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Replacement question

• To: mathgroup at smc.vnet.net
• Subject: [mg35114] Replacement question
• From: ginak <gina02122000 at yahoo.com>
• Date: Tue, 25 Jun 2002 03:41:28 -0400 (EDT)
• Sender: owner-wri-mathgroup at wolfram.com

Suppose I have something like

 In [100] := h[3]/f[2.33342]

 Out[100] := 24.12711

and now I want to evaluate h[5]/f[2.33342], i.e. the same as in
In[100] but replaceing h[3] with h[5].  This won't work

 In [101] := In[100] /. h[3]->h[5]

 Out[101] := 24.12711

because In[100] is fully evaluated to 24.12711 before the rule is
applied.  (Generally, the expressions I'm interested in are more of a
pain to type than h[5]/f[2.33342]).  How do I tell Mathematica to evaluate
In[100] only enough to apply the given substition rule, apply the
substitution rule, and only then proceed with the evaluation?

In fact, I don't even know how to do a replacement like

 In [102] := h[3]/f[2.33342] /. h[3]->h[5]

for the same reason: the LHS is evaluated before the rule can be
applied.  (Of course, in this case this replacment task is pointless,
since it is so easy to type out the desired expression, but there are
situations in which one can obtain a complicated expression by cutting
and pasting, and wants to apply a substitution rule to the complicated
expression before Mathematica evaluates it.)

Thanks!

G.