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Top 10 MATLAB code practices that make me cry 83

Posted by Doug Hull,

I was chatting with the Application Support Engineers here at MathWorks about what kind of coding practices cause avoidable pain for MATLAB users.

Without further ado: The top ten and quick ways to not do them:

10.) Not using left hand zeros

Certain things must be learned the hard way. I learned this one bleary eyed evening as an undergraduate. Well into the night working on a MATLAB homework assignment that would “take ten minutes, fifteen if you type slow.” (yes, Dr. P, 13 years later I still remember that one! -smile-)

It is really easy to mistake a .5 for a 5. That is why I always use a left hand zero like 0.5.

9.) Plotting enormous amounts of data

My computer monitor has 2.3 million pixels, total. If I try to plot datasets with huge amounts of data in them, they will very often just look like a blob and slow the machine down in the process.

There is very often a better visualization available. Here is an example of changing the visualization to make it clearer and less taxing on memory.

8.) GUIs with garish color schemes

In an effort to emphasize certain buttons on their GUI, people will change the colors of them. Very quickly they end up with several different colored buttons, a non-standard background color, extra big buttons, etc…

Sticking with the default colors is a good move. Most professionally produced software sticks with the defaults, it ends up looking better.

7.) Using ans, or any other MATLAB function as a variable name or function.

When you do this, MATLAB will call whichever one is higher on the path. Some strange behavior can occur when you redefine a function like that. Unfortunately, MATLAB does not catch you doing this for the most part.

I try to avoid using variables and function names that are common terms like, mean, filter, etc… If there is any doubt, use the which command to find out if a function exists of a given name.

6.) Not using white space to good effect in code.

Even though you can put several commands on one line if separated by a semicolon, these lines can often be hard to notice. Not putting blank lines between sections of code can also make it harder to read.

White space is free, use it to make your code look good.

5.) Bad variable names

Variable names are often the only commenting that gets added to people’s code. Meaningful variable names are a great opportunity to make the meaning of your code more clear and to some degree, self-documenting.

Avoid using variable names like temp, aaa, r247899921. These just do not convey as much information to people that have to read your code as flagPassedInspection, centroidX, fidCurrentFile.

4.) Hard coding data into the MATLAB code file

Some people like to put some of their variables directly into the MATLAB code. That makes sense for small variables (I will let you define what small means for you). For instance, I would feel fine putting a 3×3 matrix into my code. I would think twice about a 10×10, and I would start using one of our file readers for a 100 x 100.

The worst instance I ever saw of this was some MATLAB code where the .M file was 4 GIG (not a mistake) long. All but a small amount of that was data written out in ASCII. This makes your code hard to read, maintain and understand.

3.) Exceptionally long files

Even if not hard coding data into a MATLAB code file, it is easy to just add on “just a few more lines of code” until you have thousands of lines of code in a single script. This makes your code hard to understand.

I try to use the rule that I should be able to see an entire script or function in one screen. This is not entirely practical, so I will at least break the code into logical sections that do fit on screen all at once.

2.) Globals

I have never seen MATLAB code where globals were the right thing to do. Exception: functions TIC and TOC use them quite nicely. Most of the time I have seen globals being used it was a situation where the code author did not understand scoping of variables. Rather than pass variables from one function to another, they were just being made global.

Why are people cautioned against using global variables? I will leave that to the consensus on Wikipedia.

1.) Eval

EVAL is right up there with globals. MATLAB user will often string together MATLAB commands to get sequential variable names s1, s2, s3… only to then have to use another EVAL statement to work with the sequential variable names! Very often, a cell array indexed with s{1}, s{2}, s{3}… would work much better.

I will also find that people use EVAL to get at certain fields in a structure (for example data.alpha) when they do not know at the time of writing the code what field they will want. Now the “.parens” notation makes that easier.

The other most common place to see people use EVAL when it is not needed is when they are trying to load a file or some other function like that. Very often they are trying to EVAL a string like “load filename.mat” not realizing that there is a functional form where you can use fileNameString = ‘filename.mat’; load(fileNameString)

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