Dumb mistakes we make with MATLAB.
Matt Fig
on 22 Feb 2011
Latest activity Reply by Bruno Luong
on 1 Jun 2022
O.k., admit it. Who has done this (or something that resulted in the same loss of info) before?
>> laster
??? Undefined function or variable 'laster'.
>> lasterr
ans =
Undefined function or variable 'laster'.
D'Oh! I need a ERRORBEFORELASTERR function.
What's one of your dumb MATLAB mistakes?
106 Comments
Use 'end' to index an empty array.
a = [];
a(end) = 3
ugly work around
a = [];
a(max(1:end):end) = 3
I spent a moment today trying to figure out why
outarray = logical(size(inarray)); % preallocate
didn't create a logical array with the same size as inarray.
I swear I must have brain damage, and with the caliber of forehead slap I needed, I can see how it would happen.
fig = figure('CloseReqFcn',@error_prone_fcn)
presses x button
error
alt-f4
error
tries close matlab
error
task manager > end task
error
Specifying 'force' with the close command will force the figure to close, bypassing its CloseRequestFcn.
fig = figure('CloseRequestFcn',@(varargin) error('No closing this figure!'));
close(fig) % Will not close the figure
close(fig, 'force') % Will close the figure
fig % This is now a handle to a deleted figure
I needed a short and confusing command for a demonstration:
eval = 1:255
eval eval
Error: "eval" was previously used as a variable, conflicting with its use here as the name of a function or command.
This works:
eval('eval')
>> 100 117 96 107
Since I happened across this old thread, one that continually catches me out since I work a lot with plotting of complex signals is to the effect of
figure; plot( mySignal )
hilbSignal = hilbert( mySignal )
hold on;
plot( hilbSignal );
Doh! Pretty spider's web, but not quite what I was looking for. One day I will remember I have complex data and that I actually want either the real or imaginary part! In this case I keep just thinking of the hilbert transform result as the imaginary part only that it adds.
>> close al
Error using close (line 111)
Specified window does not exist.
Dammit!
for i = 1:3
disp(exp(i*pi));
end
Instead of:
for i = 1:3
disp(exp(1i*pi));
end
Corrupting years of experience in other languages...
After using MatLab for several months now, I find that when I go back to writing C code I catch myself forgetting to put parentheses on my if statements and using single-quotes for my strings.
It still does!
I believe the name at one point meant Matrix Laboratory
And MATLAB with lower-case a, t, a, b. Can anybody explain, why MATLAB is written with an upper-case L frequently? I did this myself also. Has it been the official spelling in Matlab 4?
See Stevens confusion of ~= and != above.
Oh yeah I knew there was another... Writing comments with %. Grrrr
The strings 'default', 'remove', and 'factory' are not allowed as uimenu labels, see Ref: uimenu_props. To get the string 'default' as label, '\default' must be used.
This is another example for the bad magic strings. Imagine you want to create a bunch of uimenu s automatically using a list of words. Then the exception handling for these three words will be ugly and increase the program size without a real benefit.
[EDITED] The named strings have a special meaning for other handle graphics objects also, e.g. UICONTROLs:
uicontrol('String', 'default')
This doe not display 'default' in a button, but an empty string, because this is the default value:
get(0, 'defaultUIControlString')
To display 'default', 'factory', or 'remove', a leading backslash is required, see doc: setting-default-property-values:
uicontrol('String', '\default')
It would be smarter to use the backslash for the special commands, but this cannot be changed without breaking the backward compatibility.
Maybe \\default ?
What is required to display the string '\default' in an UICONTROL???
Although I never use clear all (see Answers: Bad CLEAR ALL), it appears in user-supplied code frequently. If all is defined as a variable, the behaviour might be unexpected:
clear all
b = 1;
all = 2;
clear all
whos
% Name Size Bytes Class Attributes
% b 1x1 8 double
If "all" is a variable, it is cleared, but the other variables are not touched.
[EDITED] The "clear all" behaviour is a typical example for the bad programming practize of using magic numbers. Here the string 'all' is magic, because it triggers a totally amnesia, if there is no variable of this name. It would be smarter to use invalid symbols for special commands, e.g. "$all". See Wiki: magic numbers and Wiki: magic strings.
P.S. I've also recently requested that TMW consider making the use of 'clear all' an M-lint/Analyzer warning, because it has side effects that trip people up (like clearing breakpoints & globals, flushing the M-file cache, etc). If you like the idea, call/email support and lend your vote to this enhancement request! :)
To be honest I think a lot of the functions that were created in early MATLAB versions are the ones that tend to have syntax issues like this... Since they've been around so long it would be hard for TMW to break backwards compatibility. As TMW hired more professional programmers over the years, instead of mathematicians/scientists doubling as programmers, things have improved IMHO, like your example about '-regexp'. Of course this is all personal opinion YMMV etc, and it's not intended to be a slight at all to TMW old-timers :)
@Walter: The magic strings let us make mistakes, when we forget to consider them. E.g. I forgot the escape character in a context menu, where I can move an object to its "default" position. It is in the nature of context menus, that an automatic unit-test cannot find this bug, but only an active user.
I'm sure most programmers omit a test for {'default', 'remove', 'factory'}, when they assign the string of buttons or menus dynamically.
@Jan, my point was that I don't use the CLEAR command in any of my programs. The magic strings in UIMENU is more problematic.
Magic strings are a MathWorks mistake, not a dumb mistake on our part.
The magic strings are not bound to the functional form.
"S = 'all'; clear(S);" behaves equivalently. See my next answer concerning magic strings in labels of UIMENUs.
I do not suffer from the magic string "all" for the CLEAR command, because I never overload the builtin function ALL and avoid a CLEAR('all') at all.
But the same problem concerns "clear('variables')", "clear('global')", "clear('functions')", "clear('import')" and "clear('classes')". Clearing an M-file of the same name is not possible and using the name "import" as a variable is not unlikely.
For the newer flag "-regexp" TMW has been so smart to use the minus to _unmagicalize_ the string. The same for "which <> -all" or "save -mat". The usage of magic constants without an escape character can and will lead to bugs or to large and ugly constructions to catch the exceptions.
I am not sure I see the use of magic strings in this case as particularly bad. Mostly because I don't use clear in my programming. In fact, clear is one of the few (doc, help, and edit being the others that come to mind) functions that I rarely call in its functional form.
And I should take the time to publish my functions unclear() and totallyunclear() in the FEX.
From now on I might just have to answer homework questions with code that has variable names of all and clear (and maybe builtin and str2func just to up the challenge).
have anyone tried typing "Why" in the command prompt? MATLAB gives funny responses..just carry on typing for fun.. :)
I've listed some other Easter Eggs in http://www.mathworks.com/matlabcentral/answers/2001-what-matlab-easter-eggs-do-you-know
I often use != instead of the unusual ~=
I had a "if a~=b" instead of "if a!=b" in a C-mex function. It even produced the correct result - usually.
http://zork.net/~sneakums/no-perl.svg
The process-first-non-singelton feature can be smarter than the user:
m = randi(4);
n = randi(4);
M = rand(m, n);
ColumnSum = sum(M); % DANGEROUS
If m is 1, the sum is calulated alog the 2nd dimension. Better:
ColumnSum = sum(M, 1); % Secure
NOTE: For min, max and std the 2nd input is not the dimension!
max([1, 2; 3, 4], 2) % >> [2, 2; 3, 4], elementwise maximun
max([1, 2; 3, 4], [], 2) % >> [2; 4], maximun along 2nd dimension
permute()? I'd expect reshape() to do the equivalent operation without the smart choice of the dimension.
The squeeze() command has similar dangers, when there are unanticipated singleton dimensions. In my experience, it is almost always possible, and wiser, to use permute() rather than squeeze().
Sometimes I make a typo in structure field name, for example:
my_struct.field1 = 1
and then:
my_struct.filed1 = 2
It produces:
my_struct =
field1: 1
filed1: 2
instead of:
my_struct =
field1: 2
@Walter: We can. I'm using a selftest function, which scans the source code and checks all symbols, which contain a dot. The structs are created in a dedicted function, and no other function is allowed to add further fields.
I care for a minimal edit-distance, e.g. I avoid S.mvc and S.mcv, which would be too susceptible for typos. Of course I never create fields by complicated EVAL commands or LOAD structs directly to the workspace.
Using OO methods would be a more convenient method to limit the definition of fields. But even for functional programs, OO strategies are important for writing reliable software.
Too bad we cannot "lock down" structures to prevent new fields from being added.
When I say orally to my peers to type
rehash toolboxcache
in 99 cases out of 100 they type
rehash toolboxcash
Moving into the Figure window, but in a similar spirit to the rand(100000) thing, I use this when I need an excuse to take a break...
data = randn(2, 1e6);
plot(data)
when I meant this...
plot(data')
Check if a path name contains a certain folder name anywhere
if any(findstr(PathName, '/Sub/')) ...
Because FINDSTR searches the shorter string in the longer one, the condition is true for the path name '/' also.
Modern Matlab version use STRFIND(String, Pattern) and afaik FINDSTR will be deprecated. But the program containing the example was developped under Matlab 5.3. Fixing the bug by changing FINDSTR to STRFIND is not trivial, if the strings are created dynamically, e.g. FINDSTR(a{i}, ['@', b{j}]).
At least in Matlab 2009a the toolbox functions contains a lot of these pitfalls, e.g. Signal\PMEM: "if ~isempty(findstr(flag, 'CORR')), ...", which triggers for 'OR' also.
Not so much with MATLAB, but with MATLAB answers. This one has bitten me several times.
I will edit an answer I had given earlier, in response to more questions or comments from the OP. Often the edit will be fairly lengthy, and so take some time. When I am done I check for spelling errors (usually) then hit the Submit button - at least that is what I should do. Sometimes I hit the Edit link, which causes the page to refresh! All typing lost ..... Aargh!
It would be nice if there simply was no Edit link available when editing (what is it there for anyway?), for dummies like me!
Must ... resist ...
Look at the Edit link! It is calling to you!
Sorry Andrew, at least I am not alone ;)
Haha, maybe if I bump this post I can infect more regular posters!
If they too complain, then perhaps TMW will remove the Edit button when editing is enabled...
You're a bad influence, Matt. I had never made this particular mistake - until you wrote about it!
I wanted to add the optimization flag /arch:SSE2 in my MEX call in Matlab 2009a:
mex -O OPTIMFLAGS="$OPTIMFLAGS /arch:SSE2" func.c
For a batch processing I need the functional form (or EVAL?!):
mex('-O', 'OPTIMFLAGS="$OPTIMFLAGS /arch:SSE2"', 'func.c')
>> ERROR: flag /arch:SSE2 is not recognized.
Ugly parser! The over-complicated BAT-> M-> PERL-> Compiler pipeline of the MEX command tries to be intelligent. This works:
mex('-O', 'OPTIMFLAGS="$OPTIMFLAGS', '/arch:SSE2"', 'func.c')
The string must be split inside the double quoted section, although these double quotes are used to avoid the splitting!
>> exit
instead of
>> edit
Go to the Preferences and expand the MATLAB > General > Confirmation Dialogs section. There's an entry, "Confirm before exiting MATLAB", that if you select it will bring up a dialog each time you try to quit MATLAB (normally. If you forcibly kill it from Task Manager, or if MATLAB crashes, it won't be able to show the dialog.)
Interesting. I never use EXIT, so that probably couldn't happen to me!
I've tried to start an instance of another Matlab version, but the current directory was a toolbox folder by accident:
In Matlab 6.5, Windows:
cd(fullfile(matlabroot, 'toolbox\matlab\strfun');
system('C:\Programme\Matlab\R2009a\bin\win32\MATLAB.exe &')
==> Matlab 2009a starts with a lot of warnings about finding M-files, which are builtin functions. Then the script STRNCMP (the help text only!) is attempted to execute as a function. Because this fails, the Matlab path contains just \toolbox\local. Without toolbox function the diagnosis was tedious.
I had a similar break down, after overloading STRCMP with a damaged version.
I'm glad to hear that. It is a mess and a silly waste of time. Another version of such faults:
M-script with the help text only in one folder:
\MFiles\Folder\MyFun.m
Compiled function for different platforms in specific folders, which are included in the path dynamically:
\MFiles\ForMatlab6\MyFun.dll
\MFiles\ForMatlab7\MyFun.mexw32
Now calling MyFun from the command line or another M-function fails, if \MFiles\Folder\ is the current folder: "Attempt to call the script MyFun as a function". Solution: CD to a neutral folder.
That is one mistake I have not made.
Best answer was a toss-up between the top two vote getters.
Overlaying command with variables:
plot=1:10;
plot(1:10)
ans =
1 2 3 4 5 6 7 8 9 10
Or command with commands like creating and own contrast.m function and then using MATLAB's CONTRAST function.
All this typicall leads to errors messages that are not helpful to detect the cause and only if you think of WHICH -ALL, you find it
x = 1:10; eval('plot=x;'); plot(1:10);
This give different results when run in the command window or inside a function.
And the number of commands grows all the time - especially if you have been visiting the FEX a lot.
My apologies to everyone while I learn how to edit comments. Here is my third try:
This is like David Lodge's "Humiliation Game" http://www.guardian.co.uk/books/2008/jun/13/david.lodge, except that admitting we're dumb doesn't cost us a job (unless we work for Matlab?) and everyone believes us.
Clicking on the above link might get you "page not found". If so, remove the trailing > from the URL.