Re: With[{software=Mathematica}, Frustration]

*To*: mathgroup at smc.vnet.net*Subject*: [mg24341] Re: With[{software=Mathematica}, Frustration]*From*: "Atul Sharma" <atulksharma at yahoo.com>*Date*: Sun, 9 Jul 2000 04:52:47 -0400 (EDT)*References*: <8k3of0$428@smc.vnet.net>*Sender*: owner-wri-mathgroup at wolfram.com

In the help browser (paste and evaluate NotebookOpen[$TopDirectory <> "\\Documentation\\English\\MainBook\\2_06.nb"]), you will see the description of Block vs. Module. 'With' behaves like Module in this regard: " the variable x in a module such as Module[ {x} , body] is always set up to refer to a unique symbol, different each time the module is used, and distinct from the global symbol x. The x in a block such as Block[{x} , body] is, however, taken to be the global symbol x. What the block does is to make the value of x local. The value x had when you entered the block is always restored when you exit the block." As a result, since c is defined globally outside your 'With' statement, it is not the same as the c referred to within the statement. While you could choose to define c locally, Block will give you the desired result. c: = a b Block[{a = 2, b = 3}, c] 6 In the event that a or b have previously been assigned different values, the delayed set := will avoid the following problem a = 10; c = a b Block[{a = 2, b = 3}, c] 10 b 30 a = 10; c := a b Block[{a = 2, b = 3}, c] 6 HTH Atul AES wrote in message <8k3of0$428 at smc.vnet.net>... >Pages 359-360 of The Mathematica Book says (admittedly, taken a little >out of context), > > "You can think of With as a generalization of the /. operator. . ." > >and > > " With[{x=x0}, body] works essentially like body /. x->x0 . . . " > >Great, looks neat, let's try it for evaluating expressions without >permanently setting the variables in them: > > In[1]:= c = a b > > Out[1]= a b > > In[2]:= c > > Out[2]= a b > > In[10]:= c /. {a -> 2, b -> 3} > > Out[10]= 6 > > In[3]:= With[{a = 2, b = 3}, c] > > Out[3]= a b > >*Not* what I was hoping for . . . >