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Allegro CL

ANSI Common Lisp   12 Numbers   12.2 Dictionary of Numbers

 12.2.37 signum Function

Syntax:
signum number    signed-prototype

Arguments and Values:
number - a number.

signed-prototype - a number.

Description:
signum determines a numerical value that indicates whether number is negative, zero, or positive.

For a rational, signum returns one of -1, 0, or 1 according to whether number is negative, zero, or positive. For a float, the result is a float of the same format whose value is minus one, zero, or one. For a complex number z, (signum z) is a complex number of the same phase but with unit magnitude, unless z is a complex zero, in which case the result is z.

For rational arguments, signum is a rational function, but it may be irrational for complex arguments.

If number is a float, the result is a float. If number is a rational, the result is a rational. If number is a complex float, the result is a complex float. If number is a complex rational, the result is a complex, but it is implementation-dependent whether that result is a complex rational or a complex float.

Examples:
``` (signum 0)  0
(signum 99)  1
(signum 4/5)  1
(signum -99/100)  -1
(signum 0.0)  0.0
(signum #c(0 33))  #C(0.0 1.0)
(signum #c(7.5 10.0))  #C(0.6 0.8)
(signum #c(0.0 -14.7))  #C(0.0 -1.0)
(eql (signum -0.0) -0.0)  true
```

``` (signum x) ==(if (zerop x) x (/ x (abs x)))