# Display precision vs actual precision

3 views (last 30 days)
Mario Trevino on 30 Jul 2014
Commented: Seth Wagenman on 3 Sep 2014
Hi everyone We are running some simulations in MATLAB and storing the results afterwards. Yet, we are concerned that the actual precision of the number format used to solve the equations might be huge with respect to what we actually need. For instance, one of the output variables of the program is voltage, which is bound to -120<=V<=40 and we require a max resolution of about 0.01, no more than that....my guess is that the number precision provided by MATLAB is exceeding our needs and this is costing us computing time.....
thanks!
mario
##### 2 CommentsShowHide 1 older comment
Seth Wagenman on 3 Sep 2014
Do the toy problems below convince you that speed will increase if you use single variables vs. doubles?

Image Analyst on 2 Aug 2014
Edited: Image Analyst on 2 Aug 2014
Your only choices are to cast your variables to single or double. The calculations will take the same amount of time no matter if you use "format long" or "format short" - the number of decimal places upon output do not have any effect on what's being used in the mathematical operations. Using single will cut your memory usage down by half.
Using single will make the calculations faster. See this demo:
m1 = rand(12345,12345);
m2 = rand(size(m1));
tic;
for expt = 1 : 10
m3 = m1 .* m2;
end
toc;
m1 = single(m1);
m2 = single(m2);
tic;
for expt = 1 : 10
m4 = m1 .* m2;
end
toc;
The results:
Elapsed time is 2.756050 seconds.
Elapsed time is 1.518655 seconds.
##### 2 CommentsShowHide 1 older comment
Seth Wagenman on 3 Sep 2014
Edited: Seth Wagenman on 3 Sep 2014
I do not know what "expt" and the loop was for (and my computer froze when I tried the script above) so I tried this and got similar results:
m1 = rand(999,999);
m2 = rand(size(m1));
tic
m3 = m1 .* m2;
toc
m1 = single(m1);
m2 = single(m2);
tic
m4 = m1 .* m2;
toc
Elapsed time is 0.011342 seconds.
Elapsed time is 0.007512 seconds.

Andrew Reibold on 30 Jul 2014
Edited: Andrew Reibold on 30 Jul 2014
To round to the hundredths place
X_rounded_to_hundredth = round(X*100)/100
Matlab still stores '0's after though. Not sure if you can increase computing time.
Seth Wagenman on 3 Sep 2014
One problem with single variables...if they are ever part of any future calculation, all products become single variables as well, even if a double were part of the calculation.

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