MaplePrimes - answers and comments on Question, Questions from a simple calculation with units.

Use convert for the units, auto-load packages with maple.ini

dskoog — Thu, 17 Mar 2011 22:08:06 Z

Hello,

1.I tried running your code and Pi is not automatically converted. What I would recommend rather than approximating Pi to say 5 digits is to set the interface(displayprecision=5) which will show your answer to 5 decimal places but carry out the calculations without rounding.

2. To convert units, you could try something like the following:

convert(wiredensity, 'units', Units[Standard][`*`]('g', Units[Standard][`/`]('Units[Standard][`^`](m, 3)')));

3. If you would like to automatically load certain Maple packages, have a look at the following link: http://www.maplesoft.com/support/faqs/detail.aspx?sid=32671

system CGS (centimeter-gram-second)

acer — Mon, 21 Mar 2011 19:54:08 Z

There is a predefined system of units whose base units for mass and length are gram and centimeter (see here). There are other ways to change the default units for any given dimension, but let us know if this is not adequate for you.

Maple will keep Pi as an exact quantity by default. One can use the `evalf` command to approximate it to floating-point. By default, Maple will use the `Digits` environment variable to control the working precision (see here). One can also set the display precision for Standard GUI output (see here). It's usually better to let Maple use at least its default setting of Digits=10 for computation (and to separately control only the final display) than it is to downgrade the computation precision to something as low as Digits=5 (which might allow too much roundoff error for you on more complicated expressions). Below I use `evalf[5]` on just the final result, as a programmatic way to adjust the final displayed float value.

> restart:

> with(Units:-Standard):
> Units:-UseSystem(CGS);

> wirediameter:=.40*Unit(inch):
> wirelength:=1000*Unit(ft):
> wirevolume:=(wirediameter/2)^2 * Pi * wirelength: 
> wireweight:=61*Unit(lb):
> wiredensity:=evalf(wireweight/wirevolume):

> evalf[5](wiredensity);

                         [   g     ]
                  1.1197 [ ------- ]
                         [     3   ]
                         [   cm    ]

acer

side remark: auto-load & ini file

Axel Vogt — Thu, 17 Mar 2011 23:39:31 Z

The given link into the FAQ is saying not much, instead concrete instructions should be given:

Which directory (working directory says nothing for a new user), file name and a worked example.
Some remarks when it may be not a good idea - and whether there is any need to care for the
chosen interface.

I remember that I was not able to use such ini file, when I started with Maple.

And: for Digits it does not work for classical sheets.