# Vocabulary/under

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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`