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Re: "dereference" variable

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  • Subject: [mg82194] Re: "dereference" variable
  • From: "Ted Ersek" <ted.ersek at>
  • Date: Sun, 14 Oct 2007 06:16:02 -0400 (EDT)

With, Module, BlockTodd Johnson posted a question at:
The example he provided was convoluted.
Instead I will give simple examples that might show how to solve his problem.

Module creates local variables which are given local values. The local variables take on the specified values only after they evaluate. In the following example Hold prevents the local variable from evaluating, and this is normally not the desired result. In the output below the local variable is (s$5).

In[1]:= M1[x_]:= Module[{s=x+3}, Hold[x,s^2] ];

Out[2]= Hold[z, s$5+4]

With[{s=val},expr]  replaces each instances of (s) in (expr) with (val). This replacement is made before (expr) has a chance to evaluate. I think Todd Johnson's problem will be solved if he uses With instead of Module.

In[3]:=  M2[x_]:= With[{s=x+3}, Hold[x,s^2] ];

Out[4]=  Hold[z,(3+z)+4]

Note: When using  With[{s=val}, expr]  the value of (s) is (val) inside the With statement and you can't change it inside the With statement. On the other hand when using  Module[{x,y}, expr]  you can change the values assigned local variables (x,y) as much as you want during evaluation of (expr).


Then there is  Block[{x=0,y=y0}, expr]  which evaluates (expr) with temporary values assigned to x, and y. You can also use   Block[{x,y}, expr ]   to evaluate (expr) with no values assigned to x, and y. Another thing about Block that isn't documented is that it temporarily clears any attributes of the affected symbols. It clears the attributes even if they were Protected (you don't need to Unprotect they symbols first).

So for example in the next line the pattern matches. This is because Plus has the attributes Flat and Orderless. These attributes allow the pattern matcher to regard Plus[x,b,z] as Plus[b,Plus[x,z]].

In[5]:= MatchQ[Plus[x,b,z],Plus[b,_]]Out[5]= True

In the next line the pattern doesn't match because Block supresses all attributes of Plus.

In[6]:= Block[{Plus}, MatchQ[Plus[x,b,z],Plus[b,_]]]

Out[6]= False


Alos in the next line we give (foo) the Orderless attribute inside Block. You will notice (foo) is only Orderless inside the Block.

In[7]:= Block[{foo},Attributes[foo]={Orderless};foo[z,a,x]]

Out[7]= foo[a,x,z]


For more on this stuff see:

You could also open my Tricks notebook at
And see my sections on Evaluate and Unevaluated.

For more on working with unevaluated expressions see the notebook at

But beware that the syntax of some features such as ReplacePart is different in Mathematica 6 than it was when that notebook was written.

   Ted Ersek

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