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automatic substitution within functions?

asked 2010-08-25 12:04:25 -0500

ngativ gravatar image

updated 2010-08-25 12:28:39 -0500

what i do to do this?

sage: var('t w')
sage: f(t) = sin(w*t)
t |--> sin(t*w)
sage: w = 2
sage: f
t |--> sin(2*w)

Without doing f(w=2)!!! imagine that the function is f->f(a,b,c,d,f....,t) like doing this:

...
...
sage: aw1=aw1(m1=1,m2=0.5,l1=1,l2=0.5,g=9.8)
sage: aw2=aw2(m1=1,m2=0.5,l1=1,l2=0.5,g=9.8)
sage: aw4=aw4(m1=1,m2=0.5,l1=1,l2=0.5,g=9.8)
...
...

etc

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2 answers

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answered 2010-08-25 12:10:55 -0500

Mike Hansen gravatar image

updated 2010-08-25 16:03:16 -0500

The w in sin(t*w) does not refer to whatever the object with identifier w in the current scope. There isn't a way to get Python to work like this. You need to do something like:

sage: var('t, w')
(t, w)
sage: params = {w: 2}
sage: f(a,b,c,d,e,f,g,t) = sin(t*w)
sage: f.subs(params)
(a, b, c, d, e, f, g, t) |--> sin(2*t)

You just have to define the replacements you want once in a dictionary. You still have to call subs once for every function you want to change.

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thanks, :( thats exactly what i don't want. what if f->f(a,b,c,d,f....,t)? it would be a little annoying to do that.

ngativ gravatar imagengativ ( 2010-08-25 12:14:15 -0500 )edit

I've updated my answer. Could you explain the annoying part? Do you mean having to do the ".function(*f.arguments())" part?

Mike Hansen gravatar imageMike Hansen ( 2010-08-25 12:20:11 -0500 )edit

i have updated my question, its about doing f(a=1,b=2,c=3...)

ngativ gravatar imagengativ ( 2010-08-25 12:32:44 -0500 )edit

Thanks, your answer is very usefull

ngativ gravatar imagengativ ( 2010-08-25 16:54:36 -0500 )edit
2

answered 2010-08-25 15:07:58 -0500

kcrisman gravatar image

updated 2012-10-23 04:56:17 -0500

I think I'm just explaining Mike's answer a little, but I think that you should be able to define a dictionary

sage: params = {m1:1,m2:0.5,l1:1,l2:0.5,g:9.8}

and then do f.subs(params) as above. In his example,

var('t, w')
params = {w: 2,d:1,c:1}
f(a,b,c,d,e,f,g,t) = sin(t*w*d-c)
g=f.subs(params)
g

should do it. Mike, is that extra thing necessary, given that the arguments are the same in both cases?

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hi, you can prefix the preformatted text with four space characters, then they will not wrap

Evgeny gravatar imageEvgeny ( 2010-08-25 15:11:51 -0500 )edit

Okay, but then there should be much more information about the wiki formatting somewhere (FAQ?). Maybe even a little button for multiline code, sort of like the backticks for inline code? Sorry for us all requesting so many new things!

kcrisman gravatar imagekcrisman ( 2010-08-25 15:15:34 -0500 )edit

You are right, we keep track of all these issues and will fix them. Thanks. Actually the button with a whole bunch of zeroes and ones helps format both multiline and inline code.

Evgeny gravatar imageEvgeny ( 2010-08-25 15:38:18 -0500 )edit

Actually, you don't need. However, if you did `f(w=2) then that doesn't return a CallableSymbolicExpression, but `f.subs(params)` does return one. I'll update my answer.

Mike Hansen gravatar imageMike Hansen ( 2010-08-25 16:02:35 -0500 )edit

i got it now, thanks

ngativ gravatar imagengativ ( 2010-08-25 16:54:59 -0500 )edit

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Asked: 2010-08-25 12:04:25 -0500

Seen: 391 times

Last updated: Oct 23 '12