Making Dynamic array

Asked by Khawaja Asim on 13 Aug 2011

Latest activity Commented on by Guillaume on 3 Jan 2018

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I want to creat an array that is dynamic in size. I mean I dont know what is going to be the size of array. I just want to insert every upcoming number on the head of array. Tell me the syntax of this and show me the code to find max num in this array at the end. Thanks

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3 Answers

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Answer by Oleg Komarov on 13 Aug 2011

Edited by John Kelly on 26 Feb 2015

Accepted Answer

What do you need it for?

There's no such thing as a dynamic array but you can grow an array with concatenation which is usually not recommended (or was not recommended).

If you have:

A = rand(10,1);

You can grow it as:

A = [A; rand(5,1)];

Such practice very often can be avoided.

Read the first 4 chapters of the getting started guide to see more examples of basi array operations.

3 Comments

Khawaja Asim on 13 Aug 2011

do you know about any buit-in code for finding max num from an array..??

Oleg Komarov on 13 Aug 2011

In the command vindow type:

doc max

Oleg Komarov on 13 Aug 2011

If you know k and it's last value then you don't need to grow it and as I said this practice is (was) not recommended:

for k = 1:10

A(k) = k;

end

This copy-writes the entire array at each iteration and it lead to performance degradation. With the release 2011a things changed.

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Answer by Hooman Sedghamiz on 10 Feb 2017

Edited by Hooman Sedghamiz on 10 Feb 2017

As others correctly noted, it is not a good practice to use a not pre-allocated array as it highly reduces your running speed.

Solution 1: In fact it is possible to have dynamic structures in Matlab environment too. However, it is not a native Matlab structure. Recently, I had to write a graph traversal script in Matlab that required a dynamic stack. To do so, you can simply use a Stack from java libraries for example. It is quite powerful and you can handle almost all different types of data! You can simply borrow this from java within matlab by;

% imports stack utility from java in matlab
import java.util.*;
% then use following commands to deal with stack
A = Stack();
A.push(i); % inserts i on top of stack A
A.pop(); %pops out an element

Solution 2: When I deal with arrays that I do not have any idea how big they have to be, I normally initialize them with an overestimated large size, then assign a counter to them and remove the nonused part in the end of the script. For example:

% Let A be the array that i am going to use, Initialize to large number
  A = zeros(1,1000000);
  % assign a counter that tracks the actual size of your array 
  counter = 1;
  % an arbitrary while loop
  rn = 0;
  while rn ~= 8
   A(counter) = rn;
   rn = randi(10,1,1);
   counter = counter + 1;
  end
  A = A(1:counter-1); % removes the rest of non used array

Hope that was what you were looking for! Cheers

2 Comments

Zeljko Tomicevic on 2 Jan 2018

Hooman Sedghamiz is it the same with two dimensional array? For example:

    % Let A be the array that i am going to use, Initialize to large number
  A = zeros(1,1000000);
  % assign a counter that tracks the actual size of your array 
  counter = 1;
  % an arbitrary while loop
  rn = 0;
  while rn ~= 8
   A(:, counter) = zeroes(1,11);
   rn = rn + 1;
   counter = counter + 1;
  end
  end

That example works in Matlab but not in Simulink User defined function block Matlab fcn. Simulink returns: Subscripted assignment dimension mismatch: [1] ~= [11]

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Guillaume on 3 Jan 2018

"That example works in Matlab" Not with my version of matlab!

Note that since you defined A with one row A(:, counter) is the same as A(counter), it is a scalar. You cannot assign a 1x11 array to a scalar.

As for your question, it would be very unusual to have a 2D dimensional array whose both dimension are unknown ahead of time, so just make the unknown dimension larger and declare the other one the right size to start with:

A = zeros(100000, 11);
counter = 1; 
while rand < .95 && counter <= size(A, 1)
    A(counter, :) = randi(100, 1, 11);
    counter = counter + 1;
end
A(counter:end, :) = [];

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Answer by Khawaja Asim on 13 Aug 2011

Array=[] this leads us to a dynamic array... just run a loop and assign it a value in this way Array(k)=___;

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