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_ Negative Sign Other
As the first character of a numeric constant, (_) acts as a negative sign to produce a negative constant.
The two-character constant (__) combines these usages and represents Negative infinity.
The negative sign (_) is part of the number (constant) that follows it. Any noun that begins with a negative sign must be a valid number. This is different from applying Negate (-), which is a verb primitive that forms a phrase to be evaluated.
5 _3 22 5 _3 22 5 -3 22 NB. equivalent to: 5 - (3 22) 2 _17
Common uses
1. Embed negative numbers in a numeric vector or matrix.
Related Primitives
Negate (- y)
_ Infinity Noun
Standing by itself, (_) represents the value Infinity (an IEEE floating-point value).
Common uses
1. Use as a docile placeholder in a numeric vector or matrix.
]z=: 2 3$ 0 1 _ 3 4 5 0 1 _ 3 4 5 1+z 1 2 _ 4 5 6 _ = z 0 0 1 0 0 0 _ = 1+z 0 0 1 0 0 0
Note that (_) is a floating-point value and its use will force the entire array to be floating-point.
2. Specify (_) as the placeholder to be used for a badly-formed numeral when converting a string using Numbers (".).
_ ". '33 -44 55.5 66.6.6 _77' 33 _44 55.5 _ _77
Note: the use of (_) here is better-behaved in practice than Indeterminate (_.)
Details
1. Some normally-indeterminate forms involving infinity are defined in J:
- 0 * _ = 0
- 0 * __ = 0
- _ ^ 0 = 1
- 1 ^ _ = 1