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| The {{Forge|symbolic|symbolic package}} is part of the [[Octave Forge]] project. | | The {{Forge|symbolic|symbolic package}} is part of the octave-forge project. |
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| [[Category:Octave Forge]] | | [[Category:Octave-Forge]] |
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| === Demos and usage examples === | | === Demos and usage examples === |
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| * '''I'm trying to substitute a double value into an expression. How can I avoid getting "warning: Using rat() heuristics for double-precision input (is this what you wanted?)".'''
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| In general, you should be very careful when converting floating point ("doubles") to symbolic variables, that's why the warning is bothering you.
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| <source lang="octave">
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| ## Demo of how to use a number (which was calculated in an octave
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| ## variable) in a symbolic calculation, without getting a warning.
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| ## use octave to calculate some number:
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| a = pi/2
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| ## now do some work with the symbolic pkg
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| syms x
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| f = x * cos (x)
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| df = diff (f)
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| ## Now we want to evaluate df at a:
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| # subs (df, x, a) # this gives the "rats" warning (and gives a symbolic answer)
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| ## So instead, try
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| dfh = function_handle (df)
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| dfh (a)
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| ans = -1.5708
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| ## And you can evaluate dfh at an array of "double" values:
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| dfh ([1.23 12.3 pi/2])
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| ans =
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| -0.82502 4.20248 -1.57080
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| </source>
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| * '''Demo of how to graph symbolic functions (by converting SYMBOLIC functions into ANONYMOUS functions)'''
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| <source lang="octave">
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| ## The following code will produce the same vector field plot as Figure 1.14 from Example 1.6 (pg. 39) from A Student's Guide to Maxwell's Equations by Dr. Daniel Fleisch.
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| ## Make sure symbolic package is loaded and symbolic variables declared.
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| pkg load symbolic
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| syms x y
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| ## Write a Vector Field Equation in terms of symbolic variables
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| vectorfield = [sin(pi*y/2); -sin(pi*x/2)];
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| ## Vector components are converted from symbolic into "anonymous functions" which allows them to be graphed.
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| ## The "'vars', [x y]" syntax ensures each component is a function of both 'x' & 'y'
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| iComponent = function_handle (vectorfield(1), 'vars', [x y]);
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| jComponent = function_handle (vectorfield(2), 'vars', [x y]);
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| ## Setup a 2D grid
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| [X,Y] = meshgrid ([-0.5:0.05:0.5]);
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| figure
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| quiver (X, Y, iComponent (X, Y), jComponent (X,Y))
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| </source>
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| * '''Demo of Anonymous function to symbolic function and back to anonymous function and then the use of the interval pkg.''' | | * '''Demo of Anonymous function to symbolic function and back to anonymous function and then the use of the interval pkg.''' |
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| % have fun and change it if you want to. | | % have fun and change it if you want to. |
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| f = @(x) x.^2 + 3*x - 1 + 5*x.*sin(x); | | f=@(x) x.^2 +3*x-1 + 5*x.*sin(x); |
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| % these next lines take the Anonymous function into a symbolic formula | | % the next 2 line take the Anonymous function into a symbolic formula |
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| pkg load symbolic
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| syms x; | | syms x; |
| ff = f(x); | | |
| | ff=formula(f(x)); |
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| % now calculate the derivative of the function | | % now calculate the derivative of the function |
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| ffd = diff(ff, x); | | ffd=diff(ff); |
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| % and convert it back to an Anonymous function | | % and convert it back to an Anonymous function |
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| df = function_handle(ffd) | | df=function_handle(ffd) |
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| % this uses the interval pkg to find all the roots between -15 an 10 | | % this uses the interval pkg. to find all the roots between -15 an 10 |
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| pkg load interval
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| fzero (f, infsup (-15, 10), df) | | fzero (f, infsup (-15, 10), df) |
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| [-0.777688831121563, -0.7776888311215626] | | [-0.777688831121563, -0.7776888311215626] |
| [0.22911205809043574, 0.2291120580904359] | | [0.22911205809043574, 0.2291120580904359] |
| </source>
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| * '''Demo of inputting a function at the input prompt and making an Anonymous function.'''
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| <source lang="octave">
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| # This prog. shows how to take a
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| # string input and make it into an anonymous function
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| # this uses the symbolic pkg.
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| disp("Example input")
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| disp("x^2 + 3*x - 1 + 5*x*sin(x)")
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| str_fucn=input("please enter your function ","s")
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| fucn_sym=sym(str_fucn)
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| f=function_handle(fucn_sym)
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| # now back to symbolic
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| syms x;
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| ff=formula(f(x));
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| % now calculate the derivative of the function
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| ffd=diff(ff);
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| % and convert it back to an Anonymous function
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| df=function_handle(ffd)
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| % now lets do the second derivative
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| ffdd=diff(ffd);
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| ddf=function_handle(ffdd)
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| % and now plot them all
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| x1=-2:.0001:2;
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| plot(x1,f(x1),x1,df(x1),x1,ddf(x1))
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| grid minor on
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| legend("f","f '", "f '' ")
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| </source> | | </source> |
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| ## | | ## |
| syms y(x) | | syms y(x) |
| sqrt2=sym(1.41421);
| | de =diff(y, 3 ) +sqrt(2)*diff(y,2) + diff(y) == 0; |
| de =diff(y, 3 ) +sqrt2*diff(y,2) + diff(y) == 0; | |
| f = dsolve(de, y(0) == 0, diff(y,1)(0) == 0 , diff(y,2)(0) == 1) | | f = dsolve(de, y(0) == 0, diff(y,1)(0) == 0 , diff(y,2)(0) == 1) |
| ff=function_handle(rhs(f)) | | ff=function_handle(rhs(f)) |
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| plot(x1,y) | | plot(x1,y) |
| grid minor on | | grid minor on |
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| </source> | | </source> |