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Topic: rookie code issue
Replies: 3   Last Post: Mar 21, 2013 12:55 AM

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David Bailey

Posts: 714
Registered: 11/7/08
Re: rookie code issue
Posted: Mar 21, 2013 12:55 AM
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On 20/03/2013 08:22, robholman@gmail.com wrote:
> Hi folks:
>
> I'm working on a bit of code in which I want students to choose a trig function from a drop-down menu, and then from there have some choices for viewing the graph. All is working fine except for one thing. There are a couple of constants that depend on the function they choose from the menu, and I'm trying to define the constant based on their choice for f. The basic issue I'm having boils down to the following question:
>
> Why doesn't the following code return a 2?
>
>
> m = Cos;
> Which[m == Sin, 5, m == Cos, 2]
>
>
>
> Thanks for the help!
>
> Rob
>

What you get back is:

Which[Cos == Sin, 5, m == Cos, 2]

The problem is that a comparison that resolves to a symbol match - like
x==y is neither True nor False, but undecided until x and y get values -
*unless* the two symbols are the same - x==x. This leaves the whole
Which construct unevaluated!

You need structural comparisons:

Which[m === Sin, 5, m === Cos, 2]

or alternatively use Switch:

Switch[m, Sin, 5, Cos, 2]


David Bailey
http://www.dbaileyconsultancy.co.uk




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