Vocabulary/ico

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i: y

Steps

Rank 0 -- operates on individual atoms of y, producing a result that may require fill -- WHY IS THIS IMPORTANT?

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Like Integers (i.), except that the sequence ranges from (-y) thru (+y).

   i: 4
_4 _3 _2 _1 0 1 2 3 4

   i: _4
4 3 2 1 0 _1 _2 _3 _4

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More Information

1. i: y operates on each atom of y independently, and produces a list for each one.

2. If y is a complex number a+bi (ajb or a j. b in J notation), its imaginary part b gives the number of steps between -a and a i.e. the result will contain b+1 atoms. In this case a may be any number.

3. If y has no imaginary part or the imaginary part is 0, the size of each step is 1. In this case y must be an even multiple of 0.5 (using tolerant comparison).

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x i: y

Index Of Last

Rank Infinity -- operates on x and y as a whole, by items of x -- WHY IS THIS IMPORTANT?

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Like Index Of (i.), except that it finds the last occurrence not the first.

   'abracadabra' i: 'a'
10

   'abracadabra' i: 'abc'   NB. several search terms at once
10 8 4

If the item is not found, the result is still #x, same as for x i. y.

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Related Primitives

Index Of (x i. y)

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More Information

1. x i: y is a member of the i.-family.

2. The internal rank of  x i: y uses items whose rank is the rank of items of x.

3. If x and y are of different classes, or if their items couldn't possibly match because of differing shapes, no error is signaled: each search simply fails to match.

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Use These Combinations

Combinations using x i: y that have exceptionally good performance include:

What It Does	Type; Precisions; Ranks	Syntax		Primitives permitted in place of f	Variants; Restrictions	Benefits; Bug Warnings
Find last place where  x f y is true	Permitted: Boolean, integer, floating point, byte, symbol (not unicode). x and y need not be the same precision.	x (f i: 1:) y	x i:&1@:f y	= ~: < <: > >: e.	Permitted: (f!.0) (parentheses obligatory!) to force exact comparison. J recognizes FLO only if f returns an atom or list.	Avoids computing entire  x f y Bug warning: if f is e. it does (,@e.) rather than e. regardless of ranks of arguments
Find last place where  x f y is false		x (f i: 0:) y	x i:&0@:f y	= ~: < <: > >: e.

What it does	Type; Precisions; Ranks	Syntax	Variants; Restrictions	Benefits; Bug Warnings
Find first/last match		m&i. y	i: in place of i. for last match !.0 for exact comparison
Find index of first/last cell of y that does/does not match an m-item		(e. i. 1:)&m y	i: in place of i. for last cell 0: for mismatch	Bug warning: it does (,@e.) rather than e.
Find index of first/last occurrence of largest/smallest value	integer or floating-point list	(i. <./) y (i. >./) y	or i:	it actually does (i. >.!.0/) etc.; is faster than 0 ({ /:~) y

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