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Re: Mantaining the same form

On Mon, 09 Jan 2012 08:21:52 -0000, Chris Young <cy56 at> wrote:

> On 2012-01-07 10:16:36 +0000, Miguel Gil said:
>> At a function in which one parameter is an expression "expr",
>> Mathematica 8.0 evaluates or simplifies the value of "expr" when it is
>> introduced.
>> How is that Mathematica keep the same form of the "expr" introduced?.
>> Example:
>> In []: MyFunction [expr_]: = expr;
>> MyFunction [(Sin [2x], Tan [y]) / (x * Sec [y]^2)]
>> Out []: (Cos [y]^2 (Sin [2x], Tan [y]) / x
>> I want to get the same expression (Sin [2x], Tan [y]) / (x * Sec [y]^2)
>> Clearly, the input expression and modified expression are equivalent,
>> but are not equal.
>> For example, if we were to apply the rule of L'Hopital or the theorem
>> Schwarz  the results would be erroneous.
> I tried everything I could think of, but couldn't write any function
> that would just pass on the expression unchanged:
> In[264]:= exprHeld[e_] := Hold[e]
> In[265]:= exprHeld[(Sin[2 x] * Tan[y])/(x * Sec[y]^2)]
> Out[265]= Hold[(Cos[y] Sin[2 x] Sin[y])/x]

You forgot your Attributes:

In[1] :=
exprHeld[e_] := Hold[e];
SetAttributes[exprHeld, HoldAll];

In[3]:= exprHeld[(Sin[2 x]*Tan[y])/(x*Sec[y]^2)]
Out[3]= Hold[(Sin[2 x] Tan[y])/(x Sec[y]^2)]

(Note that HoldAll is an Attribute, not a function.)

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