Is there an equivalent to Lisp's “runtime” pri

问题:

According to SICP section 1.2.6, exercise 1.22:

Most Lisp implementations include a primitive called runtime that returns an integer that specifies the amount of time the system has been running (measured, for example, in microseconds).

I'm using DrScheme, where runtime doesn't seem to be available, so I'm looking for a good substitute. I found in the PLT-Scheme Reference that there is a current-milliseconds primitive. Does anyone know if there's a timer in Scheme with better resolution?

回答1:

current-milliseconds is a function that returns the current millisecond count from the system, but it might decrease. current-inexact-milliseconds is similar, but returns a guaranteed-to-increase floating point number.

There are also a bunch of similar functions that you can find on that page, but if all you need is to time a certain function, then just use (time expr) and it will print out the time it took to evaluate the expression.

Another thing that is relevant here is the profiler, in case you need some more verbose analysis of your code.

回答2:

I too came across this problem today. I am using DrRacket, as it seems to have superseded DrScheme. Though this is an old thread, I am adding my findings for anyone new who stumbles across this thread.

With R5RS as selected language, add following two lines before the program to make it work

(#%require (only racket/base current-milliseconds))
(define (runtime) (current-milliseconds))