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Substituting function value in an expression

asked 2011-06-29 23:27:56 +0200

Ondra gravatar image

updated 2011-06-29 23:40:08 +0200

I have an expression like uR(t) == 3*iL(0) + uC(0)/2 - 4

how can I substitute for iL(0) and uC(0), if I'm given, that iL(0) = 0 and uC(0) = 0.

uR, iL, uC are a functions of var t:

t = var('t')
uR = function('uR', t)
iL = function('iL', t)
uC = function('uC', t)

thank you :)

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answered 2011-07-01 01:49:26 +0200

benjaminfjones gravatar image

The simplest way is just to use the subs (or substitute) method of your symbolic expression like so:

sage: t = var('t')
sage: uR = function('uR', t)
sage: iL = function('iL', t)
sage: uC = function('uC', t)
sage: SE = uR(t) == 3*iL(0) + uC(0)/2 - 4
sage: SE.subs(iL(0)==0)
uR(t) == 1/2*uC(0) - 4
sage: SE.subs(uC(0)==0)
uR(t) == 3*iL(0) - 4
sage: SE
uR(t) == 3*iL(0) + 1/2*uC(0) - 4

You see from the last line that the object SE is not changed during the substitution, so you should assign the result of the substitution. Also, you can do both (or arbitrarily many) substitutions using a dictionary:

sage: R = SE.subs({iL(0):0, uC(0):0})
sage: R
uR(t) == -4
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works, thanks! :), *but* sage gives: __main__:4: DeprecationWarning: Substitution using function-call syntax and unnamed arguments is deprecated and will be removed from a future release of Sage; you can use named arguments instead, like EXPR(x=..., y=...) Is there an unobsolete way?

Ondra gravatar imageOndra ( 2011-07-01 04:57:49 +0200 )edit

@Ondra: Your original syntax had that problem too. It's hard to see without formatting, so I will put this in an answer, but @benjaminfjones has answered your question :)

kcrisman gravatar imagekcrisman ( 2011-07-01 10:51:43 +0200 )edit

answered 2011-07-01 10:53:14 +0200

kcrisman gravatar image

Expanding on my remark:

Functions defined in the way above don't say they have just one variable for input (their expression could, in theory, have some constants like c or a that shouldn't be substituted, only t), so we have to do this:

sage: t = var('t')
sage: uR = function('uR', t).function(t)
sage: iL = function('iL', t).function(t)
sage: uC = function('uC', t).function(t)
sage: SE = uR(t) == 3*iL(0) + uC(0)/2 -4
<no deprecation message>
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thanks a lot! :)

Ondra gravatar imageOndra ( 2011-07-01 11:11:57 +0200 )edit

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Asked: 2011-06-29 23:27:56 +0200

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Last updated: Jul 01 '11