How to select the columns of a matrix which are a multiple of 5?

Question

Ellen De Jonghe 2020-1-9

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回答： Ellen De Jonghe 2020-1-9

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Hye

I don't know how to solve the following exercise:

M is a 3x5000 matrix. A student wants to select the elements on the third row in all columns which are a multiple of 5.

The student writes the following script.

N = zeros(1,1000)
for i = 1:5000
    if rem(i,5) == 0
        N(i/5) = M(3,i);
    end
end 

I have to simplify it using only N = M(...). How do I do that?

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Guillaume 2020-1-9

It's hard to give you any help without giving the solution away (which would be cheating).

For which i values would the if be true? Couldn't you pass these values directly as indices of M and not bother with the loop?

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Answer 1

Alex Mcaulley 2020-1-9

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https://ww2.mathworks.cn/matlabcentral/answers/499588-how-to-select-the-columns-of-a-matrix-which-are-a-multiple-of-5#answer_409371

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N = M(3,5:5:end)

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Guillaume 2020-1-9

In my opinion (as an academic), it'd be better if you didn't post outright solution to homework. Particularly, when it's that easy.

Alex Mcaulley 2020-1-9

Oops you are right, I didn't realise it was homework

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Answer 2

Ellen De Jonghe 2020-1-9

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Hi guys, thanks a lot for your help!

I managed to solve exercises which are way more difficult, but my mind is still full of my previous exam stuff, so I didn't see that the solution ofr this case was so simple. Hope that makes sense ;)

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