how to do a command line only if it is possible?

Dear all, sorry for the "stupid" question. I am writing a code and sometimes happens that i want to recall a previous setting only if it exists, so i am using try as following

try setting=previous_setting; catch end

in this way if the previous setting exists it will used, if not nothing happens. Now i think that using try and catch is not so nice since it should be used for error tracing. Do you have any advice (i would not like to use if)? thank you all catch end

in this way if the previous setting exists it will used, if not nothing happens. Now i think that using try and catch is not so nice since it should be used for error tracing. Do you have any advice (i would not like to use if)? thank you all

Answer 1

[Thorsten]

if exist('previous_setting')

end

If previous_setting is variable, use

exist('previous_setting', 1)

if it is an m-file, use

exist('previous_setting', h)

See

help exist

for further options.

[John D'Errico]

While this is a good answer (that I up-voted) there is no reason why try/catch is not acceptable. In fact, you may find that it is actually more efficient than a test for existence.

The best thing to do, is to test it out. So, here I've written a simple function to compare times.

function testfun()
%{
try
  setting=previous_setting;
catch
  setting = 17;
end
%}
if ~exist('previous_setting','var')
  setting = 17;
end

In my first test, I block commented the exist call out, using the try/catch construct.

timeit(@() testfun)
ans =
           0.0010885090595

Then I swapped the block comments, so using the exist test.

timeit(@() testfun)
ans =
      1.32021526428571e-05

So exist was faster, nearly by a factor of 100-1. But it is always a good idea to test these things.

[Luigi Bruno]

thank you for the answer. i would use the lowest number of strings in the code (this is why i dont want to use if). So if according to you there is no error to use try w/o catch i will proceed like that.

[John D'Errico]

As I pointed out in the test though, exist is faster. If this is a question of milliseconds, it may not matter. Much time is spent by programmers looking for an optimal solution, when the cost is only a few milliseconds. Of course, if the code will be executed thousands or millions of times you should worry about the difference. But for something that will happen once, even one second of programmer time spent on the choice is a net waste of time.