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Re: Sort problem

On Oct 1, 5:02 am, Donald DuBois <don... at> wrote:
> 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}}

According to the documentation, the default ordering function
for Sort is OrderedQ[{#1,#2}]& (as opposed to Less, which you use).
Thus, to emulate this behavior with the restriction to the first
element, you could use:
Sort[{{r, 2}, {a, 3}, {x, 4}},
 OrderedQ[{#1[[1]], #2[[1]]}] &]

Conversely, you could force the more confusing behavior in the
first example by switching the input to
Sort[{r, a, x}, #1 < #2 &]

I suppose the reason is that Less compares numerical quantities,
while OrderedQ is more general.


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