MaplePrimes - answers and comments on Question, Functions as matrix elements???? I want to see the algebraic expressions behind the matrix elements, but also do calculations

eval

acer — Thu, 09 Jul 2009 10:16:08 Z

This is called 2-argument eval, or evalat ("eval at", because it evaluates at some values). Notice that the instantiation can be obtained without having to actually assign to alpha. That can be useful because it leaves the original object as is, while also leaving name alpha unassigned and thus free immediately for continued symbolic use (no need to unassign it).

> eval(Result,alpha=Pi/2);
                                 [0    0    1]
                                 [           ]
                                 [1    0    0]
                                 [           ]
                                 [0    0    1]

I should mention that lowercase matrix and linalg are deprecated in favour of capitalized Matrix and LinearAlgebra.

acer

Hi, thanks a lot for your

mapleStarter — Thu, 09 Jul 2009 10:35:44 Z

Hi,

thanks a lot for your help....one further stupid question: I have noticed that If I say:

eval(Result, alpha = Pi/2), it calculates the matrix elements........

but when I say, for example, eval(Result, alpha=Pi/10), then it shows me cos(pi/10).....I mean, not that I couldnt calculate this myself, but i find it unusal...

Thanks anyway ...:)

Trig functions of rational multiples of Pi

Thomas Richard — Thu, 09 Jul 2009 12:58:24 Z

First, note that Maple interprets pi as a greek character with no special connotation. For the mathematical constant, use Pi instead.

As for the simplification of cos(Pi/2) versus cos(Pi/10), some rational multiples of Pi are treated automatically, while others are not because the resulting expression could be rather large. If you explicitly want that, try

convert(cos(Pi/10),radical);

convert, evalf

acer — Thu, 09 Jul 2009 15:54:14 Z

> R := Matrix([[cos(b/10),sin(b/10)]]);
                              [     b            b   ]
                         R := [cos(----)    sin(----)]
                              [     10           10  ]
 
> eval(R,b=evalf(Pi));
                        [0.9510565163    0.3090169944]
 
> evalf(eval(R,b=Pi));
                        [0.9510565163    0.3090169944]
 
> map(convert,eval(R,b=Pi),radical);
                     [ 1/2       1/2 1/2             1/2]
                     [2    (5 + 5   )               5   ]
                     [------------------    - 1/4 + ----]
                     [        4                      4  ]

acer