Re: Mystifying Scoping of Piecewise Variable?

*To*: mathgroup at smc.vnet.net*Subject*: [mg116136] Re: Mystifying Scoping of Piecewise Variable?*From*: DrMajorBob <btreat1 at austin.rr.com>*Date*: Thu, 3 Feb 2011 05:29:37 -0500 (EST)

(BUG ALERT. See the last line of code.) The function myFunc[y_] = Piecewise[{{x^2, x < 0}, {x, x >= 0}}] depends only on y on the left, but it depends only on x on the right. That obviously cannot work, but how, exactly, should we expect it to fail? If x has no value, Piecewise has no numeric value: x=. Piecewise[{{x^2,x<0},{x,x>=0}}] \[Piecewise] x^2 x<0 x x>=0 0 True so the first plot is empty. In the second plot, x is given values across the range from -10 to +10, so Piecewise also gets numeric values: Table[myFunc[x], {x, -10, 10, 1}] {100, 81, 64, 49, 36, 25, 16, 9, 4, 1, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, \ 10} hence the second plot is not empty. That doesn't mean your definition of myFunc is suddenly a good one. It's still completely wrong. But it works when you're passing x to the function AND varying x at the same time. So this plot is empty (because x still has no value): Plot[myFunc[x], {y, -10, 10}] But this plot is a constant: x = -7; Plot[myFunc[x], {y, -10, 10}] When passing z and varying z, we see what I'd call a bug: z =. Plot[myFunc[z], {z, -10, 10}] It plots the value of myFunc[-7] from the previous plot, even though x is not involved, and z doesn't have a value. Bobby On Wed, 02 Feb 2011 05:08:08 -0600, Frank Iannarilli <frankeye at cox.net> wrote: > Perhaps I'm tired, but this is weird: > > Clear[myFunc] > > myFunc[y_] = Piecewise[{{x^2, x < 0}, {x, x >= 0}}] > (Same behavior below for either Set(=) or SetDelayed(:=) ) > > Plot[myFunc[y], {y, -10, 10}] > (returns empty plot) > > Plot[myFunc[x], {x, -10, 10}] > (returns "expected" piecewise function plot) > > What is the scoping of the Piecewise variable, in this case "x"? > > How can one write a Module that creates and returns a Piecewise > function? The scope of its argument is unclear to me. > > Thanks > -- DrMajorBob at yahoo.com

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**Re: Mystifying Scoping of Piecewise Variable?**

**Re: Mystifying Scoping of Piecewise Variable?**