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MathGroup Archive 2007

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Re: Sort problem

  • To: mathgroup at smc.vnet.net
  • Subject: [mg81739] Re: Sort problem
  • From: Szabolcs Horvát <szhorvat at gmail.com>
  • Date: Tue, 2 Oct 2007 05:46:59 -0400 (EDT)
  • References: <fdqd33$n6m$1@smc.vnet.net>

Donald DuBois wrote:
> Hello,
> 
> This Sort works
> 
> In[32]:= Sort[{r, a, x}]
> 
> Out[32]= {a, r, x}
> 
> but this one doesn't:
> 
> In[33]:= Sort[{{r, 2}, {a, 3}, {x, 4}}, #1[[1]] < #2[[1]] &]
> 
> Out[33]= {{r, 2}, {a, 3}, {x, 4}}
> 
> 
> What am I doing wrong?
> 

This is because "r < a" does not evaluate to False or True.  You don't 
need anything special to make Sort[] work in this example:

In[1]:= Sort[{{r, 2}, {a, 3}, {x, 4}}]
Out[1]= {{a, 3}, {r, 2}, {x, 4}}

The correct version of In[33] from your message would be:

In[2]:= Sort[{{r, 2}, {a, 3}, {x, 4}}, OrderedQ[{#1[[1]], #2[[1]]}] & ]
Out[2]= {{a, 3}, {r, 2}, {x, 4}}

-- 
Szabolcs


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