Declaring a matrix with null dimension, or checking if a matrix exists

Inside a for loop, I want to do a calculation that produces new results, and then append these results to an existing matrix called "my_matrix":

for k=1:big_number
    new_results = func(blah blah blah)
    my_matrix = [my_matrix   new_results];
end

The problem is: the first time the calculations are done, "my_matrix" does not exist, so I get an error in Matlab.

--> Is there a way to declare "my_matrix" with no dimensions, so that I don't get the above error on the first loop iteration?

--> Is there a way to check if a matrix called "my_matrix" exists? For instance:

If "my_matrix" exists, then my_matrix = [my_matrix new_results];

If "my_matrix" does NOT exist, then my_matrix = new_results;

[Stephen23]

"Is there a way to declare "my_matrix" with no dimensions, so that I don't get the above error on the first loop iteration?"

There is nothing stopping you from declaring a matrix to have any of its dimensions to have zero size, e.g.:

M = nan(2,0,3,0);

but probably all you need is this:

M = [];

"Is there a way to check if a matrix called "my_matrix" exists?"

You could use EXIST, but coding using EXIST is slow and best avoided:

https://www.mathworks.com/help/matlab/matlab_prog/techniques-for-improving-performance.html#buwj1nn

Really, that approach is best avoided. Here is a much better approach:

M = func(blah blah blah);
for k = 2:big_number
    R = func(blah blah blah)
    M = [M,R];
end

However this is all rather moot, because if your code iterates up to a BIG_NUMBER then you really need to preallocate the entire array before the loop:

https://www.mathworks.com/help/matlab/matlab_prog/preallocating-arrays.html

Answer 1

[Matt J]

There is, but it is highly inadvisable to iteratively grow a matrix like you are doing. Here is one alternative:

my_matrix=cell(1,big_number);

for k=1:big_number
    my_matrix{k} = func(blah blah blah);
end

my_matrix=cell2mat(my_matrix);