(/:/:m){c0,c1
Based on an old APL idiom.
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Zsbán Ambrus
Sent: 28 January 2013 16:37
To: Programming forum
Subject: [Jprogramming] Conditional by filtering then merging an array
This question is about evaulating a conditional expression on each element
of a list.
Let's take a simple working example. Let's say we have this input,
]a =: 10+i.10
10 11 12 13 14 15 16 17 18 19
and we want to invoke one of two different operations on its elements
according to this mask:
]m =: 1 p: a
0 1 0 1 0 0 0 1 0 1
We can of course do the operations individually, like this:
m (-@])`(*:@])@.["0 a
_10 121 _12 169 _14 _15 _16 289 _18 361
Or we can compute both operations on the whole list then merge them like
this:
]b =: m} (-a) ,: *:a
_10 121 _12 169 _14 _15 _16 289 _18 361
Octave (at least a recent enough version) does support this latter method as
well:
octave> a = 10:19,
a =
10 11 12 13 14 15 16 17 18 19
octave> m = isprime(a),
m =
0 1 0 1 0 0 0 1 0 1
octave> b = ifelse(m, a.^2, -a),
b =
-10 121 -12 169 -14 -15 -16 289 -18 361
But it turns out the octavers have a different idiom for conditional
expressions, which works by breaking the list to two parts according to the
mask, invoke a list function on each part, then merge the two lists.
octave> c1 = a(m).^2, c0 = -a(~m), c = []; c(m) = c1; c(~m) = c0,
c1 =
121 169 289 361
c0 =
-10 -12 -14 -15 -16 -18
c =
-10 121 -12 169 -14 -15 -16 289 -18 361
Now breaking the list to two parts and running a list function on each part
is easy enough in J:
]c0 =: -(-.m)#a
_10 _12 _14 _15 _16 _18
]c1 =: *:m#a
121 169 289 361
But how do you merge the two resulting lists? There are some ugly ways,
such as these
]c =: (I.-.m) C.^:_1 c1,c0
_10 121 _12 169 _14 _15 _16 289 _18 361
(m#^:_1 c1) + ((-.m)#^:_1 c0) NB. works only for numeric lists
_10 121 _12 169 _14 _15 _16 289 _18 361
m} ((-.m)#^:_1 c0) ,: (m#^:_1 c1)
_10 121 _12 169 _14 _15 _16 289 _18 361
c0 (I.-.m)} m#^:_1 c1
_10 121 _12 169 _14 _15 _16 289 _18 361
but is there a nicer way to merge two lists according to a mask like this?
The mask tells which of the two lists each element of the result comes from
but with the inputs containing only as many elements as necessary?
Ambrus
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