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This question already has an answer here:

I'm having trouble defining piecewise functions. What I have so far is a function that outputs something in a suitable format for Piecewise:

envpart = getpsipieces[LUs[[1]], LUs[[2]], eenv, ms, hbar]
{{(0.707118 + 0. I) E^((-0.138441 + 0. I) x) + (0.707082 + 0.00436397 I) E^((0.138441 + 0. I) x), 
  0 < x < 0.47619}, 
 {(0.708113 + 0.0127916 I) E^((0. - 0.617118 I) (-0.47619 + x)) + 
      (0.709159 - 0.00813024 I) E^((0. + 0.617118 I) (-0.47619 + x)), 
  0.47619 < x < 0.524036}, 
 {(0.755281 + 0.00032215 I) E^((-0.138441 + 0. I) (-0.524036 + x)) + (0.661991 + 
      0.00436804 I) E^((0.138441 + 0. I) (-0.524036 + x)), 
  0.524036 < x < 1.}}

What I want to be able to do is make a function that looks like

f[x_]:=Piecewise[envpart] 

for simple plotting, integrating, etc.

When I try this, f[x0] returns Piecewise[envpart], f[x_]=Piecewise[envpart] works, but it locks up Piecewise and I want three of these types of functions working at once.

f[x_]=Piecewise[(writing out actual argument)] works perfectly, but one of the functions I want to build has hundreds of cases, so this isn't feasible.

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marked as duplicate by Kuba, Yves Klett, m_goldberg, rm -rf Feb 11 '14 at 14:13

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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Use Evaluate for this purpose:

 f[x_] := Evaluate@Piecewise[envpart]

Make sure that x has no value assigned. ClearAll[x] before will handle this.

This operation is described in Evaluate documentation. 3rd/4th example in Applications section.

Why it does not worki without it? x is scoped when creating the function and if you later provide envpart it's just different x. Evaluate provides it before it is scoped.

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  • $\begingroup$ This is exactly what I needed. Thanks for your quick response, concise solution and explanation. $\endgroup$ – user12308 Feb 11 '14 at 7:38
  • $\begingroup$ @user12308 Please take a look at the link I've just found. Do not worry if question is closed. It will stay here as a link to the generic case. $\endgroup$ – Kuba Feb 11 '14 at 7:46

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