Increment and Decrement operators in scheme programming language

10,429

Solution 1

They are not defined as such since Scheme and Racket try to avoid mutation; but you can easily define them yourself:

(define-syntax incf
  (syntax-rules ()
    ((_ x)   (begin (set! x (+ x 1)) x))
    ((_ x n) (begin (set! x (+ x n)) x))))

(define-syntax decf
  (syntax-rules ()
    ((_ x)   (incf x -1))
    ((_ x n) (incf x (- n)))))

then

> (define v 0)
> (incf v)
1
> v
1
> (decf v 2)
-1
> v
-1

Note that these are syntactic extensions (a.k.a. macros) rather than plain procedures because Scheme does not pass parameters by reference.

Solution 2

Your reference to “DrRacket” somewhat suggests you’re in Racket. According to this, you may already be effectively using #lang racket. Either way, you’re probably looking for add1 and sub1.

-> (add1 3)
4
-> (sub1 3)
2

Solution 3

The operators 1+ and -1+ do /not/ mutate, as a simple experiment in MIT Scheme will show:

1 ]=> (define a 3)
;Value: a
1 ]=> (1+ a)
;Value: 4
1 ]=> (-1+ a)
;Value: 2
1 ]=> a
;Value: 3

So you can implement your own function or syntactic extensions of those functions by having them evaluate to (+ arg 1) and (- arg 1) respectively.

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Updated on June 14, 2022

Comments

  • unknownerror
    unknownerror 3 months

    What are the increment and decrement operators in scheme programming language. I am using "Dr.Racket" and it is not accepting -1+ and 1+ as operators. And, I have also tried incf and decf, but no use.

  • unknownerror
    unknownerror over 5 years
    This will not change that value of variable you are passing. Have a look here: ideone.com/KEgO9g