Graphing and converting sym p to long (or double) p.

>>x=linspace(-1,1,100) >>p = 1/26 + 25/26*x- (25/26*x-25/26)*x + ((625/1313*x-1250/1313)*x-625/1313)*x >> plot(x,double(p))

??? Error using ==> eval Undefined function or variable 'x'. Error in ==> sym.double at 45 D = reshape(eval(X),siz);

Questions: (1) How do I convert sym x (lower case variable x) to long (because I wanted it to be to the 16th decimal places)? (2) How do I get this Polynomial expression called 'p' to graph along a linspace between -1 and 1 of 100 intervals between them?

[Andrew Newell]

@Brutus, tags are supposed to be really short (one or two words).

Answer 1

[Brutus Egor]

Thank you so much. You answered 2 questions in 1 command. You R-O-C-K!!!

[Christopher Creutzig]

You really should click the “accept answer” next to Walter's answer, not next to your own comment.

[Walter Roberson]

I figure it balances out the times when I get a simple reference to the FAQ Accepted ;-)

Answer 2

[Walter Roberson]

The code you show is not your actual code. Your actual code would give an error,

 ??? Error using ==> mtimes
 Inner matrix dimensions must agree.

In order to fix that you would have to use .* instead of * in your definition of p.

The code you show does not use symbolic variables at all.

The function you give cannot be expressed accurately to 16 decimal places using double precision floating point numbers -- only to 15 1/2 decimal places. Generally speaking, if you need more than 15 decimal places you need to use symbolic numbers.

If what you have is

x = linspace(-1,1,100);
sym X
p = 1/26 + 25/26.*X- (25/26.*X-25/26).*X + ((625/1313.*X-1250/1313).*X-625/1313).*X;

then you can use:

plot(x, double(subs(p,x,X)));

[Brutus Egor]

Thank you so much. You answered 2 questions in 1 command. You R-O-C-K!!!