Conditional function on matrix

Hello,

This is a tough one to explain.

I am analysing a big chunk of data and now I have a equation defined by intervals.

Kd=1 for Kt<0.2
Kd=e for 0.2<=Kt<=1.09*c2
Kd=f for Kt>1.09*c2

Kt=rand(8784,89);
c2=ones(size(Kt))/2;
e=rand(size(Kt));
f=rand(size(Kt));

I've simplified it quite a bit, because it envolves other matrixes (all with the same size (8784,89)) on the conditions.

My question is, how to execute it? I've tried piecewise, but I'm stuck with the symbolic. I would need to get Kd as double.

Kd(Kt)=piecewise(Kt<0.2,1, 0.2<=Kt<=1.09*c2,10, Kt>1.09*c2,5);

I've tried with If and elseif but as far as I've seen it will only analyze the first element of the array to verify if the condition is true:

if Kt<0.2
    Kd=1;
elseif 0.2<=Kt<=1.09*c2
    Kd=e;
elseif Kt>1.09*c2
    Kd=f;
end

Other ideas or something I'm missing?

edit: edited after trying solution given by Matt J.

thanks.

Answer 1

[Matt J]

vals=[1,10,5];
Kd=vals(   discretize(Kt,[-inf,0.2,1.09*c2,+inf])   );

[Osnofa]

Did you try that solution? I cannot run it, I get an error.

Error using horzcat

Dimensions of matrices being concatenated are not consistent.

Error in test (line 8)

Kd=vals(discretize(Kt,[-inf,0.2,1.09*c2,+inf]));

[Matt J]

Sorry, I didn't notice that c2 was non-scalar. In that case, I would just use simple logical indexing.

 Kd=ones(size(Kt));
 thresh=1.09*c2;
 
 Kd(Kt>=0.2 & Kt<=thresh)=10;
 Kd(Kt>thresh)=5;

[Osnofa]

No worries, I was trying to figure it out and decided to ask if you did try just to check if there was something I was not seeing.

Thank you, it works. But I see now that I didn't gave the best example because I set the answers as a value when in fact I have arrays.

Kd=1 for Kt<0.2
Kd=10 for 0.2<=Kt<=1.09*c2 ->this one should be Kd=e for 0.2<=Kt<=1.09*c2
Kd=5 for Kt>1.09*c2 -> this one should be Kd=f for Kt>1.09*c2. Where: 
e=rand(size(Kt));
f=rand(size(Kt)); *

Once I change the ouctomes to arrays in case of true condition I get a new problem: In an assignment A(:) = B, the number of elements in A and B must be the same.

I've tried to change all to same sized arrays, just to check but I'm still missing something.

*I've editted this in the 1st post.

[Matt J]

 Kd=ones(size(Kt));

 condition1 = Kt>=0.2 & Kt<=1.09*c2;
 condition2 = Kt>1.09*c2;
 
 Kd(condition1)=e(condition1);
 Kd(condition2)=f(condition2);

See also,

https://www.mathworks.com/help/matlab/math/array-indexing.html#MatrixIndexingExample-3

[Osnofa]

Thanks Matt! I was reading a bit further into logical indexing. Thanks for the link also!