Thanks, Raul, but who/what were you commenting to/on?
Cheers,
Mike
On 25/07/2015 20:11, Raul Miller wrote:
1 0 1 +/ .* 10 100 1000
1010
The dot product is arguably a key feature of J. This allows you to
multiply matrices:
1+i.2 3
1 2 3
4 5 6
7+i.3 2
7 8
9 10
11 12
(7+i.3 2)+/ .*1+i.2 3
39 54 69
49 68 87
59 82 105
Note also that you can often use %. to go the other way:
((7+i.3 2)+/ .*1+i.2 3)%.7+i.3 2
1 2 3
4 5 6
But let's save that issue for later...
Anyways, what . does is pair up the last dimension of the argument on
the left with the first dimension of the argument on the right -
combining things with the verb on the right and then applying the verb
on the left to that.
For example:
7 8 +/ .* 1+i.2 3
39 54 69
7 8 * 1+i.2 3
7 14 21
32 40 48
+/7 8 * 1+i.2 3
39 54 69
And we can do the same thing with the other rows of 7+i.3 2:
+/9 10 * 1+i.2 3
49 68 87
+/11 12 * 1+i.2 3
59 82 105
And you can see all of these in the result:
(7+i.3 2)+/ .*1+i.2 3
39 54 69
49 68 87
59 82 105
Other operations are also possible. For example we can detect
collisions between binary lists using *.
0 1 0 1 *. 0 0 1 1
0 0 0 1
Or, of course, we could do exactly the same thing with *
0 1 0 1 * 0 0 1 1
0 0 0 1
And we can check if there were any collisions using +./
+./ 0 1 0 1 * 0 0 1 1
1
So basically, it's just:
dot=:2 :'u@(v"1 _)'
And u is usually formed using the / adverb.
I hope this helps, but I suspect I'm not doing a very good job of
describing the process.
Oh well...
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