MATLAB Answers

Dumb mistakes we make with MATLAB.

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Matt Fig
Matt Fig on 22 Feb 2011
Commented: Rik on 14 Aug 2018
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?

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Walter Roberson
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
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
Matt Fig on 25 Feb 2011
Best answer was a toss-up between the top two vote getters.

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Accepted Answer

the cyclist
the cyclist on 22 Feb 2011
Entering
>> rand(100000)
instead of
>> rand(100000,1)
Hm. Why has my whole machine come to a grinding halt?

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Bjorn Gustavsson
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
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.
Steven Lord
Steven Lord on 21 Sep 2016
The array size limit preference introduced in release R2015a can help avoid that.

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More Answers (31)

Andrew Newell
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!

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jiro
jiro on 22 Feb 2011
+1 vote on "editing Preview window".
Sean de Wolski
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
Andrew Newell on 22 Feb 2011
It's a greedy algorithm.

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Paulo Silva
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
Matt Fig on 22 Feb 2011
Oh yeah. Been there, done that.
Sean de Wolski
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.

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Andreas Goser
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
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
Jan 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.

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Oliver Woodford
Oliver Woodford on 22 Feb 2011
clear
Oops, I wanted to keep one of those variables.

  2 Comments

Paulo Silva
Paulo Silva on 22 Feb 2011
I wonder if it's possible to undo such operations?
Sean de Wolski
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

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Matt Tearle
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)

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Matt Tearle
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::

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Andrew Newell
Andrew Newell on 22 Feb 2011
But the Command History window makes it so easy to patch that boo-boo!
Matt Tearle
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
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.

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Matt Fig
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
Matt Tearle on 23 Feb 2011
Oh yeah, done that. Not often, but at least once... It's memorable
Matt Fig
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
Jan on 23 Feb 2011
@Matt Fig: Use a German keyboard where the key are neighboring. I use it to create "zeros(1:100)" frequently.

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Aurelien Queffurust
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

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Jan
Jan 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.

  1 Comment

Rik
Rik on 14 Aug 2018
That really is a terrible debug situation...

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Matt Fig
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!

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Matt Fig
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
Matt Fig on 2 May 2011
Look at the Edit link! It is calling to you!
Andrew Newell
Andrew Newell on 2 May 2011
Must ... resist ...

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Andrew Newell
Andrew Newell on 22 Feb 2011
The case of the misplaced transpose:
v1'*v2
instead of
v1*v2'

  2 Comments

Matt Tearle
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
Andrew Newell on 22 Feb 2011
At least if there's no semicolon, you notice right away!

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Jan
Jan on 22 Feb 2011
Edited: Jan on 2 Oct 2017
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.

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Michael Quinn
Michael Quinn on 4 Mar 2011
>> exit
instead of
>> edit

  2 Comments

Matt Fig
Matt Fig on 4 Mar 2011
Interesting. I never use EXIT, so that probably couldn't happen to me!
Steven Lord
Steven Lord on 2 Oct 2017
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.)

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Steven
Steven on 4 Nov 2011
I often use != instead of the unusual ~=

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Walter Roberson
Walter Roberson on 4 Nov 2011
http://zork.net/~sneakums/no-perl.svg
Jan
Jan 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.

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Knut
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

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Adam
Adam on 21 Sep 2016
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.

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Andrew Newell
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
Matt Fig on 22 Feb 2011
Except I was using MATLAB 7.01!
jiro
jiro 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

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Andrew Newell
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.)

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Jan
Jan 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.

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Jan
Jan 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
Matt Fig on 3 Mar 2011
That is one mistake I have not made.
Jan
Jan 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.

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Jan
Jan 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!

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Jan
Jan 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.

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Ben Mitch
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')

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Grzegorz Knor
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
Walter Roberson on 15 Sep 2011
Too bad we cannot "lock down" structures to prevent new fields from being added.
Jan
Jan 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.

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Jan
Jan 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
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
Jan on 31 Oct 2011
permute()? I'd expect reshape() to do the equivalent operation without the smart choice of the dimension.

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Shatrughan
Shatrughan on 4 Nov 2011
have anyone tried typing "Why" in the command prompt? MATLAB gives funny responses..just carry on typing for fun.. :)

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Image Analyst
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

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Jan
Jan 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.

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Jan
Jan 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
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
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! :)

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Jan
Jan on 8 May 2012
Edited: Jan 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.

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Jan
Jan on 24 Apr 2013
What is required to display the string '\default' in an UICONTROL???

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Geoff
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.

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Jan
Jan 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 Shub
Daniel Shub on 10 May 2012
I believe the name at one point meant Matrix Laboratory

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

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Jan
Jan on 3 Oct 2017
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

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