlab gives me the error message that X and Y need to be vectors.
I know there is a solution to this problem... any suggestion/thoughts about how to execute the interpolation without using loops?
Thanks!!
  2 个评论
Jan
Jan 2012-7-23
What exactly is the problem? As far as I understand your loop method runs successfully. Do you want to run it faster? If so, avoiding the slow INTERP1 is the first thing I'd suggest.
Angela
Angela 2012-7-23
Yeah, I am just trying to make it faster.

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采纳的回答

Jan
Jan 2012-7-23
编辑：Jan 2012-7-23
Try this instead of INTERP1:
function Yi = LeanInterp(X, Y, Xi)
X = X(:);
Xi = Xi(:);
Y = Y(:);
nY = numel(Y);
[dummy, Bin] = histc(Xi, X); %#ok<ASGLU>
H = diff(X);
if Bin(length(Bin)) >= nY
Bin(length(Bin)) = nY - 1;
end
Ti = Bin + (Xi - X(Bin)) ./ H(Bin);
% Interpolation parameters:
Si = Ti - floor(Ti);
Ti = floor(Ti);
% Shift frames on boundary:
d = (Ti == nY);
Ti(d) = Ti(d) - 1;
Si(d) = 1;
% Now interpolate:
Yi = Y(Ti) .* (1 - Si) + Y(Ti + 1) .* Si;
For [1x100] vectors this is about 3 times faster than INTERP1 of Matlab 2009a.

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