A Range Function

The range function in Python is quite useful for generating lists of numbers quickly. It is often used with 'for' as a basic iterative construct. The same effect can easily be achieved with Lisp and the Loop macro. Here is an example of how range works

Python 2.2.3 (#1, Nov  8 2003, 23:39:13) 

[GCC 3.3 20030304 (Apple Computer, Inc. build 1493)] on darwin

Type "help", "copyright", "credits" or "license" for more information.

>>> range(5)

[0, 1, 2, 3, 4]

>>> range(2,10)

[2, 3, 4, 5, 6, 7, 8, 9]

>>> range(2,10,2)

[2, 4, 6, 8]

>>> range(-10,-2)

[-10, -9, -8, -7, -6, -5, -4, -3]

>>> range(-10,-2,2)

[-10, -8, -6, -4]

>>> def square (x): return x * x 

... 

>>> square



>>> for x in range(5): print square(x)

... 

0

1

4

9

16

>>> 

Here is my Lisp function for the same using Lisp with Loop

(defun range (n &rest parms)

  (case (length parms)

    ((0) (loop for x from 0 to n collect x))

    ((1) (loop for x from n to (first parms) collect x))

    (t (let ((start n)

             (stop (first parms))

             (step (second parms)))

         (loop for x = start then (+ x step) until (>= x stop)

              collect x)))))

Its a little different because I have inclusive of the highest number. That is the easiest thing to modify. here is a test

[6]> (range 5)

(0 1 2 3 4 5)

[7]> (range 2 10)

(2 3 4 5 6 7 8 9 10)

[8]> (range 2 10 2)

(2 4 6 8)

[9]> (range -10 -2)

(-10 -9 -8 -7 -6 -5 -4 -3 -2)

[10]> (range -10 -2 2)

(-10 -8 -6 -4)

[11]> (defun square (x) (* x x))

SQUARE

[12]> (loop for x in (range 5) do (print (square x)))



0 

1 

4 

9 

16 

25 

NIL

[13]> 

The Loop macro is much more powerful than the usage shown here but it can look ugly from a Lisp perspective.