-

Conformance: R5RS Scheme

Purpose: Subtract numbers. If only one argument is given, return its negative value.

Arguments:
A - number
B... - numbers (to be subtracted)

Implementation:

(define (- a . b)
  (letrec
    ((idiff
       (lambda (a b)
         (cond ((negative? b)
             (+ a (abs b)))    ; B<0
           ((negative? a)
             (+ a (negate b))) ; B>=0, A<0
           ((n< (abs a) (abs b))
             (negate (n- (abs b) (abs a))))  ; A>=0, B>=0, A<B
           (else (n- (abs a) (abs b))))))    ; A>=0, B>=0, A>=B
     (i-
       (lambda (a b)
         (idiff (integer a) (integer b)))))
    (cond ((null? b) (negate a))
      (else (fold-left i- a b)))))

Example:

(- 5 7) 
=> -2

See also:
digits, +, *, quotient, remainder, n-.