Re: [Jprogramming] Spare Matrix from Index Matrix

[Roger Hui]

You can use the  1$.y  verb:

   ] i=: 3 2$0 0 1 1 2 1
0 0
1 1
2 1
   ] s=: 1 (<"1 i)}1$.3 3;0 1;0
0 0 | 1
1 1 | 1
2 1 | 1

Using  1$.y  is required if the dense matrix
would have been too big.  For example:

   ] s1=: 1 (<"1 i)}1$.123456789 213456789;0 1;0
0 0 | 1
1 1 | 1
2 1 | 1

   $s1
123456789 213456789



----- Original Message -----
From: June Kim <[EMAIL PROTECTED]>
Date: Saturday, February 3, 2007 4:16 am
Subject: [Jprogramming] Spare Matrix from Index Matrix

>   ]m=: 1 0 0, 0 1 0,: 0 1 0
> 1 0 0
> 0 1 0
> 0 1 0
>   ]i=:4$.$. m
> 0 0
> 1 1
> 2 1
>   ]m2=:1 (<"1 i)} 3 3$0
> 1 0 0
> 0 1 0
> 0 1 0
>   ]s=:$.m2
> 0 0 &#9474; 1
> 1 1 &#9474; 1
> 2 1 &#9474; 1
> 
> Can I get s from i without going through the dense matrix form?
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