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Lesson 2: Build and Test the Project

Continuing to Build the Project 

  • Now the condition of the if else statement needs to be set. To use the Distance Sensor to detect castles, the found_object command can be used. Add the found_object command to the condition. Your project should now look like this:

    def main():
	# Does the Distance Sensor detect a castle?
	
	if front_distance.found_object():
		# Crash castle detected by a Distance Sensor
		pass
		
	else:
		# Turn to find a castle using the Distance Sensor
		pass

  • If the Distance Sensor finds an object, the VR Robot should drive forward to knock over the castle. Add a drive command into the 'if' branch of the if else statement, and set the parameter to drive the VR Robot forward. (This command should replace the 'pass'). 

    def main():
	# Does the Distance Sensor detect a castle?
	
	if front_distance.found_object():
		# Crash castle detected by a Distance Sensor
		drivetrain.drive(FORWARD)
		
	else:
		# Turn to find a castle using the Distance Sensor
		pass

  • When the found_object command reports False, the 'else' branch will run, so the VR Robot will need to turn to find a castle on the Playground. Add a turn command inside the 'else' branch, and set the parameters to turn the VR Robot to the right. 

    def main():
	# Does the Distance Sensor detect a castle?
	
	if front_distance.found_object():
		# Crash castle detected by a Distance Sensor
		drivetrain.drive(FORWARD)
		
	else:
		# Turn to find a castle using the Distance Sensor
		drivetrain.turn(RIGHT)

For Your Information

The 'else' branch of the if else statement only functions in a project as part of an if else statement. While you can have an if statement stand alone, there is not an 'else' statement in VEXcode VR Python. The 'else' branch is run when the condition of the 'if' branch is not met. Without a defined condition (the 'if' branch), the 'else' branch would never run. Three code snippets demonstrating conditional statements in Python, each featuring an if statement with a condition followed by the pass keyword. The first two snippets have green checkmarks above them, indicating correct usage. The first snippet includes only an if statement, while the second includes both an if and an else statement with the pass keyword in each. The third snippet, marked with a red cross, consists of only an else statement with pass, indicating incorrect usage, as an else statement should follow an if statement.

Add a Loop and Test the Project

  • Before the project is tested, one more command needs to be added. If else statements will only check the condition once before moving to the next command outside the statement. In Unit 7, an infinite while loop was added to the project to instruct the VR Robot to repeatedly check the condition of the Down Eye Sensor. To ensure the Distance Sensor condition is being checked repeatedly, drag in or type an infinite while loop, then copy the if else statement inside the infinite while loop. Your project should now look like this:

    • Remember to be mindful of indentation when copying and pasting commands into a loop, so that the project works as intended when it is run. 

      def main():

	while True:
		# Does the Distance Sensor detect a castle?
	
		if front_distance.found_object():
			# Crash castle detected by a Distance Sensor
			drivetrain.drive(FORWARD)
		
		else:
			# Turn to find a castle using the Distance Sensor
			drivetrain.turn(RIGHT)
		
		wait(5, MSEC)
  • Open the Playground Window if it is not already open. Be sure the Dynamic Castle Crasher Playground opens, and run the project. A top down view of the Dynamic Castle Crasher Playground showing that only some of the castles have been knocked over. The VR Robot is in the top right of the playground after its movement around the playground.
  • Once the VR Robot has knocked over at least two castles, reload the Playground and run the project on another layout of the Dynamic Castle Crasher Playground. A top down view of the Dynamic Castle Crasher with the playground interface. The reset button in the bottom left of the playground interface is highlighted with a red box.
  • On each run of the project, the VR Robot will turn until a castle is detected by the Distance Sensor, then drive towards that building to knock it over.An angled view of the Dynamic Castle Crasher playground showing the VR Robot pushing a piece of a castle over the red border line and off the edge of the playground.
  • During the run of the project, the VR Robot may push a castle piece all the way to the edge and fall off the Playground. This is because the found_object command is still reporting that there is an object in front of the VR Robot. When this command reports True, the VR Robot drives forward, leading the VR Robot to fall off the Playground.

Project Flow with an If Else Statement

The if else statement is used so the VR Robot can make a decision. If the Boolean condition is True, then the commands inside of the 'if' branch will run. If the Boolean condition is False, then the commands inside of the 'else' branch will run.

For instance, in this iteration of the Dynamic Castle Crasher Playground, when the project is started the Distance Sensor detects an object – the condition of the found_object command reports True. Now the 'if' branch of the project will run, and the VR Robot will drive forward. The VR Robot will skip the 'else' branch of the project.On the left is the project's current code snippet. On the right is the top down view of the Dynamic Castle Crasher playground with the playground interface visible. The Distance Sensor's measurement on the playground interface reads 568 mm and is highlighted with a red box. This shows that the castle in the middle of the playground is 568 millimeters from the VR Robot.

  • Notice that the Distance Sensor data is also reported in the Dashboard. When an object is detected, the distance value will be displayed.

However, in this iteration of the Dynamic Castle Crasher Playground, when the project is started the Distance Sensor does NOT detect an object – the condition of the found_object command reports False. Now the VR Robot will skip the 'if' branch of the project, and the 'else' branch of the project will run, so the VR Robot will turn right.On the left is the project's current code snippet. On the right is the top down view of the Dynamic Castle Crasher playground with the playground interface visible. The Distance Sensor's measurement on the playground interface reads 1354 mm and is highlighted with a red box. This shows that the castle at the top of the playground is 1354 millimeters from the VR Robot.

  • Notice that the Distance Sensor value in the Dashboard is reported as  > 3000 mm. The Distance Sensor can detect objects within a 3000 millimeter (mm) range, so when an object is not detected, the value is reported as greater than (>) 3000 mm. 

Because the if else statement is inside of an infinite while loop, the project loops back to the top of the infinite while loop to check the condition of the if else statement repeatedly. This loop will continue forever, instructing the VR Robot to continually check the if else condition hundreds of times per second.This is the project's code snippet surrounded by an orange arrow that begins and ends on the while True: statement at the top of the code snippet. There is a note on the arrow that says Loops continuously, showing that anything nested in the while True loop will repeat forever.

  • Note that the drive and turn commands inside the if else statement are non-waiting commands. Non-waiting commands do not stop the flow of a project while they are running.  A non-waiting behavior that is executed in an if else statement (such as 'drive forward if object is detected') will continue while the condition is checked, and will only change when the condition changes.

For Your Information

An if else statement is used to ensure that only one branch in the project is run when the condition is checked. Only one instance or condition can be true at any time. This makes the project more efficient.

The Python project code with arrows pointing to the if and else statements. The top of the arrows read Checks condition of Distance Sensor to show the distance.found_object if statement checks the condition. If the object is detected, the command under the if statement will be executed, but if there is no object detected, the command under the else statement is executed.

Compare the current project with the project created in Unit 7.  The Unit 7 project uses four if statements.

The project code from Unit 7 with a large orange arrow surrounding the entire code, beginning and ending at the while True statement. A note on the orange arrow reads If statements contained in the infinite while loop are repeatedly checked. Green arrows inside the while True statement show the project's flow, starting with individual if statements on if a certain color is seen and the behavior that should be done accordingly, whether that's to to do a behavior or move on to the next if statement until the end where everything loops once more.

When running through a project with multiple if statements, it could be possible that multiple conditions would report True (or False) as the project runs through the commands in the infinite while loop. Conflicts like this could cause the project to stop running or run more slowly. 

In other words, multiple if statements in a project is like traveling down a road with multiple exit ramps. Depending on where you are trying to go, you would turn at different places. For instance, if you were going to school, you would turn left; if you were going to the store, you would turn right. But what if you weren't going to school or the store? Both conditions would report False, and you would stay on the straight path. Or what if you were going to both school and the store? Both conditions would report True, but you cannot turn at two locations at the same time. A diagram showing a gray T-shaped intersection with three directional arrows - one pointing left, one pointing straight up, and one pointing right. In the center of the intersection is a VR Robot. To the right of the diagram are two conditional statements in code format: if going to school: Turn left and if going to store: Turn right. The if statements are written in black and blue text, with if in blue and the directional instructions in blue.

 

To continue the analogy, an if else statement is like traveling to a fork in the road – it forces you to make a decision. If you are going to school, turn left; else, turn right. No matter where you are going, you either turn left or right. Using an if else statement would avoid bugs like those above, because it would force you to make a decision, as only one condition can be True at a given moment.A diagram showing a gray Y-shaped intersection with two directional paths - one leading left and one leading right. In the center of the intersection is the VR Robot. Above the diagram is code showing conditional statements in code format: if going to school: Turn left and else: Turn right. The if and else statements are written in black and blue text, with if and else in blue and the directional instructions in lighter blue.  

Questions

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