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Dumb mistakes we make with MATLAB.

Asked by Matt Fig on 22 Feb 2011

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?

5 Comments

Walter Roberson on 22 Feb 2011

Clicking on the above link might get you "page not found". If so, remove the trailing > from the URL.

Andrew Newell on 23 Feb 2011

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.

Matt Fig on 25 Feb 2011

Best answer was a toss-up between the top two vote getters.

Matt Fig

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30 Answers

Answer by the cyclist on 22 Feb 2011
Accepted answer

Entering

 >> rand(100000)

instead of

 >> rand(100000,1)

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

7 Comments

Andrew Newell on 20 Jun 2011

Did I ever get bitten by this one today. I had the OS complaining that the startup disk had run out of space for memory (600 GB?!) and telling me to close some applications.

Bjorn Gustavsson on 20 Jun 2011

My preferred version of this stumble is something along these lines:
idx = find(M(:)<this_or_that);
N = randn(idx);
instead of
N = randn(size(idx));

Naz on 5 Nov 2011

rand(10000) creates matrix of 10000x10000
rand(10000,1) creates matrix of 10000x1
In the first case you are likely to run out of memory. I think there is a default setting that defines how much memory Matlab can use.

the cyclist
Answer by Andrew Newell on 22 Feb 2011

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!

8 Comments

Jiro Doke on 22 Feb 2011

+1 vote on "editing Preview window".

Sean de Wolski on 22 Feb 2011

Andrew Newell about 2 hours ago
assert(Yup=vote)
Error: The expression to the left of the equals sign is not a valid target for an assignment.

+1 to Preview Window

Andrew Newell on 22 Feb 2011

It's a greedy algorithm.

Andrew Newell
Answer by Andreas Goser on 23 Feb 2011

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

2 Comments

Andrew Newell on 23 Feb 2011

And the number of commands grows all the time - especially if you have been visiting the FEX a lot.

Jan Simon on 31 Oct 2011

x = 1:10; eval('plot=x;'); plot(1:10);
This give different results when run in the command window or inside a function.

Andreas Goser
Answer by Paulo Silva on 22 Feb 2011

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

2 Comments

Matt Fig on 22 Feb 2011

Oh yeah. Been there, done that.

Sean de Wolski on 22 Feb 2011

I do that all of the time. But just forgetting the ';' ONCE, isn't the bad part. I use the up arrow a lot and so then I rerun the same command w/out the damn ';'. One lost semicolon can stick around for awhile.

Paulo Silva
Answer by Matt Tearle on 22 Feb 2011

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)

0 Comments

Matt Tearle
Answer by Oliver Woodford on 22 Feb 2011
clear

Oops, I wanted to keep one of those variables.

2 Comments

Paulo Silva on 22 Feb 2011

I wonder if it's possible to undo such operations?

Sean de Wolski on 22 Feb 2011

I almost never use clear. I downloaded 'keep' on the FEX and it is far better.
That is of course unless I take the "Nuke it from orbit" approach; for which I wrote a script: 'cll.m' >>clear;close all;clc

Oliver Woodford
Answer by Matt Fig on 22 Feb 2011

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.

3 Comments

Matt Tearle on 23 Feb 2011

Oh yeah, done that. Not often, but at least once... It's memorable

Matt Fig on 23 Feb 2011

I do this about twice a year. I would like to blame fat fingers, but the ":" isn't that close to the "," on the keyboard!

Jan Simon on 23 Feb 2011

@Matt Fig: Use a German keyboard where the key are neighboring. I use it to create "zeros(1:100)" frequently.

Matt Fig
Answer by Aurelien Queffurust on 15 Sep 2011

When I say orally to my peers to type

   rehash toolboxcache

in 99 cases out of 100 they type

 rehash toolboxcash

0 Comments

Aurelien Queffurust
Answer by Matt Tearle on 22 Feb 2011

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::

4 Comments

Andrew Newell on 22 Feb 2011

But the Command History window makes it so easy to patch that boo-boo!

Matt Tearle on 23 Feb 2011

As I reenter commands I chide myself "thought you'd save those few keystrokes, didn't you..."

Daniel Armyr on 20 Jun 2011

Yeah, I was tought not to use ans by some teacher way back. Using ans simply gets you into trouble.

Matt Tearle
Answer by Andrew Newell on 22 Feb 2011

The case of the misplaced transpose:

v1'*v2

instead of

v1*v2'

2 Comments

Matt Tearle on 22 Feb 2011

especially when these are very long row vectors... and there's no semicolon at the end of that line.

Andrew Newell on 22 Feb 2011

At least if there's no semicolon, you notice right away!

Andrew Newell
Answer by Jan Simon on 22 Feb 2011
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.

0 Comments

Jan Simon
Answer by Jan Simon on 22 Feb 2011

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.

0 Comments

Jan Simon
Answer by Michael Quinn on 4 Mar 2011
    >> exit

instead of

    >> edit

1 Comment

Matt Fig on 4 Mar 2011

Interesting. I never use EXIT, so that probably couldn't happen to me!

Michael Quinn
Answer by Matt Fig on 1 May 2011

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!

4 Comments

Matt Fig on 2 May 2011

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...

Matt Fig on 2 May 2011

Look at the Edit link! It is calling to you!

Andrew Newell on 2 May 2011

Must ... resist ...

Matt Fig
Answer by Steven on 4 Nov 2011

I often use != instead of the unusual ~=

2 Comments

Walter Roberson on 4 Nov 2011

http://zork.net/~sneakums/no-perl.svg

Jan Simon on 4 Nov 2011

I had a "if a~=b" instead of "if a!=b" in a C-mex function. It even produced the correct result - usually.

Steven
Answer by Knut on 5 Mar 2013
for i = 1:3
 disp(exp(i*pi));
end

Instead of:

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

0 Comments

Knut
Answer by Andrew Newell on 22 Feb 2011

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

2 Comments

Matt Fig on 22 Feb 2011

Except I was using MATLAB 7.01!

Jiro Doke on 22 Feb 2011

Even with this, you can make the same mistake:

>> MException.lst
??? No appropriate method, property, or field lst for class MException.

>> MException.last
ans =
MException

Properties:
identifier: 'MATLAB:noSuchMethodOrField'
message: 'No appropriate method, property, or field lst for class MException.'
cause: {}
stack: [0x1 struct]
Methods

Andrew Newell
Answer by Andrew Newell on 22 Feb 2011

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

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

0 Comments

Andrew Newell
Answer by Jan Simon on 22 Feb 2011

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.

0 Comments

Jan Simon
Answer by Jan Simon on 3 Mar 2011

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.

2 Comments

Matt Fig on 3 Mar 2011

That is one mistake I have not made.

Jan Simon on 3 Mar 2011

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.

Jan Simon
Answer by Jan Simon on 4 Apr 2011

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!

0 Comments

Jan Simon
Answer by Jan Simon on 27 May 2011

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.

0 Comments

Jan Simon
Answer by Ben Mitch on 27 May 2011

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')

0 Comments

Ben Mitch
Answer by Grzegorz Knor on 15 Sep 2011

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

2 Comments

Walter Roberson on 15 Sep 2011

Too bad we cannot "lock down" structures to prevent new fields from being added.

Jan Simon on 15 Sep 2011

@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.

Grzegorz Knor
Answer by Jan Simon on 31 Oct 2011

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

2 Comments

the cyclist on 31 Oct 2011

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().

Jan Simon on 31 Oct 2011

permute()? I'd expect reshape() to do the equivalent operation without the smart choice of the dimension.

Jan Simon
Answer by Shatrughan on 4 Nov 2011

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

1 Comment

Image Analyst on 4 Feb 2012

I've listed some other Easter Eggs in http://www.mathworks.com/matlabcentral/answers/2001-what-matlab-easter-eggs-do-you-know

Shatrughan
Answer by Jan Simon on 4 Feb 2012

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.

9 Comments

Jan Simon on 5 Mar 2013

@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.

Eric Sampson on 24 Apr 2013

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 :)

Eric Sampson on 24 Apr 2013

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! :)

Jan Simon
Answer by Jan Simon on 8 May 2012
Edited by Jan Simon on 16 Sep 2012

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.

2 Comments

Jan Simon on 24 Apr 2013

What is required to display the string '\default' in an UICONTROL???

Walter Roberson on 24 Apr 2013

Maybe \\default ?

Jan Simon
Answer by Geoff on 10 May 2012

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.

4 Comments

Jan Simon on 10 May 2012

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.

Daniel on 10 May 2012

I believe the name at one point meant Matrix Laboratory

Sean de Wolski on 10 May 2012

It still does!

Geoff
Answer by Kye Taylor on 24 Apr 2013
>> close al
Error using close (line 111)
Specified window does not exist. 

Dammit!

0 Comments

Kye Taylor

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