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An (undocumented?) feature of CompoundExpression and Out

  • To: mathgroup at
  • Subject: [mg76090] An (undocumented?) feature of CompoundExpression and Out
  • From: Szabolcs <szhorvat at>
  • Date: Wed, 16 May 2007 05:26:20 -0400 (EDT)
  • Organization: University of Bergen

I always thought that % was set to the result of the last evaluation. 
But it seems that this is not always the case.

In[1]:= 1
Out[1]= 1

In[2]:= %
Out[2]= 1

In[3]:= 2;

In[4]:= %
Out[4]= 2

In[5]:= 3;Null

In[6]:= %
Out[6]= 3

In[7]:= 4;Print[]

In[8]:= %
Out[8]= 4

In[9]:= 5;; (* Mathematica 5.2 *)

In[10]:= %
Out[10]= 5

Note that a; and a;Null are both equivalent to CompoundExpression[a, 
Null]. It seems that all those arguments at the end of 
CompoundExpression that evaluate to Null are ignored when % is set.

I never expected this behaviour, therefore I have never before tried to 
use % when the previous expression was terminated by a semicolon.

I hope that this information will be useful for someone who thought 
about ; the same way as I did. :-)


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