Can I create a symbolic variable with units and assign an initial value of 0

11 views (last 30 days)
Paul Buckman on 21 Nov 2019
Commented: Walter Roberson on 7 Dec 2019
G'day, new to Matlab and just been through symbolic equations tutorial.
If I do the following:
syms dia circ u
u=sysunit
dia=10*u.m %10 is assigned with the unit of a metre
If I then say
dia = 0
dia
0 %zero is returned with no units.
Can I assign units to a sym reqardless of quantity and have the units carry through all calculations?
That is, can I define dia and circ with the units of metre or kg etc without assigning a numeric value?
Thanks and reqards
Paul

Priyanshu Mishra on 1 Dec 2019
According to MATALAB documentation, you need to assign a value to speciy unit. For more information on symunit, you may refer to the following link

Paul Buckman on 4 Dec 2019
Thank you Priyanshu
I sort of solved this by creating a function and assigning the units to the function
syms d x u
u = symunit;
fArea(d)=(pi * (d/2)^2)*u.meter^2; %Area of a circle
x = 3;
A = fArea(x);
%That returns
A =
((9*pi)/4)*[m]^2
%if I then do
>> A=0
A =
0 %no units from here on
I would humbly suggest that 0 (zero) is a valid quantity for many if not most variables. Example, velocity calculations on a four-bar linkage crank/ slider mechanism. Both the velocity and acceleration variables goto zero at some point through their position calculations; velocity goes to zero when the slider reaches its maximum and minimum travel, with acceleration going to maximum (+ or -). Motion then reverses and velocity begings to increase from zero. Therefore is a valid value and consequently it should retain its units.
Thanks and Regards
Paul

Walter Roberson on 5 Dec 2019
Units are implemented by multiplying by a symbolic variable that can be recognized by the units manipulation package. Unfortunately because it is a multiplication, when the value being multiplied by is 0, the result simplifies to just 0 -- with no unit.
You have to use cell array representation to hold the units along with a value of 0. See the temperature 0 discussion at https://www.mathworks.com/help/symbolic/units-conversion.html

Paul Buckman on 6 Dec 2019
Thank you Walter
I do accept the way it works, and I must say that this is an incredible application.
Still getting to grips with its power. Thank you one and all
Walter Roberson on 7 Dec 2019
It is a nuisance that 0 acts this way, and I filed a bug report about it years ago. I do recommend that other people file as well, as Mathworks prioritizes problems that multiple people complain about.

R2019a

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