how to choose k values

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Nikolas Spiliopoulos 2017-2-24

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https://ww2.mathworks.cn/matlabcentral/answers/326747-how-to-choose-k-values

评论： Star Strider 2017-2-25

采纳的回答： Star Strider

hi all,

I have an array of 365 values and I do some calculations with an index for k=1:365

However, I need to do some different calculations for some k (let's say k=7, k=9,k=11, k=12)

how can I select only these?

I imagine is something like that: for k=1:365

if k=7.....

else...

thanks

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Star Strider 2017-2-24

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https://ww2.mathworks.cn/matlabcentral/answers/326747-how-to-choose-k-values#answer_256178

在 MATLAB Online 中打开

It is easiest to address them directly:

v = randn(1,365);                               % Create Data
k = 1:365;                                      % Index Vector
select_k = [7 9 11 12];                         % Select ‘k’ Values
select_v = v(select_k);                         % Select ‘v’ Values

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Star Strider 2017-2-25

在 MATLAB Online 中打开

You implied a row vector in your Question. I do not know what error you are getting, or the sizes of your arrays. You must index the arrays correctly in order to use them with my logical vectors.

This works:

k = 1:365;                                      % Index Vector
select_k = [7 9 11 12];                         % Select ‘k’ Values
select_v = v(select_k);                         % Select ‘v’ Values
Lv = logical(ones(size(k)));                    % Logical Vector For All 365 Indices
Lkk(select_k) = Lv(select_k > 0);               % Create Limited Logical Vector Of Selected Values
Lk = Lv & [Lkk ~Lv(length(Lkk)+1:end)];         % Full Logical Vector Of Selected Values
Lnk = ~Lk;                                      % Full Logical Vector Of Non-Selected Values
A = randi(99, 365, 2);                          % Create Data Array (365 rows, 2 columns)
RowsToDoSomething = A(Lk,:);
RoswToDoSomethingElse = A(Lnk,:);

I chose ‘A’ to be (365x2) to illustrate the correct approach. The new variables are self-documenting.

This will work for any (365xN) matrix.

See the documentation on Matrix Indexing to understand the indexing into ‘A’ I use here.

Nikolas Spiliopoulos 2017-2-25

the new question is a different one,

thanks a lot

you are very helpful!

sorry for being a pain..I just started using matlab!

I am learning a lot from you

thanks!

Star Strider 2017-2-25

My pleasure.

You are not a pain. If you have previous programming experience, learning MATLAB will be relatively easy, once you get used to the conventions and programming environment. If you have no programming experience, learning any programming language for the first time will be a challenge.

Learning MATLAB will be much easier if you use the online tutorials. Begin with Getting Started.

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