RE: My posting about Optional command

• To: mathgroup at yoda.physics.unc.edu (Mathgroup)
• Subject: RE: My posting about Optional command
• From: Scott Herod <sherod at newton.colorado.edu>
• Date: Sat, 2 Jul 1994 23:12:02 -0600 (MDT)

```I think I figured out the problem I was having with the Optional
command.  Briefly, it was the fact that I was assigning an undefined
as the optional argument.  This variable was then receiving all
of the definitions.  Use of the Trace command details this.

In[3]:= ??f
Global`f

f[x_, y_:temp] := Block[{}, If[y == temp, y = Pi]; Print[x, y]]

In[3]:= y

Out[3]= y

In[4]:= temp

Out[4]= Pi

In[5]:= Trace[f[b]]
bPi

Out[5]= {f[b], Block[{}, If[temp == temp, temp = Pi]; Print[b, temp]],

>    {If[temp == temp, temp = Pi]; Print[b, temp],

>     {If[temp == temp, temp = Pi], {{temp, Pi}, {temp, Pi}, Pi == Pi, True},

>      temp = Pi, Pi}, {{temp, Pi}, Print[b, Pi], Null}, Null}, Null}

In[6]:= bye

As you can see, temp (not y) is being set to Pi.  The next time the function
f is loaded, temp has a value assigned to it.  Sorry to make two post
out of this.

Scott Herod