avarage running time problem

Hello, i want to calculte an avarage running time amongst all 100 iterations.

for some reason my avarage running time is astronomicly high

k = 3.6888e+11

where did i go wrong

for i=1:100
tic
1+1
2+2
toc
t(i)=tic-toc;
end
k=mean(t)

Answer 1

[Adam Danz]

Here are three ways to use tic/toc to measure the timing of a loop. Each method has pros and cons.

Method 1: Least overhead & resolution

The tic() & toc() functions require processing time, albeit a very small amount. Storing the results of toc() also requires processing time. This method contains the least overhead but you don't get the single-iteration times.

start = tic(); 
for i=1:100
    1+1;
    2+2;
end
k= toc(start)/100; 

Method 2: Single iterations times

This method gives you the single-iteration times but the processing time is added by tic/toc and storing the toc output.

t = nan(1, 100); 
for i=1:100
    start = tic(); 
    1+1;
    2+2;
    t(i)=toc(start);
end
k=mean(t)

Method 3: Single iterations times, less overhead

This method removes tic() from the loop which doesn't reduce the overhead by much but it's worth sharing. The single-iteration times are computed outside of the loop.

t = nan(1, 101); 
start = tic(); 
t(1) = toc(start); 
for i=1:100
    1+1;
    2+2;
    t(i+1)=toc(start);
end
k=mean(diff(t))
% Compare that to (t(end)-t(1))/100

Reasons your timing was off

• tic() returns an integer (uint64) that has meaning only for the toc function. Example: t = tic(); t = 1943576784277
• toc() returns the elapsed time since the previous tic() which is very quick in your loop so you're subracting a tiny amount from a large, meaningless integer.
• The 1+1 and 2+2 lines display the result of each iteration in the command window since those lines are not supressed with a semicolon (;). That takes a lot of time.
• The t variable was not pre-allocated so the length of t grows with each iteration. That also takes a lot of time.

Since the computations are so simple, you'll find a wide range of mean(t) if you run that script many times. Consider using the timeit function.

[fima v]

Hello, i need to measure the time of each iteration.Why did you put the tic out side the loop?

Thanks

[Adam Danz]

I've updated my answer to show several different way to think about how the tic/toc functions can be used to time a loop. Method #2 is probably what you're looking for.

[fima v]

Thank you very much