How can I create an dynamically increasing array to store positions?

load iris.dat
for i:size(iris,2)-1
  if (iris(:,i)==iris(1,1)
         %store the row number in a dynamically increasing array (could be a cell array, or any %other matlab structure

%Please explain in simple terms. I am new to matlab

[mizuki]

Do you want to find elements in iris(:,1) (the first column) that are same as iris(1,1)? If so, the following code can:

load iris.dat
j = 1;
for i=1:size(iris,1)-1
  if ( iris(i,1)==iris(1,1) )
      idx(j) = i;   % store the row# which has the same element with iris(1,1)
      j = j+1;
  end
end

I do not recommend you to make a code which includes the variable that increases the size of the variable dynamically because it takes much more time than creating a variable in one time like:

find(iris(:,1) == iris(1,1))