Cannot use expression with multiple variables

Dario Mangoni

I have two variables of 'length' type.

If I set a dimension as #woodWidth or #tireDiameter they both work fine,
but as soon as I use

#woodWidth*#tireDiameter

I get the message "Insert a valid expression"

Why is that?
I also tried with #woodWidth*1*#tireDiameter.
I also made sure to have same measurement units.

What's happening?

john_mcclary

Values with units suck, I try not to add units until the last step.

You will need to remove the units of all but 1 variable

Ex:
X=12 in
Y=2 in
Z=3 in

A=#x/in * #y/in * #z

#A=72 in


Or you can create a new variable as type 'any'
A=#x*#y*#z

#A=72 in

Dario Mangoni

Thanks a lot. Good to know!

ilya_baran

When you multiply two lengths, you get an area, not a length.  This is to help catch formulas that are wrong because units don't match up, like adding a meter to a kilogram.  See e.g. https://en.wikipedia.org/wiki/Dimensional_analysis -- the dimensional homogeneity section.

mahir

john_mcclary said:

Values with units suck, I try not to add units until the last step.

Personally, I prefer enforcing units. It's less flexible in some ways, but if done properly it saves a lot of hassle downstream. It just requires a little forethought. Scaling factors should be unitless, but actual dimensions should have real units. This way I don't need to remember whether I used metric or imperial, deg or rad. The unit conversion math happens in the background. And as @ilya_baran mentioned, it prevents you from creating nonsensical formulas with inconsistent units.

Dario Mangoni

I realized that there was an issue on my equation.
The example was just a piece of it.

I agree on the unit enforcement.

Thanks a lot