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minusp, plusp

minusp, plusp Function

Syntax:

minusp real → generalized-boolean

plusp real → generalized-boolean

Arguments and Values:

real—a real.

generalized-boolean—a generalized boolean.

Description:

minusp returns true if real is less than zero; otherwise, returns false.

plusp returns true if real is greater than zero; otherwise, returns false.

Regardless of whether an implementation provides distinct representations for positive and negative float zeros, (minusp -0.0) always returns false.

Examples:

(minusp -1) → true 
(plusp 0) → false 
(plusp least-positive-single-float) → true

Exceptional Situations:

Should signal an error of type type-error if real is not a real.

Expanded Reference: minusp, plusp​

Testing for negative and positive numbers​

minusp returns true if the real number is less than zero. plusp returns true if the real number is greater than zero. Neither returns true for zero.

(minusp -1)
=> T
(minusp 0)
=> NIL
(minusp 1)
=> NIL

(plusp 1)
=> T
(plusp 0)
=> NIL
(plusp -1)
=> NIL

With floating-point numbers​

These predicates work with all real number types, including floats.

(plusp 0.001)
=> T
(minusp -0.001)
=> T
(plusp least-positive-single-float)
=> T
(minusp most-negative-double-float)
=> T

Negative zero​

Regardless of whether an implementation supports negative zero, (minusp -0.0) always returns false.

(minusp -0.0)
=> NIL
(plusp -0.0)
=> NIL

With rationals​

minusp and plusp work with ratios as well as integers.

(plusp 1/1000000)
=> T
(minusp -1/1000000)
=> T
(plusp 0/5)
=> NIL

Practical use: classifying numbers​

These predicates are useful for branching logic based on sign.

(defun describe-sign (n)
  (cond ((plusp n) "positive")
        ((minusp n) "negative")
        (t "zero")))

(describe-sign 5)
=> "positive"
(describe-sign -3)
=> "negative"
(describe-sign 0)
=> "zero"