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

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Ellen De Jonghe on 9 Jan 2020
Answered: Ellen De Jonghe on 9 Jan 2020
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?

#### 1 Comment

Guillaume on 9 Jan 2020
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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### Answers (2)

Alex Mcaulley on 9 Jan 2020
N = M(3,5:5:end)

#### 2 Comments

Guillaume on 9 Jan 2020
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 on 9 Jan 2020
Oops you are right, I didn't realise it was homework

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Ellen De Jonghe on 9 Jan 2020
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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