# Use conditional statements to separate elements from a single array into 2 new arrays

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Vance Blake
on 15 Aug 2019

Commented: Vance Blake
on 28 Aug 2019

Hello, I am trying to separate elements from a matrix A (10x2) using conditional statements. I compare the distances between the all the points in A and if they are smaller than the threshold (16) then the pair of points that created that produced that result are isolated in another matrix (HS_elim). I want to use that newly created elim matrix to determine the points that wont be eliminated and instead sent to another matrix (B). I am able to isolated the points that meet my if condition but I cannot isolate the points that fail the condition. the snippet of code I am using is below. Any help would be greatly appreciated.

HS_HS_threshold = 16;

for i = 1:n

for j = 1:n

elim_dist2(i,j) = sqrt((x(i)-x(j)).^2 + (y(i)-y(j)).^2);

if (elim_dist2(i,j) ~= 0 && elim_dist2(i,j) < HS_HS_threshold)

HS_elim_x = x(j); HS_elim_y = y(j);

HS_elim(j,:) = [HS_elim_x, HS_elim_y];

elseif HS_elim(:,1) ~= x(i) && HS_elim(:,2) ~= y(i)

keep_x = x(i); keep_y = y(i);

B(i,:) = [keep_x, keep_y];

end

end

end

##### 3 Comments

Walter Roberson
on 15 Aug 2019

### Accepted Answer

Bjorn Gustavsson
on 16 Aug 2019

Are you trying to separate points that have no neighbours closer than HS_HS_threshold from those that have?

If so this should do something like that:

elim_dist2 = nan(numel(x)); % this to have nans on the diagonal after distance calculation

HS_HS_threshold = 16;

for i = n:-1:1 % looping from largest index lets you avoid calculating the size of the elim_dist2 matrix without pying the price of dynamic growing of a matrix

for j = (i-1):-1:1 % distance symmetric (l(i1,i2) == l(i2,i1)) so calculate it once

elim_dist2(i,j) = sqrt((x(i)-x(j)).^2 + (y(i)-y(j)).^2);

elim_dist2(j,i) = elim_dist2(i,j);

end

end

% find the points that have its nearest neighbour further away than HS_HS_threshold:

i2keep = find(nanmin(elim_dist2)>HS_HS_threshold);

% put those into one pair of arrays

keep_x = x(i2keep);

keep_y = y(i2keep);

% and the others into another pair of arrays

x_close_neighbours = x;

y_close_neighbours = y;

x_close_neighbours(i2keep) = [];

y_close_neighbours(i2keep) = [];

HTH

##### 6 Comments

Guillaume
on 28 Aug 2019

None of the pieces of code you were given assume anything about the size of the inputs x and y (other than they are vectors). For a vector of N points, all the example will give you a NxN matrix for elim_dist2.

If that's not the case for you, then you're doing something different. It's clearly different since your original code worked with two vectors of the same length and you're now telling us that you have two matrices of different size instead. Please clarify what you're doing by showing us the actual code.

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