Re: Piecewise inside a Module or Block, I don't understand this behavior.

*To*: mathgroup at smc.vnet.net*Subject*: [mg83296] Re: Piecewise inside a Module or Block, I don't understand this behavior.*From*: Jean-Marc Gulliet <jeanmarc.gulliet at gmail.com>*Date*: Sat, 17 Nov 2007 05:15:22 -0500 (EST)*Organization*: The Open University, Milton Keynes, UK*References*: <fhjs9v$4vg$1@smc.vnet.net>

W. Craig Carter wrote: > I have a Piecewise function calculated in a module: > Here is a simplifed example of something which has a > behavior that puzzles me. > > a[c_, d_] := > Module[{e, f}, e = d^2; f = c^2; > Return[Piecewise[{e, 0 < x < 1/2}, {f, 1/2 < x <= 1}]]]; > a[1,2] (*doen't return what I had anticipated*) > > (*However this does*) > a[c_, d_] := > Module[{e, f}, e = d^2; f = c^2; > Return[Piecewise[{d^2, 0 < x < 1/2}, {c^2, 1/2 < x <= 1}]]]; > a[1,2] <snip> All this is just about a syntax error you made. First, note that, as posted, none of the above examples works: they both return the same error message complaining about the erroneous structure of the argument given to Piecewise. In[1]:= a[c_, d_] := Module[{e, f}, e = d^2; f = c^2; Return[Piecewise[{e, 0 < x < 1/2}, {f, 1/2 < x <= 1}]]]; a[1, 2] a[c_, d_] := Module[{e, f}, e = d^2; f = c^2; Return[Piecewise[{d^2, 0 < x < 1/2}, {c^2, 1/2 < x <= 1}]]]; a[1, 2] During evaluation of In[1]:= Piecewise::pairs: The first argument {4,0<x<1/2} of Piecewise is not a list of pairs. >> Out[2]= 1 1 Piecewise[{e$65, 0 < x < -}, {f$65, - < x <= 1}] 2 2 During evaluation of In[1]:= Piecewise::pairs: The first argument {4,0<x<1/2} of Piecewise is not a list of pairs. >> Out[4]= 2 1 2 1 Piecewise[{2 , 0 < x < -}, {1 , - < x <= 1}] 2 2 The correct syntax for Piecewise is a list of lists. (Note that Return is useless in these functions.) The following will work as expected. In[5]:= a[c_, d_] := Module[{e, f}, e = d^2; f = c^2; Piecewise[{{e, 0 < x < 1/2}, {f, 1/2 < x <= 1}}]]; a[1, 2] a[c_, d_] := Module[{e, f}, e = d^2; f = c^2; Piecewise[{{d^2, 0 < x < 1/2}, {c^2, 1/2 < x <= 1}}]]; a[1, 2] Out[6]= 1 1 Piecewise[{{4, 0 < x < -}, {1, - < x <= 1}}] 2 2 Out[8]= 1 1 Piecewise[{{4, 0 < x < -}, {1, - < x <= 1}}] 2 2 Regards, -- Jean-Marc