tic toc inside a timeit

Hi,

I have a function "test". Inside this test function, I have a for loop that goes from 1 to N. Since I want to know the average time for each iteration, I put tic /toc, and a variable called "Time_for" will give me the average time, as you can see in the code below

test(x)  
tic  
  for i=1:N
      some operations
      % code
    end
  Time_for=toc/N % this result is displayed in the command window, and not saved as an output of the function

Now, I want to compute the time execution of the function. For this purpose, in the main of the program, I use the timeit function, as you can see below

f=@()test(x);

time_function=timeit(f);

My problem is that, when I run timeit, the variable "Time_for", that should just be displayed only once, it is actually displayed 10 times. And for each of these 10 times, the value slightly changes.

Why the toc function, inside a timeit, is computed multiple times?

Best,

Maria

Answer 1

[Adam]

I've never tried outputting to screen in a function wrapped in a timeit call, but I suspect the reason is simply because timeit runs your function multiple times in order to give a more accurate estimate of the function time than if you just ran it once with a tic toc. So if you output to the screen in your function it will likely do it however many times timeit runs your function.

Before timeit I used to create my own for loop, call my function many times and take an average, make sure each time had equal conditions and all that, but now timeit takes care of that for you.

Answer 2

[Sean de Wolski]

This is actually mentioned in the tips at the bottom of the doc for timeit

http://www.mathworks.com/help/releases/R2014b/matlab/ref/timeit.html

Unexpected results occur when you call tic and toc inside of timeit. If you are going to do this, be sure to assign an output to tic

myt = tic;
mystuff
toc(myt)

Of course printing to the command line takes some time too, so I would also disable that if really trying to time the code.

[Maria]

Thank you, I have not noticed the "Tips" section.