How to use "if statement" for different matrix?

Question

sky walker 2021-7-14

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https://ww2.mathworks.cn/matlabcentral/answers/878368-how-to-use-if-statement-for-different-matrix

评论： sky walker 2021-7-17

i dont know how to explain my question,

lets say i have 4 matrix

a = [1 3 5; 2 4 7];

b = [1 2 5; 2 4 6];

c = [1 4 5; 4 4 7];

d = [1 3 1; 2 4 1];

i want use like

for i=1:4

if a, b and d then

x=2+n

AK= a*n*x

BK= b*n*x

DK=b*n*x

else (only c)

x=7+n

CK=c*n*x

end

kind of like that?

hope you understand. Thanks

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Steven Lord 2021-7-14

What do you mean by "if a, b and d then"? If a, b, and d what?

If a, b, and d exist
If a, b, and d are the same size
If a, b, and d all contain the same values
If a, b, and d all contain nonzero values

Or do you want to perform the same steps for a, b, and d and to perform a different set of steps for c?

sky walker 2021-7-14

yes, i want to exclude c, because c have different step than other

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Answer 1

Stephen23 2021-7-14

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https://ww2.mathworks.cn/matlabcentral/answers/878368-how-to-use-if-statement-for-different-matrix#answer_746653

编辑：Stephen23 2021-7-14

The MATLAB approach is to use arrays and indexing, e.g. using one simple cell array:

V = {[1,3,5;2,4,7];[1,2,5;2,4,6];[1,4,5;4,4,7];[1,3,1;2,4,1]};
W = V;
n = 1;
for k = 1:numel(V)
    if k<4
        x = 2+n;
    else
        x = 7+n;
    end
    W{k} = V{k}*n*x;
end
W{:}
ans = 2×3
     3     9    15
     6    12    21
ans = 2×3
     3     6    15
     6    12    18
ans = 2×3
     3    12    15
    12    12    21
ans = 2×3
     8    24     8
    16    32     8

Or by just using numeric arrays (which is probably what you should be doing, compare how much simpler and more efficient your MATLAB code would be):

A = cat(3,V{:});
U = cat(3,2,2,2,7);
B = A.*n.*(U+n)
B = 
B(:,:,1) =

     3     9    15
     6    12    21


B(:,:,2) =

     3     6    15
     6    12    18


B(:,:,3) =

     3    12    15
    12    12    21


B(:,:,4) =

     8    24     8
    16    32     8

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Stephen23 2021-7-14

编辑：Stephen23 2021-7-14

"My code is actually simple,but just generates a lot of new matrices."

Hopefully in one cell array or structure, otherwise you really have a major design problem.

"I've seen your script, but what i need to be different is matrix c, not d."

My answer does not have matrix c nor d.

Instead of having lots of separate variables (bad data design, difficult to extend to lots of matrices) I simply placed all of your matrices into one cell array (much better data design, works automatically for any number of matrices) or into one ND-array (ditto). Both of these data designs makes it trivial and efficient to loop over your matrices (unlike your complex approach) and also to apply different steps to whichever of those matrices that you want (in fact, which ones does not even need to be hardcoded).

In my answer the fourth matrix has a different step, just as you requested.

You can make any matrix/matrices that you want have different step sizes.

As far as I can tell, my does what you need**, just simpler, more efficient, and more versatile than your approach (and also trivially extends to any number of matrices, unlike your approach).

** or atleast something very similar to it, so similar that you should not have any problems tweaking it.

sky walker 2021-7-17