Asked by James Morris
on 20 Aug 2019 at 19:49

I know how to take the first 4 values of an n size array (1:4)

and the last 4 (end-4:end)

but how would I take the middle 4 values of an n size array no matter the length?

Thanks

Answer by Jon
on 20 Aug 2019 at 19:56

Edited by Jon
on 20 Aug 2019 at 19:59

Accepted Answer

You could use something like, for an array A

n = length(A(:)) % : treats as column even if this is actually a multidimensional array

midpoint = round(n/2)

select = A(midpoint-1:midpoint+2); % slightly unsymmetrical 1 before, 2 after

Note, you may have to further clarify, what your really mean by "middle 4". There are two sources of asymmetry inherent in taking the "middle 4". If there are an even number of elements then the "middle" one is either closer to one end or the other. Also as you are taking an even number (4) elements then either they are offset one way or the other from the selected middle element

Jon
on 21 Aug 2019 at 12:54

I agree, middle four for n even is well defined. For n odd you end up with it biased one to the left or or one to the right depending how you choose.

Here is a simple way of doing it which will be symmetrical for n even, and biased to the left for n odd

n = length(A(:)) % or you could use n = numel(A) if you prefer that style

midpoint = floor(n/2)

select = A(midpoint-1:midpoint+2);

or if you want it biased to the right you can use

n = length(A(:)) % or you could use n = numel(A) if you prefer that style

midpoint = ceil(n/2)

select = A(midpoint-1:midpoint+2);

Note that in my original solution I used round instead of floor or ceil. Since the rounding rules round up for values where the decimal part is greater than or equal to 0.5 round will give the same result as ceil (bias to the right) . Using ceil makes what is happening clearer.

Also my comment in the original solution, was not really clear. It is infact symmetrical for n even.

Jon
on 23 Aug 2019 at 14:37

Are you still looking for more help with this?

James Morris
on 23 Aug 2019 at 21:41

Sorry for the late reply. No I figured it out in the end. You guys are awesome, thank you.

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Answer by the cyclist
on 20 Aug 2019 at 19:58

Edited by the cyclist
on 20 Aug 2019 at 20:13

Here is one way, assuming the number of elements is even:

% Input

a = rand(1,8);

numberElements = numel(a);

numberToRemove = numberElements - 4;

numberToRemoveFromEachEnd = numberToRemove/2;

output = a(numberToRemoveFromEachEnd+1:end-numberToRemoveFromEachEnd)

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Answer by the cyclist
on 20 Aug 2019 at 20:09

Edited by the cyclist
on 20 Aug 2019 at 20:13

Here is one way, which will work for either even- or odd-length arrays. It is not efficient.

% Input

a = rand(1,9);

whichEnd = 1;

while numel(a) > 4;

a = a(whichEnd:(end+whichEnd-2))

whichEnd = 3-whichEnd;

end

end

It will choose the "slightly left of center" for odd-length array.

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