       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:= FoldList[g[#2, #1] &, x, {a, b, c, d}]
Out= {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:= P = hexPts;
list = {1, 2};

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

Out=
{
{{0, 0}, {1, 0}, {1/2, Sqrt/2}, {-(1/2), Sqrt/2}, {-1, 0},
{-(1/2), -(Sqrt/2)}, {1/2, -(Sqrt/2)}},
{{-1,  0}, {-(1/2), Sqrt/2}, {-(1/2), -(Sqrt/2)}, {0, 0}, {1/2,
Sqrt/2}, {1/2, -(Sqrt/2)}, {1, 0}},
{{-(1/2), -(Sqrt/2)}, {1/2, -(Sqrt/2)}, {-1, 0}, {0, 0}, {1, 0},
{-(1/2), Sqrt/2}, {1/2, Sqrt/2}}
}

```

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