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_

Negative Sign

Other

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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

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Common uses

1. Embed negative numbers in a numeric vector or matrix.

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Related Primitives

Negate (- y)

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_

Infinity

Noun

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Standing by itself, (_) represents the value Infinity (an IEEE floating-point value).

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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 (_.)

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Details

1. Some normally-indeterminate forms involving infinity are defined in J:

• 0 * _ = 0
• 0 * __ = 0
• _ ^ 0 = 1
• 1 ^ _ = 1

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