Matlab Producing Different Values Running Entire Script VS Single Line Runs

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John 2023-6-18

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编辑： dpb 2023-6-21

As the title states, I am getting different values from Matlab's length function depending on whether I run the entire script (Using the "Run" button) or just line by line manually. I have no clue why this is occuring but the Matlab functions are not producing correct results themselves when this occurs. I have Matlab on two seperate machines and tried testing the code on both and found the same issue persists. I tried simplifying the code with a very trvial example with the same input structures and the error does not apply to the simpler case. It is just occuring within my main code. For context, I am generating a custom mesh in cylindrical coordinates. The issue occurs on lines 104-105:

stemp = [length(nodesPhis) length(nodesPhiMSim)];
maxNodesPhi = max(length(nodesPhis),length(nodesPhiMSim)); % Finds the max dimension required for the
% phi-component 3D matrix to be populated

If we run the script using the run button, we can see in the variable window that the vector for stemp is [40,53]:

But when I highlight the text to manually run the line, it says [60,53]:

For context, I only define stemp once and in that line 104. Nowhere else is it redefined. Not only this, but even more contradictions come up later in the code from these lines. The code uses the max element of that vector to produce a NAN matrix with one of the dimensions lengths being that max number. This matrix is called "meshPhi", it's size is (x,max(stemp),z) and it is defined as:

meshPhi = NaN(length(nodesR),maxNodesPhi,max(length(nodesZ1),length(nodesZ2))); % Create "empty" phi-mesh to populate

When I run the code with the button, it actually creates the correctly sized meshPhi matrix:

But as can be seen, the meshR and meshZ matrices which are supposed to be the same size, are using 53, and not 60. Those matrices are defined like this on lines 142 and 156:

meshZ = NaN(length(nodesR),maxNodesPhi,max(length(nodesZ1),length(nodesZ2))); % Create "empty" z-mesh to populate
meshR = NaN(length(nodesR),maxNodesPhi,max(length(nodesZ1),length(nodesZ2))); % Create "empty" r-mesh to populate

Exactly the same... so the only thing I can think of is it seems that Matlab is bugged in some way here. Nonetheless, I still wanted to post this question to see if possibly it's not a bug and instead I am potentially missing something or not using an internal function correctly.

Main Code: "TEST_DynCylMesh.m"

Supporting function: "sigdigits.m"

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dpb 2023-6-19

编辑：dpb 2023-6-19

length() probably wouldn't make it into the MATLAB stable if it were to be brought up as a new addition now; it existed in first releases as is and so is carried along for compatibility purposes, but really should be avoided owing to the above issues.

If one is extremely careful, one can get by, but it's very possible for an unexpected result to occur when/if data sizes change beyond those that were expected when the code was written. Simply reading an array that now has N+1 variables will return the column size where it is expected to be the row size if there are <=N observations. This case may have been thought to be impossible originally, but things changed.

IMO, the height(), width() functions for tables ought to be extended to accept matrices/arrays as wel(*)l...before they were introduced I built and keep in my Utilities folder nrows(), ncols() as syntax sugar for the size(x,1), size(x,2) idioms; I've suggested those should also be builtins, but has fallen on deaf ears for 30 year or more...

(*) Of course table is constrained to be a 2D object; trying to create an array of tables results in simply appending the table content instead so the English meaning of width and height is maintained whereas for arrays of more than three dimensions there's not such a correlation for the higher dimensions.

I notice that length() actually isn't defined for table objects although one can still use max(size(t)) to get the same end result (although I see no reason why one would want to do that given the problems outlined).

Rik 2023-6-19

If you want the length of a vector, length does what it says on the tin. When you get to how people actually use it, dragons pop up around every corner.

Adding to the discussion at hand: I maintain that any code you want to use after next week should be in a function. That provides a stable interface that you can document. Of course this is a rule you can break, but only if you follow it first. Problem like this get a lot easier to track down.

I personally compose larger functions from parts (a poor man's package management), so I don't actually need to remember which functions do what. Sometimes I come across a function I wrote years ago. I usually need to wipe some tears and rewrite a lot of code, but only to improve style and performance (or to fix edge cases). The beauty of functions is that it works whether you wrote it terribly or well. And once you update, you shouldn't need to worry about other code depending on it, because every function is a black box to the functions calling it.

dpb 2023-6-19

编辑：dpb 2023-6-21

在 MATLAB Online 中打开

@Bruno Luong -- that's the edge case, yes.

The doc actually reads as " For arrays with more dimensions, the length is max(size(X)). The length of an empty array is zero." I did leave off the for zero-length arrays part, yes, since they weren't pertinent in this particular thread.

So strictly in compliance, the form would be max(size((x))*~isempty(x)

x=zeros(1,0);
length(x)
ans = 0
max(size(x))*~isempty(x)
ans = 0

dpb 2023-6-19

@John, while you've probably lost interest long ere now, with the update that height and width now operate on arrays as well, I'll change my advice to recommend using them instead.

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