What is the difference between the effect of clear and clearvars?

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What is the difference between the effect of clear and clearvars?
  1 Comment
Jan
Jan on 29 Jan 2013
I love to see, that somebody cares about the details of the clear command, because I've seen too many clear all without any use.

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Accepted Answer

Shashank Prasanna
Shashank Prasanna on 29 Jan 2013
clearvars just clears variables you specify, in the way you specify.
clear is more powerful in the sense you can clear variables, variables, functions, compiled scripts, MEX-functions
Check our the doc for more info:
  4 Comments
Yudhishthir
Yudhishthir on 17 Dec 2023
@Stephen23 is there a way to provide a hint to the jit that a variable is no longer used? Or it already knows that ? Is this true for the GPU memory variables as well?
Stephen23
Stephen23 on 18 Dec 2023
"is there a way to provide a hint to the jit that a variable is no longer used?"
Yes: CLEAR or CLEARVARS. But most of the time this is not required:
"Or it already knows that ?"
The MATLAB engine tracks the scoping of all variables and knows when they are no longer accessible.

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More Answers (3)

Walter Roberson
Walter Roberson on 30 Jul 2021
@Kien Pham asked above whether there was a timing difference between clear and clearvars.
The answer astonished me: there is a big timing difference between the two!!
In the below code, I generate random assignment statements and write them to files. The files differ only in function name, and in whether they use "clear" or "clearvars" or have no clearing (keeping in mind that local variables are supposedly cleared when a function returns, so using a function is another form of clearing.)
Because the exact same assignment statements are used, there will be differences between the variations due to differences in how long the clearing takes; there should also be normal execution variances, so if the values are close then it is worth testing repeatedly to see if any particular relative order is accidental.
Note that it would normally be expected by programmers that the great majority of the execution time would be due to the assignment statements. Programmers tend to expect that clearing variables is fast. Mathworks does however warn that if you have a loop with a local variable that is being overwritten, then clearing the local variable is not good for performance
The results are:
  • not clearing is fastest
  • using "clear" is about 14 times slower -- too much difference for it to be due to chance
  • using "clearvars" is much slower. In this test, 25000 times slower!! And I could tell from my tests that the time taken is proportional to roughly the square of the number of variables !!
When I analyze the code for clearvars, I can see that in the case that no argument is provided, that it would execute
evalin('caller', 'clear -regexp ^.')
which tells us that the problem isn't exactly with clearvars, but rather that the speed problem is when using clear with -regexp .
The poor performance in this case is astounding, and I will create a support case about it.
N = 12000;
tn = tempname;
tn1 = tn + "clear.m";
tn2 = tn + "clearvars.m";
tn3 = tn + "noclear.m";
[folder, fn1, ext] = fileparts(tn1);
[~, fn2, ~] = fileparts(tn2);
[~, fn3, ~] = fileparts(tn3);
addpath(folder)
[fid1, msg] = fopen(tn1, 'w');
if fid1 < 0; error('Failed to open file "%s" because "%s"', tn1, msg); end
cleanme1 = onCleanup(@()delete(tn1));
[fid2, msg] = fopen(tn2, 'w');
if fid2 < 0; error('Failed to open file "%s" because "%s"', tn2, msg); end
cleanme2 = onCleanup(@()delete(tn2));
[fid3, msg] = fopen(tn3, 'w');
if fid3 < 0; error('Failed to open file "%s" because "%s"', tn3, msg); end
cleanme3 = onCleanup(@()delete(tn3));
fprintf(fid1, "function %s\n", fn1);
fprintf(fid2, "function %s\n", fn2);
fprintf(fid3, "function %s\n", fn3);
AL = 'A':'Z';
rn = AL(randi(length(AL), N, 62));
rv = compose(" = %.15g;", rand(N,1));
rl = rn + rv;
fprintf(fid1, "%s\n", rl);
fprintf(fid2, "%s\n", rl);
fprintf(fid3, "%s\n", rl);
fprintf(fid1, "\nclear\n");
fprintf(fid2, "\nclearvars\n");
fprintf(fid3, "\n");
fclose(fid1); fclose(fid2); fclose(fid3);
clear(fn1); clear(fn2); clear(fn3); %needed because m files were modified
dir(folder)
. hsperfdata_wguser tp06f0eb7b_8a00_4a71_9ec0_ee7b62775d90noclear.m .. jetty-0.0.0.0-8080-worker-webapp-_-any- worker.properties Editor_yhfth matlabpref ws_override.properties GDS-CACHE tp06f0eb7b_8a00_4a71_9ec0_ee7b62775d90clear.m hsperfdata_mcguser tp06f0eb7b_8a00_4a71_9ec0_ee7b62775d90clearvars.m
which(fn1)
/tmp/tp06f0eb7b_8a00_4a71_9ec0_ee7b62775d90clear.m
which(fn2)
/tmp/tp06f0eb7b_8a00_4a71_9ec0_ee7b62775d90clearvars.m
which(fn3)
/tmp/tp06f0eb7b_8a00_4a71_9ec0_ee7b62775d90noclear.m
fh1 = str2func(fn1);
fh2 = str2func(fn2);
fh3 = str2func(fn3);
fprintf('timing with clear\n');
timing with clear
timeit(fh1, 0)
ans = 0.0043
fprintf('timing with clearvars\n');
timing with clearvars
timeit(fh2, 0)
ans = 8.2146
fprintf('timing with no clear or clearvars\n');
timing with no clear or clearvars
timeit(fh3, 0)
ans = 2.9454e-04
  5 Comments
Walter Roberson
Walter Roberson on 31 Jul 2021
Edited: Walter Roberson on 1 Aug 2021
Use clear or clearvars:
  • outside of any function, when you are at the command prompt and want to reset your MATLAB session because you are done with the previous task and want to start a new task
  • outside of any function, when you are at the command prompt, and you are working through debugging a script and you are worried that changes you made in the code are incompatible with values you already have in your workspace and your code does not have proper initialization
  • inside of a function when you have finished dealing with a variable and you do not have enough memory to store that variable and the next variable you need. If you are not short on free space, much of the time it is better to leave the variable in place.
  • Every once in a while, it can be easier to write code that depends upon whether a variable exists at all, and so sometimes you clear a variable so that the code recognizes that situation. However, most of the time this situation is better handled by setting a flag saying whether to do that processing or not
  • Sometimes you might be paying for memory use in a circumstance where you no longer need a large variable because you are finished with that phase of the program; in such a case it might be worth clearing the variable. However, typically it would be better to write such code as functions so that the unused variables are automatically removed.
Jan
Jan on 1 Aug 2021
Thanks for this detailed explanation. It would be useful to find such hints in doc clervars .

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Jab re
Jab re on 19 Jan 2017
if you define a global variable, clearvars will remove it as well, however, somehow its value is still accessible in the memory and next time you define a global variable with the same name, the value of the previously removed variable will be assigned to the new one! Which is very strange!
global Test
Test = 10;
clearvars
if you check the workspace after above, you will see there is no variable called Test at this point, however:
global Test
display(Test)
Now Test has a value of 10 again! This strange behavior does not happen if you use clear all to remove the variables.
I am not sure if it is a bug or something designed, but clearvars did cause me problems and I found it the hard way!
  4 Comments
DZ
DZ on 29 Jul 2021
Because you need the (-)global flag to remove global variables. Or you can make your example an implicit complaint about the lack of alternative to global variables.

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Fady Dawoud
Fady Dawoud on 28 Sep 2021
Edited: Fady Dawoud on 28 Sep 2021
I also would like to know the answer perhpas from a mathwork developer privy to the underlying code differences. I am encountering a scenario where if I use 'clearvars' from within a running script, memory is not released (as seen on task manager) but when I stop script and run 'clearvars' manually (using F9), magically memory is released. I tried the same with 'clear all' and in both cases (within running script vs manually) memory is released.
I know there are other ways to re-write code to be more memory efficient (most obviously using functions instead of scripts) but it would be helpful to know the difference in mechanics between 'clear all' and 'clearvars'.
  4 Comments
Fady Dawoud
Fady Dawoud on 30 Sep 2021
Thanks Walter for the suggestion. I also tried 'pack' but it didn't seem to make a difference if called from within running script or from command line. I also tried other clear('itemtype') with arguments functions, global and import with no luck.

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