cs2010 Notes: 10-17 More Programming

Tree Chopper

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Then go open it in a real editor (folder, minecraft/saves/World/computercraft/…)

more = true
while more do
  more, data = turtle.inspect()
  if string.find(data.name, "log") then
    turtle.dig()
    turtle.digUp()
    turtle.up()
  end
end

Let’s walk through this:

• This is a text file.
• It contains a sequence of “lines of code”, which are almost but not quite the same as lines of text.

Example (this is two “lines of code” and three “lines of text”):

a = 3 +
    5
print(a)

Example (two lines of code, three lines of text):

print("giant")

print("squid")

• When we run the program (a text file containing code), each line of code is run in order, top to bottom.
• Some lines of code do something right away.
• Other lines of code are part of multi-line statements, and do something conditionally, repeatedly, or eventually.

So how does this work?

• We have a boolean variable, more, that starts as true.
• The next line is “while”, which has special rules.
• While repeats its body (the code between do and the matching end) as long as the condition (here “more”) is true.
• Then we inspect, seeing what block is in front of the turtle.
• If the block is a log, we chop it and go up.
• The “if” line is special: we execute the code in the body only if the condition is true.
• Repeat until there’s no more block in front of the turtle.

Simple Sequence

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print(1)
print(2)
print(3)

That’s interesting.

• Three lines of code, no tricks, they run in order.
• We’re doing the same thing three times.
• We’re using an incrementing number.
• We want a for loop.

for i = 1,3 do
  print(i)
end

• Again, three lines of code.
• Now we’ve got a for / end pair and an indented middle line.
• A “for” block has special rules:
  • All the lines until the matching “end” are our loop body.
  • Loop body is typically indented.
  • The loop body runs several times, once for each value in the range 1,3 (inclusive).
  • Those values are assigned to the variable i for each loop body execution.

turtle.dig()
turtle.forward()
turtle.dig()
turtle.forward()
turtle.dig()
turtle.forward()

We’re ding the same thing three times again, so we can use a for loop again. We still need to name the variable though. The name “_” is typically used for variables that we don’t plan to use.

for _ = 1,3 do
  turtle.dig()
  turtle.forward()
end

Let’s dig a rectangle of size N * M.

n = 3
m = 5

for _ = 1,n do
  turtle.digDown()
  turtle.forward()
end

turtle.turnLeft()

for _ = 1,m do
  turtle.digDown()
  turtle.forward()
end

turtle.turnLeft()

for _ = 1,n do
  turtle.digDown()
  turtle.forward()
end

turtle.turnLeft()

for _ = 1,m do
  turtle.digDown()
  turtle.forward()
end

That’s a lot of repeated, almost identical code.

We need the key trick of computer programming: Abstraction

We want to take things that are almost the same, name them, and name the differences.

Functions

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Our tool for that in Lua is a function

n = 3
m = 5

function digTrench(x) do
  for _ = 1,m do
    turtle.digDown()
    turtle.forward()
  end
end

digTrench(n)
turtle.turnLeft()
digTrench(m)
turtle.turnLeft()
digTrench(n)
turtle.turnLeft()
digTrench(m)