Raspberry Pi and Parallax Ping))

sonar2.py

import RPi.GPIO as GPIO
import time

# NOTE: don't use GPIO02 or GPIO03, they have
# internal pullup resistors that prevents results.

try:
  # Setup
  GPIO.setmode(GPIO.BOARD)

  while(1):
    GPIO.setup(11, GPIO.OUT)
    # Set to low
    GPIO.output(11, False)

    # Sleep 2 micro-seconds
    time.sleep(0.000002)

    # Set high
    GPIO.output(11, True)

    # Sleep 5 micro-seconds
    time.sleep(0.000005)

    # Set low
    GPIO.output(11, False)

    # Set to input
    GPIO.setup(11, GPIO.IN)

    # Count microseconds that SIG was high
    while GPIO.input(11) == 0:
      starttime = time.time()

    while GPIO.input(11) == 1:
      endtime = time.time()

    duration = endtime - starttime
    # The speed of sound is 340 m/s or 29 microseconds per centimeter.
    # The ping travels out and back, so to find the distance of the
    # object we take half of the distance travelled.
    # distance = duration / 29 / 2
    distance = duration * 34000 / 2
    print distance, "cm"
    time.sleep(0.5)

finally:
  GPIO.cleanup()

after execution it`s empty. What could be the error?