Time¶

(time/elapsed) is the number of seconds since START was pressed on the Driver Hub:

user=> (time/elapsed)
1.164393094
user=> (format "%.1f s" (time/elapsed))
"1.2 s"

It is the same as the SDK's ElapsedTime, reset right after waitForStart(), so it tells you how far into the match you are: autonomous lasts 30 seconds and TeleOp 2 minutes. Before START it counts from INIT, and every press of INIT starts a new count.

It is the only clock. Java's System/currentTimeMillis and System/nanoTime are not available, so with time/sleep every time in your code is in seconds.

Pause¶

(time/sleep s) pauses the current thread for s seconds, so (time/sleep 0.02) is 20 milliseconds. On the robot it is also how a loop slows down: a loop with no sleep would run as fast as the CPU allows and starve the rest of the OpMode. A time of zero or less does not wait at all, so (time/sleep (- deadline (time/elapsed))) is safe even after the deadline.

Fixed duration¶

Set a deadline once, then compare against it each round:

(let [end (+ (time/elapsed) 2)]
  (while (< (time/elapsed) end)
    (println "driving")                 ; e.g. set the motor power
    (time/sleep 0.02)))

This is the usual "drive for two seconds" shape. The check is at the top, so the body always runs at least once.

Timeout¶

(let [deadline (+ (time/elapsed) 3)]
  (loop []
    (cond (:a (gamepad 1)) :pressed
          (> (time/elapsed) deadline) :timed-out
          :else (do (time/sleep 0.02) (recur)))))

The loop returns :pressed or :timed-out, which is easier to test than a nil from a loop that just stops.

Timing¶

(time expr) evaluates the expression, prints how long it took, and returns the value:

user=> (time (+ 1 2))
"Elapsed time: 0.02035 msecs"
3.0

The duration varies from run to run; the important part is that 3.0 is still the value, so time can wrap any expression.

Round time¶

A PID controller needs the time since the last round. Carry the previous (time/elapsed) through recur and subtract:

(defn control-loop []
  (loop [last (time/elapsed)]
    (let [now (time/elapsed)
          dt (- now last)]               ; seconds since the last round
      ;; ... use dt in the controller ...
      (time/sleep 0.02)
      (recur now))))

dt is in seconds, about 0.02 here.