Vocabulary/tildeco

~: y (Nub Sieve)

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A boolean vector relating a given vector to its Nub (~.).

It has 1 for the first occurrence of each unique element, and 0 for a repeated element.

   z=: 'abracadabra'
   ~.z  NB. Nub z
abrcd
   ~:z  NB. Nub Sieve z
1 1 1 0 1 0 1 0 0 0 0

Common uses

1. Wherever data is held in 2 parallel lists, Nub Sieve (~:) yields a vector capable of pruning the second list to match the Nub (~.) of the first.

   NAME=: 'Joe' ; 'Bob' ; 'Jim' ; 'Mary' ; 'Jim'
   AGE=: 25 60 32 21 33
   
   z=. ~: NAME
   ] z=. ~: NAME        NB. "sieve" to apply to AGE
1 1 1 1 0
   ] NAME=: ~. NAME
+---+---+---+----+
|Joe|Bob|Jim|Mary|
+---+---+---+----+
   ] AGE=: z # AGE
25 60 32 21

See Also

• Nub (~.)

• Entry in the J Dictionary for ~:

x ~: y (Not-Equal)

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The boolean result of comparing two items, or lists of items.

See the J Dictionary for the tolerance rules governing equality between numbers. By default the tolerance is 2^_44 when recognizing two numbers as being "equal" under =.

   ]z=: i.5
0 1 2 3 4
   3 ~: z
1 1 1 0 1
   z ~: z
0 0 0 0 0
   z ~: |.z
1 1 0 1 1

Not-Equal (~:) works between boxed items too:

   z=: 'alpha' ; 'bravo' ; 'charlie'
   z ~: |.z
1 0 1
   z ~: <'bravo'
1 0 1

To avoid x~:y giving length error, the Shape Of ($) x and y must follow the rules for Plus (+).

Common uses

1. To construct criteria for conditional expressions in verb definitions

 if. '.' ~: y do. return. end.
NB. instead of:
 if. -. '.' = y do. return. end.

See Also

• Equal (=)

• Entry in the J Dictionary for ~:

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