Best Practices with Initialization or Pre-allocation - MATLAB

Question

My question doesn't depend expressly on one snippet of code, but is more conceptual.

Unlike some programming languages, MATLAB doesn't require variables to be initialized expressly before they're used. For example, this is perfectly valid to have halfway through a script file to define 'myVector':

myVector = vectorA .* vectorB

My question is: Is it faster to initialize variables (such as 'myVector' above) to zero and then assign values to them, or to keep initializing things throughout the program?

Here's a direct comparison of what I'm talking about:

Initializing throughout:

varA = 8;
varB = 2;

varC = varA - varB;
varD = varC * varB;

Initializing at start:

varA = 8;
varB = 2;
varC = 0;
varD = 0;

varC = varA - varB;
varD = varC * varB;

On one hand, it seems a bit of a waste to have these extra lines of code for no reason. On the other hand, though, it makes a little bit of sense that it would be faster to allocate all the memory for a program at once instead of spread out over the runtime.

Does anyone have a little insight?

Answer 1

Copy and paste your Initializing at start: code into MATLAB Editor Window and you would get this warning that looks like this -

And if you go into the Details, you would read this -

Explanation 
The code does not appear to use the assignment to the indicated variable. This situation occurs when any of the following are true:
Another assignment overwrites the value of the variable before an operation uses it.
The specified argument value contains a typographical error, causing it to appear unused.
The code does not use all values returned by a function call...

In our case, the reason for this warning is The code does not use all values. So, this clarifies that initialization/pre-allocation won't help for that case.

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When should we pre-allocate?

From my experience, pre-allocation helps when you need to later on index into part of it.

Thus, if you need to index into a portion of varC to store the results, pre-allocation would help. Hence, this would make more sense -

varC = zeros(...)
varD = zeros(...)
varC(k,:) = varA - varB;
varD(k,:) = varC * varB;

Again, while indexing if you are going beyond the size of varC, MATLAB would spend time trying to allocate more memory space for it, so that would slow things a bit. So, pre-allocate output variables to the maximum size which you think would be used for storing results. But, if you don't know the size of results, you are in a catch there and have to append results into the output variable(s) and that would slow down things for sure.