MATLAB: Taking middle 4 values of n size array

indexingmiddle values of array

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

Best 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

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MATLAB: Array elements not equal despite only difference being their respective end points

I received an answer from Tech Support which clarifies this issue:

This is a very interesting question. This is an expected behaviour, to understand why this happens we need to understand how the colon operator works.

v = a:d:b is not constructed using repeated addition (as one would think) as that might accumulate round-off errors.

In fact, this kind of round-off error is inherent to software or hardware systems that perform calculations using floating point numbers. Since floating point numbers have to be represented within a limited number of bits, the result of some calculations is rounded off in order to fit the result into the finite representation (sign, exponent, mantissa) for floating-point numbers.

An example of a number that cannot be represented exactly in binary floating point is the number 0.1, which, while finite in decimal representation, would require an infinite number of bits to be exactly represented in floating-point notation. For instance, executing the following in MATLAB:

0.1+0.1+0.1 == 0.3

will not return true due to such round-off errors.

To counteract such error accumulation, the algorithm of the COLON operator dictates that:

1. The first half of the output vector (in this example ‘v’) is calculated by adding integer multiples of the step ‘d’ to the left-hand endpoint ‘a’. 2. The second half is calculated by subtracting multiples of the step ‘d’ from the right-hand endpoint.

The COLON operator ensures symmetry of the output vector about its midpoint and also that the round off error is minimized at both endpoints. For example, even though 0.01 is not exactly represented in binary,

v = -1 : 0.01 : 1

consists of 201 floating points numbers centered symmetrically about zero.

For more detailed information on how the COLON operator works, refer to the attached function (COLONOP).

function v = colonop(a,d,b)
% COLONOP  Demonstrate how the built-in a:d:b is constructed.
%




%   v = colonop(a,b) constructs v = a:1:b.
%   v = colonop(a,d,b) constructs v = a:d:b.
%
%   v = a:d:b is not constructed using repeated addition.  If the
%   textual representation of d in the source code cannot be
%   exactly represented in binary floating point, then repeated
%   addition will appear to have accumlated roundoff error.  In
%   some cases, d may be so small that the floating point number
%   nearest a+d is actually a.  Here are two imporant examples.
%
%   v = 1-eps : eps/4 : 1+eps is the nine floating point numbers
%   closest to v = 1 + (-4:1:4)*eps/4.  Since the spacing of the
%   floating point numbers between 1-eps and 1 is eps/2 and the
%   spacing between 1 and 1+eps is eps,
%   v = [1-eps 1-eps 1-eps/2 1 1 1 1 1+eps 1+eps].
%
%   Even though 0.01 is not exactly represented in binary,
%   v = -1 : 0.01 : 1 consists of 201 floating points numbers
%   centered symmetrically about zero.
%
%   Ideally, in exact arithmetic, for b > a and d > 0,
%   v = a:d:b should be the vector of length n+1 generated by
%   v = a + (0:n)*d where n = floor((b-a)/d).
%   In floating point arithmetic, the delicate computatations
%   are the value of n, the value of the right hand end point,
%   c = a+n*d, and symmetry about the mid-point.
if nargin < 3
   b = d;
   d = 1;
end
tol = 2.0*eps*max(abs(a),abs(b));
sig = sign(d);
% Exceptional cases.
if ~isfinite(a) || ~isfinite(d) || ~isfinite(b)
  v = NaN;
  return
elseif d == 0 || a < b && d < 0 || b < a && d > 0
  % Result is empty.
  v = zeros(1,0);
  return
end
% n = number of intervals = length(v) - 1.
if a == floor(a) && d == 1
  % Consecutive integers.
  n = floor(b) - a;
elseif a == floor(a) && d == floor(d)
  % Integers with spacing > 1.
  q = floor(a/d);
  r = a - q*d;
  n = floor((b-r)/d) - q;
else
  % General case.
  n =  round((b-a)/d);
  if sig*(a+n*d - b) > tol
     n = n - 1;
  end
end
% c = right hand end point.
c = a + n*d;
if sig*(c-b) > -tol
  c = b;
end
% v should be symmetric about the mid-point.
v = zeros(1,n+1);
k = 0:floor(n/2);
v(1+k) = a + k*d;
v(n+1-k) = c - k*d;
if mod(n,2) == 0
  v(n/2+1) = (a+c)/2;
end

MATLAB: Getting the central n numbers in array..

To extract the middle N elements of the vector x:

x((end-N)/2+1:(end+N)/2)

Warning: this will cause an error if you ask for an odd number of elements from a vector of even length, or an even number of elements from a vector of odd length.

Best Answer

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MATLAB: Array elements not equal despite only difference being their respective end points

MATLAB: Getting the central n numbers in array..

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