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Re: Sorting point-arrays by rows and columns, cont.

  • To: mathgroup at smc.vnet.net
  • Subject: [mg124091] Re: Sorting point-arrays by rows and columns, cont.
  • From: Chris Young <cy56 at comcast.net>
  • Date: Sun, 8 Jan 2012 04:28:19 -0500 (EST)
  • Delivered-to: l-mathgroup@mail-archive0.wolfram.com
  • References: <je6e3j$q6k$1@smc.vnet.net> <je96m6$j6c$1@smc.vnet.net>

It's a little hard to get used to the conventions about which slot gets 
which argument in a lot of these functions. For example, in the Help 
for FoldList, it just has an example, which doesn't even use the usual 
sort order.

Fold to the right:
In[1]:= FoldList[g[#2, #1] &, x, {a, b, c, d}]
Out[1]= {x, g[a, x], g[b, g[a, x]], g[c, g[b, g[a, x]]],
 g[d, g[c, g[b, g[a, x]]]]}

I wish the documentation would at least say what the slots are for. One 
self-documenting way to write the code is with the tee-arrow notation 
for funcitons (via Esc-fn-Esc). Then we can just put in "P" and "list" 
for the arguments and everything is a lot clearer.

In[78]:= P = hexPts;
list = {1, 2};

FoldList[
 {P, list} \[Function]
  P[[Ordering[P[[All, list]], All, LessEqual]]],
 P,
 list
 ]

Out[80]=
 {
{{0, 0}, {1, 0}, {1/2, Sqrt[3]/2}, {-(1/2), Sqrt[3]/2}, {-1, 0}, 
{-(1/2), -(Sqrt[3]/2)}, {1/2, -(Sqrt[3]/2)}},
{{-1,  0}, {-(1/2), Sqrt[3]/2}, {-(1/2), -(Sqrt[3]/2)}, {0, 0}, {1/2, 
Sqrt[3]/2}, {1/2, -(Sqrt[3]/2)}, {1, 0}},
{{-(1/2), -(Sqrt[3]/2)}, {1/2, -(Sqrt[3]/2)}, {-1, 0}, {0, 0}, {1, 0}, 
{-(1/2), Sqrt[3]/2}, {1/2, Sqrt[3]/2}}
}




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