Page 56 - The Unofficial Guide to Lego Mindstorms Robots

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if I'm on the line, go straight forward
if I'm off the line, find the line and start over

It's the "find the line" part that's difficult. When Trusty's light sensor goes off the black line, Trusty has no way of knowing if
he's on the right or the l eft side of the line. Ideally, Trusty would turn back to the line and start going straight again. He would
p roceed in a zigzag fashion along the line.

State

Even if Trusty doesn't know which side of the line he's on, he can make a pretty good guess. If he knows he drove off the left
si de of the line last time, it's a pretty good bet he'll drive off the right side the next time. Figure 3-2 shows a likely path as
Trusty tries to stay on the line.

Figure 3-2.
Trusty zigzags along the black line

With this in mind, Trusty's algorithm can be more specific:

if I'm on the line, go straight forward
if I'm off the line {
turn back the opposite direction from the way I turned last time
if I still don't find the line, turn farther back the other direction
}

Another way of looking at this is to say that Trusty now has two possible states:

1. Just turned left (turn right next)

2. Just turned right (turn left next)

By keeping track of this state, Trusty can figure out the best way to turn the next time he drives off the black line.

Could I Please Have a Variable?

Some kind of variable is needed if Trusty is to keep track of his state. Said another way, Trusty needs some kind of memory to
remember which way he last turned. This highlights one of the weak points of RCX Code (the RIS programming
environment), its lack of variables.