# How to determine if a row appears more than once in an array in MATLAB

1. Jan 23, 2013

### roldy

I'm working on a project for myself that I started some time ago. This question only covers a small part of what I am doing. In my program I have vertices that I read in from a text file. They are read and stored in a matrix where each row of the matrix is a facet (triangle) created by the 2 (x,y) vertices. I end up with a N X 3 matrix of vertices where N is the number of facets. I then pass these through a function that determines what the intersection points are for each facet that is intersected by a plane at x = some value. The intersection points are stored in a N X 2 cell array (say B) where each row is a line segment created by the intersection points of each facet. I wrote a function that takes this cell array and "connects" the line segments by finding the common point (cell) of a row to another row. I have this working to a certain extent. The catch is that it only works if the first row in B is a line segment that has only one point in common with one other row. What I want to do is figure out a way to find which row in B has only one point in common with the one other row. There should be 2 rows in B for this (one for the start and one for the end of the line). Below is the code that I have written thus far. A represents the starting line segment to begin the sorting. Right now A is set as the first row in B in order for the program to run. Try changing A to any other row besides an end line segment to understand what I am getting at.

Code (Text):

function [line] = connect(B)
clear all
clc

% Build the example cell arrays
A = {[4,-1],[3,0]};     %Initial test row value
B = {[4,-1],[3,0];     %end line segment
[-1,4],[-3,5];     %end line segment
[3,0],[2,1];
[2,1],[-1,4]};

ANumVec = cell2mat(A);
ANum = reshape(ANumVec, 2, 2)';
Bmat = cell2mat(B);

line = A;
m = length(Bmat);
count = 2;

while  m ~= 1

I1 = ismember(Bmat(:, 1:2), ANum, 'rows');      %check first two columns
I2 = ismember(Bmat(:, 3:4), ANum, 'rows');      %check last two columns
I = [I1, I2];
B(all(I == 1,2),:) = {[], []};      %replace start row with empty cells to remove from list for next iteration
I(all(I == 1,2),:) = 0;             %Find indice of the row in B that contains a matching vertice of the vertices in A

A = Bmat(I > 0,:);                  %Get next test row vertices
ANum = reshape(A,2,2)';
line(1,count:count+1) = {A(1,[1 2]), A(1,[3 4])};      %Assemble connectivity array

empty_row = ~cellfun('isempty',B);
B = B(empty_row(:,1),:);                %Resize B array by removing empty rows
Bmat = cell2mat(B);
[m n] = size(Bmat);

count = count+1;
end

2. Jan 24, 2013

### kreil

I think the 'isequal' operator might be of some help?

>> A = [1 1 1; 1 1 1;0 0 0;0 0 0]

A =

1 1 1
1 1 1
0 0 0
0 0 0

>> B = [1 1 1]

B =

1 1 1

>> for i=1:size(A,2)
C(i) = isequal(A(i,:),B);
end
>> C

C =

1 1 0 0

3. Jan 24, 2013

### roldy

Thanks for the reply. I actually figure this out last night using a combination of ismember and if/then logic statements. This works, however it is in a separate while loop than the one that does the "connecting". Right now I'm trying to optimize the code and trying to figure out how to combine the two while loops.