XML CO538 Anonymous Questions and Answers Keyword Index

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Keyword reference for shift

Question 43 (2007):

Submission reference: IN1372

Hi, I'm changing the compute.speed function, but I had some trouble understanding the first one. Can you explain the code below because I can't find any explanations for the operators?

    IF i = 0 FOR 7
      range <= (robot.radius << i)
        speed := i

Cheers.

Answer 43:

The left and right shift operators, << and >>, are defined is slide 33 of basics and in section 2.2 ("Built-in Operators") of the occam-pi Quick Reference Wiki, which is linked from this module's web page.

See also Question 42 (2007), which observes that shifting the bit-pattern of a positive integer leftwards multiplies it by 2 for each bit shifted. So, expanding the above replicator fully (it's only 7 times) and replacing the shifts with equivalent multiplies, we get:

    IF
      range <= (robot.radius * 1)
        speed := 0
      range <= (robot.radius * 2)
        speed := 1
      range <= (robot.radius * 4)
        speed := 2
      range <= (robot.radius * 8)
        speed := 3
      range <= (robot.radius * 16)
        speed := 4
      range <= (robot.radius * 32)
        speed := 5
      range <= (robot.radius * 64)
        speed := 6

So long as range is not more than 64 times the radius of the robot, a speed will be assigned. In brain.0, that range is the minimum clear distance reported by the laser scanner. It sets the speed on a logarithmic scale, stopping it if it's bumped into something.

The greatest value the laser scanner can return is the length of its rays (see the variable laser.range in the main process). In that main process, laser.range is set to 32 times the robot.radius – so the above code is safe. If you change that ratio, you may need to protect that replicated IF – see Question 30 (2007).

Keywords: cylons , operators , shift