The if...else if...else statement in the loop function checks the whiskers for any states that require attention. The statement starts with if((wLeft == 0) && (wRight == 0)). Translated to English, it reads “if the wLeft variable AND the wRight variable both equal zero.” If both variables are zero, the two calls in the if statement’s code block get executed: backward(1000) and turnLeft(800).
if((wLeft == 0) && (wRight == 0)) // If both whiskers contact
{
backward(1000); // Back up 1 second
turnLeft(800); // Turn left about 120 degrees
}
In the if…else if…else statement, the sketch skips code blocks with conditions that are not true, and keeps checking until it either finds a condition that’s true or runs out of conditions. When the sketch finds a true statement, it executes whatever is in its code block, then it skips to the end of the if…else if…else statement without checking any more conditions, and moves on to whatever else comes next in the sketch.
So, if both whiskers are not pressed, that first if statement is not true and its code block is skipped. The sketch will check the first else if statement. So, maybe the left whisker is pressed and the calls in this statement’s code block will run. After backing up for one second and turning left for 0.4 seconds, the sketch skips the rest of the conditions and moves on to whatever comes after that last else statement.
else if(wLeft == 0) // If only left whisker contact
{
backward(1000); // Back up 1 second
turnRight(400); // Turn right about 60 degrees
}
If it’s the right whisker that detects an obstacle, the first two code blocks will be skipped, and the if(wRight == 0) block will run.
else if(wRight == 0) // If only right whisker contact
{
backward(1000); // Back up 1 second
turnLeft(400); // Turn left about 60 degrees
}
An else condition functions as a catch-all for when none of the statements preceding it were true. It’s not required, but in this case, it’s useful for when no whiskers are pressed. If that’s the case, it allows the BOE Shield-Bot to roll forward for 20 ms. Why so little time before the loop repeats? The small forward time before rechecking allows the BOE Shield-Bot to respond quickly to changes in the whisker sensors as it rolls forward.
else // Otherwise, no whisker contact
{
forward(20); // Forward 1/50 of a second
}
The forward, backward, turnLeft and turnRight functions were introduced in Chapter 4, Activity #5, and are used in the MovementsWithSimpleFunctions sketch. These functions certainly simplified the coding. (Hopefully, they also help demonstrate that all the navigation coding practice from Chapter 4 has its uses!)
Your Turn
You can also modify the sketch’s if...else if...else statements to make the LED indicators broadcast which maneuver the BOE Shield-Bot is running. Just add digitalWrite calls that send HIGH and LOW signals to the indicator LED circuits. Here is an example:
if((wLeft == 0) && (wRight == 0)) // If both whiskers contact
{
digitalWrite(8, HIGH); // Left LED on
digitalWrite(2, HIGH); // Right LED on
backward(1000); // Back up 1 second
turnLeft(800); // Turn left about 120 degrees
}
else if(wLeft == 0) // If only left whisker contact
{
digitalWrite(8, HIGH); // Left LED on
digitalWrite(2, LOW); // Right LED off
backward(1000); // Back up 1 second
turnRight(400); // Turn right about 60 degrees
}
else if(wRight == 0) // If only right whisker contact
{
digitalWrite(8, LOW); // Left LED off
digitalWrite(2, HIGH); // Right LED on
backward(1000); // Back up 1 second
turnLeft(400); // Turn left about 60 degrees
}
else // Otherwise, no whisker contact
{
digitalWrite(8, LOW); // Left LED off
digitalWrite(2, LOW); // Right LED off
forward(20); // Forward 1/50 of a second
}
- Modify the if...else if...else statement in RoamingWithWhiskers to make the BOE Shield-Bot broadcast its maneuver using the LED indicators.
- Remember to set the digital pins to outputs in the setup function so they can actually supply current to the LEDS:
pinMode(8, OUTPUT); // Left LED indicator -> output pinMode(2, OUTPUT); // Right LED indicator -> output