# Vocabulary/curlyrtdot

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`}. y`Behead

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

Drop the leading item from `y`

}. 'abc' bc

### Related Primitives

Head (`{. y`),
Tail (`{: y`),
Curtail (`}: y`)

### More Information

1. ` }.y ` is equivalent to ` 1 }. y` .

### Details

1. The rank of ` }.y ` is the same as the rank of `y` unless `y` is an atom,
in which case ` }.y ` is an empty list (with the same type as `y`).

2. If `y` has no items, ` }.y ` is the same as `y`.

`x }. y`Drop

Rank 1 _ *-- operates on lists of x and the entirety of y --*
WHY IS THIS IMPORTANT?

Drop the leading `x` items of `y`

if `x` is negative, the *trailing* `x` items are dropped

2 }. 'abcde' cde _2 }. 'abcde' abc

### Common uses

1. Remove data from a list

datatosend =: sentlength }. datatosend NB. remove sent data from the send queue 21 }. 'This is sentence 1. This is sentence 2.' This is sentence 2.

2. (With Take `{.`)
Chop out an interval from a list

C/f `MID$` in BASIC, `substr()` in other languages

az=: 'abcdefghijklmnopqrstuvwxyz' 3 {. 5 }. az fgh

Subarray `];.0` is a better way to do this.

3. (With Take `{.`)
Split a list into head and beheaded list

split NB. defined in stdlib as... {. ,&< }. lst=: 'abcde' 2 split lst +--+---+ |ab|cde| +--+---+ 'hd lst'=: split lst hd ; lst +-+----+ |a|bcde| +-+----+

`split` is a

- Standard Library word(s) residing in the 'z'-locale.
- Defined in the factory script
`stdlib.ijs`. - View definition(s) by entering in the J session:
`open'stdlib'`

### Related Primitives

Take (`x {. y`)

### More Information

1. If `x` is a list, each atom of `x` specifies a *drop* operation
along the corresponding axis of `y`

]a =: _4 ]\ 'abcdefghijkl' abcd efgh ijkl _1 1 }. a NB. Drop the last row, and the first column bcd fgh

### Details

1. When `x` is a list, axes of `y` beyond the length of `x` are taken in full.

2. If `y` is an array, `x` must not be longer than the rank of `y`

2 3 {. i. 4 |length error

4. If `y` is an atom, it has leading axes of length 1 added to bring its rank up to the same as the length of `x`,
and then the *drop* operations are applied.

5. If `x` is empty then ` x }. y ` is the same as `y`.
This is the only way that the result of ` x }. y ` can be an atom.

6. In accordance with the description above, values in `x` that are larger than the length of the corresponding axis
cause all atoms to be dropped, leaving an empty array

Infinity (`_` or `__`) are valid values in `x` for this purpose

$ _ }. i. 4 5 0 5