Recursion for a certain amount of time

October 20, 2009
by Lee Spector (lspector)

I was asked about whether it was possible to use the time to terminate an otherwise endless recursion, and I thought the answer might interest other people as well:

Sure. There are a couple of ways.

If you are flexible enough in your interpretation of “time” and “how long it’s been running” then you could do something very simple by setting up a step counter, something like this:

(define ticks 0)

(define my-function
 (lambda ()
   (unless (>= ticks 40)
     (print '*) ;; or whatever else you want to do
     (set! ticks (+ ticks 1))


If you really mean clock time then you can use the function current-seconds. This takes no arguments and it “Returns the current time in seconds. This time is always an exact integer based on a platform-specific starting date (with a platform-specific minimum and maximum value).”

The upshot of this is that (current-seconds) will return a big number but the differences between its values in subsequent calls will reflect the number of seconds called. So you could do something like this:

(define starting-time (current-seconds))

(define print-stuff-for-5-seconds
 (lambda ()
   (print '*)
   (unless (> (- (current-seconds) starting-time) 4)


If you need more resolution there’s also (current-milliseconds).

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