Which way is better?

Note: This is not about preallocating variable in a loop. This is for general case.

For instance:

a(10,10) = 10; % the rest are filled with zeros
%or
a = zeros(10,10);
a(10,10) = 10;

[Walter Roberson]

There was a Question in which people did some timing. The answer is not always obvious.

The larger question at the time also asked about the timing of assigning a constant to the matrix. For example is

5*ones(10,10)

faster than

5+zeros(10, 10l

Or

repmat(5, 10, 10)

[madhan ravi]

Thank you for the response sir Walter. Even a while ago , did some experiments and found out that multiplying a constant against ones() were always faster. Forgot this point , sorry :/. So with my above two approaches, is the first option a good way of coding?

[Walter Roberson]

See also https://www.mathworks.com/matlabcentral/answers/58055-faster-way-to-initilize-arrays-via-empty-matrix-multiplication

https://www.mathworks.com/matlabcentral/answers/433108-preallocate-array-without-initializing

https://www.mathworks.com/matlabcentral/answers/51411-initialize-a-mxn-matrix-with-the-same-number#

[madhan ravi]

Thank you sir Walter :)

Answer 1

[Adam Danz]

In terms of speed, the 2nd option is 3.0 times faster on average (p<0.0001, Wilcoxon Signed Rank; 1 million individually timed iterations of the following 2 options).

a = [];  % to reset a upon each loop
a(10,10) = 10;  

% versus 

a = []; % to reset a upon each loop
a = zeros(10,10);
a(10,10) = 10;

Declaring 'a' as zeros and then filling in the index (10,10) could also avoid other errors that option 1 may impose if 'a' is already a variable or function that exists.

Lastly, I think the 2nd option is more readable. Upon first glace of the first option, I'm wondering where 'a' came from.

[madhan ravi]

Thanks Adam for the answer. I knew the differences in speed. I mean't to ask whether if the first option also considered as a good way to code.

[Rik]

There is nothing inherently wrong with the first option, but because it can cause confusion, you should add a comment there. It is also important to make sure that a doesn't exist before this executes.

[madhan ravi]

Thanks Rik.