Subscript indices must either be real positive integers or logicals.

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Kelly 2013-2-15

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Can someone please explain this error. Here is my code and function.

bD = [0.1, 1.0, 10, 100];
R = (1:1.25:5);
c = R;
a = 1/2*bD;
b = abs(sqrt(c.^2-a.^2));
theta = abs(atan(b./a));
[F] = ratio_rp_bf(R,theta,bD);
function [F] = ratio_rp_bf(R,theta,bD)
F(bD,R) = (16/3).*(1./bD).*(1./R).^3.*abs(sqrt((1./R).^4-2.*(1./R).^2.*cos(2.*theta)+1));

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Image Analyst 2013-2-15

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https://ww2.mathworks.cn/matlabcentral/answers/63652-subscript-indices-must-either-be-real-positive-integers-or-logicals#answer_75246

编辑：Image Analyst 2013-2-15

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R takes on fractional values and thus can't be an index into an array. You don't need to put indexes for F in your function. Just do:

F = (16/3).*(1./bD).*(1./R).^3.*abs(sqrt((1./R).^4-2.*(1./R).^2.*cos(2.*theta)+1));

and it will make F a 1 by 4 array automatically. It will take the first values from all 3 arrays (bD, R, and theta), and then the second values, and then the third values, and then the last values. Is that what you want?

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Image Analyst 2013-2-15

编辑：Image Analyst 2013-2-15

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So you have a 2D array where bD is independent, but R and theta indexes are matched (both 2, both 3, or both 4)? Then have the function look like this:

function F = ratio_rp_bf(R,theta,bD)
for r = 1 : length(R)
  for b = 1 : length(bD)
    F(b, r) = (16/3).*(1./bD(b)).*(1./R(r)).^3.*abs(sqrt((1./R(r)).^4-2.*(1./R(r)).^2.*cos(2.*theta(r))+1));
  end
end

And call it like this in the main program:

F = ratio_rp_bf(R,theta,bD)

In the command window:

F =
5332    5.5303    1.2148    0.4980
6533    0.5530    0.1215    0.0498
0653    0.0553    0.0121    0.0050
1065    0.0055    0.0012    0.0005

Kelly 2013-2-16

I'm trying to compute F and plot a contour 2x2 subplot, bD is independent, R and theta are dependent, for each R there is a corresponding theta.

bD(1) R(1) theta(1)

bD(1) R(2) theta(2)

...

bD(2) R(1) theta(1)

bd(2) R(2) theta(2)

...

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Walter Roberson 2013-2-15

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https://ww2.mathworks.cn/matlabcentral/answers/63652-subscript-indices-must-either-be-real-positive-integers-or-logicals#answer_75248

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R includes numbers with fractions, but inside ratio_rp_bf, you are trying to assign to F(bD,R) which is an attempt to assign use R as a subscript.

If you are trying to define a function f(x,y) by using an assignment

f(x,y) = <something>

then the only time you can do that is if you are using R2012a or later and you have already defined the function as symbolic

syms f(x,y)

MATLAB does not use assignment to f(x,y) as the notation for creating functions.

My guess as to what you want to do would be

function F = ratio_rp_bf(R,theta,bD)
  [Rmat, bDmat, thetamat] = ndgrid(R, bD, theta);
  F = (16/3).*(1./bDmat).*(1./Rmat).^3.*abs(sqrt((1./Rmat).^4-2.*(1./Rmat).^2.*cos(2.*thetamat)+1));
end

Notice that this will return a 3D array, as your R, theta, and bD are all vectors.

Possibly you want to use R and theta linked together, always using R(K), theta(K) together? But that you want bD to be an independent axis?