Dumb mistakes we make with MATLAB.

Entering

 >> rand(100000)

instead of

 >> rand(100000,1)

Hm. Why has my whole machine come to a grinding halt?

Trying to edit a command already visible in the Command Window:

>> x=3

x =

     3

Now put the cursor up beside x=3 and try to change it to 4:

>> 4

Oh, right, I should be down here!

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

forget to put ; in some vector and array operations and get the command line spammed.

I've been using MATLAB for 15 years or so - I teach people how to use it - and, of course, I stress the important difference between * and .* ...

So guess what once caused me a couple of hours of frustrated debugging of an image reconstruction algorithm...

Physician, heal thyself! (And don't test your algorithm on square images)

clear

Oops, I wanted to keep one of those variables.

Along the lines of the cyclist's popular mistake, I do this one often enough:

x = rand(1:100);  % I meant:  x = rand(1,100);
??? Error using ==> rand
Maximum variable size allowed by the program is exceeded.

When I say orally to my peers to type

   rehash toolboxcache

in 99 cases out of 100 they type

 rehash toolboxcash

More in keeping with the OP, and another in the "physician heal thyself" category:

[fiddling about at the command line, trying to work out some kinks in an algorithm]

x1 - x2        % looks mostly near zero, but big vector, so:
max(ans)       % oops, I meant...
max(abs(ans))  % no, not *that* ans! ::sigh::

The case of the misplaced transpose:

v1'*v2

instead of

v1*v2'

c = {'a', 'b', 'c'}
c(2) = ''
>> c = {'a', 'c')

Ups. I meant C{2} = ''. Another variant:

c = cell(1, 100);
c(:) = ''
>> c = {}

Better: c(:) = {''} to get a cell of empty strings.

The old EVAL bugs, only a little bit more concealed:

Marker = {'lank', 'rank'};  % Ankle markers
Trajectory = rand(100, 3, 2);
MagicAutoAssign(Marker, Trajectory);
plot(rank);  % FAIL

% ---------------
function MagicAutoAssign(Marker, Trajectory)
for i = 1:numel(Marker)
  assignin('caller', Marker{i}, Trajectory(:, :, i));
end

Now plotting "rank" fails, because Matlab calls the toolbox function RANK instead. But it works, if "rank" is accessed using an index:

plot(1:100, rank(:, 1));   % OK

It was even worse in Matlab < 2008a: Even "RANK" in uppercase letters was forwarded to the function in non-debug mode, but in debug mode the uppercase "RANK" was recognized as variable. So you can create an error, which disappears at debugging - worst case!

Simple conclusion: Never create variables dynamically.

    >> exit

instead of

    >> edit

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!

I often use != instead of the unusual ~=

for i = 1:3
 disp(exp(i*pi));
end

Instead of:

for i = 1:3
 disp(exp(1i*pi));
end

Rejoice - now you can do it object-oriented style!

>> ME = MException
??? Error using ==> MException
Not enough input arguments

>> ME = MException.last

ME = 

MException

Properties:
  identifier: 'MATLAB:minrhs'
     message: 'Not enough input arguments.'
       cause: {0x1 cell}
       stack: [0x1 struct]

Methods

While in the debugger, typing quit instead of dbquit kills Matlab!

(My earlier version of this answer may have been too cryptic.)

Working:

n = datenum(datestr(now, 29), 29)

Failing (Matlab 2009a):

n = datenum(datestr(now, 30), 30)
>> Failed to lookup month of year.

DATENUM(S, F) needs a string as format specifier F. If F is numerical, it is interpreted as pivot year. With F=29 DATENUM works on accident only.

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 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!

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.

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')

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

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

have anyone tried typing "Why" in the command prompt? MATLAB gives funny responses..just carry on typing for fun.. :)

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.

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.

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.

>> close al
Error using close (line 111)
Specified window does not exist. 

Dammit!