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Learn How to Think with Karel the Robot. Chapter 4. Conditions

Chapter 4
Conditions

In this chapter you will learn:

  • What are conditions and why they are used in computer programming.
  • Why does Karel need conditions.
  • How to use the if statement and the else branch
  • How to detect obstacles, collectible objects, and containers.
  • New function print and a new command pass.
  • About the sensors north, empty and home.
  • How to use the full if-elif-else statement.
  • How to combine two or more conditions using the keywords not, and, and or.

4.1. What are conditions?

Sometimes we need our program to contain parts which are not executed always, but only in certain situations (conditionally). Imagine, for example, that our program asks the user to enter a credit card number. Users are known to make mistakes. So, when the credit card number is received, the program needs to execute a condition, checking whether the number has 16 digits. If so, then the program can proceed to other tests (is the credit card number valid?) But if not, then the program needs to ask the user to correct his/her input. Conditions are present in every programming language, not only to check user data but for many other types of tests as well.

4.2. Why does Karel need them?

Karel needs conditions to check his sensors. You will learn more about them soon. With the help of his sensors, he can:

  • Detect collectible objects and containers on the ground beneath him,
  • detect and avoid obstacles in front of him,
  • find out which way is North (and from there, any other direction),
  • check his GPS coordinates and know where he is in the maze,
  • check whether he is at his home square,
  • check whether his bag is empty,
  • pass through unknown mazes, etc.

4.3. Karel’s wall sensor and the if statement

Karel’s mazes can feature many different types of walls:

PIC

Figure 4.1: *

Various types of walls in Karel’s mazes.

The robot has a sensor named wall which helps him detect a wall in front of him. Here "in front of him" means that the wall is right in front of him - he would crash into it if he made one more step forward. picture

Now imagine that Karel is in a new environment that he does not know. He wants to make one step forward. Keep in mind that he does not have eyes, so he cannot see what you see. So, he cannot know if there is a wall in front of him or not:

PIC

Figure 4.2: *

Karel is in the jungle.

He knows that he can’t just execute

1go

because if there was a wall in front of him, he would blindly crash into it. So, instead, he uses the keyword if to check his sensor wall before making the step:

Program 4.1: Karel is checking for a wall before making a step
1if wall 
2  left 
3go

And this was a very good decision indeed, because otherwise he would have crashed! The outcome of Program 4.1 is:

PIC

Figure 4.3: *

Karel detected and avoided a wall.

Notice that the keyword left was indented in Program 4.1 - that’s because it forms the body of the condition. picture

4.4. What happens if there is no wall?

Now let’s see what happens when the condition is not satisfied:

PIC

Figure 4.4: *

Karel’s wall sensor cannot detect a wall which is one step away.

When Program 4.1 is executed, the condition is not satisfied. Therefore the conditional code - in this case just one command left on line 2 - is skipped. Hence, Karel just executes the command go on line 3 and makes one step forward:

PIC

Figure 4.5: *

Karel safely made one step forward.

4.5. Other types of obstacles

Besides walls, one can find in Karel’s mazes various other obstacles:

PIC

Figure 4.6: *

Other types of obstacles.

Karel has a dedicated sensor to detect each one. The names of the sensors exactly correspond to the names of the obstacles. For example, the fire sensor helps him detect fire, the water sensor helps him detect water, etc. All obstacle sensors work analogously to the wall sensor, so the robot can detect fire only when the fire is right in front of him, etc. picture

All of the thin ones are walls. Some of the thick ones are walls as well, but mainly these are other objects such as fire, water, plant, scorpion, tools, skull, etc.

4.6. Combining loops and conditions

As you have seen in Section 3.10 (page 193), loops can contain other loops. Then we talk about nested loops. In this section we want to show you that loops can also contain conditions. To illustrate it, let’s have Karel play the Game of Arrows. How is this game played?

PIC

Figure 4.7: *

The Game of Arrows.

Karel knows that his home is exactly 33 steps away. Dangerous carnivorous plants are located randomly in the maze. He only knows that when he encounters one, he must turn left - so in fact, each plant serves as a left arrow.

Here is a program which solves this puzzle:

Program 4.2: Karel plays the Game of Arrows
1repeat 33 
2  go 
3  if plant 
4    left

The point of this example was to show you that when a loop contains a condition, then the body of the condition must be indented twice. We are talking about the command left on line 4. You already have seen the accumulation of indents in nested loops in Section 3.10 (page 193). Of course, conditions can contain loops, and conditions can also contain other conditions. And all these can be nested. We will see more examples later.

4.7. Collectible objects

In addition to obstacles, one can find various collectible objects in Karel’s mazes:

PIC

Figure 4.8: *

Collectible objects - part 1.

Detecting collectible objects works differently from obstacles - Karel must stand above a collectible object (in the same square) to be able to detect it. He has a dedicated sensor for each collectible object, and the name of the sensor exactly corresponds to the name of the object. For example, he can use the sensor beeper to detect beepers on the ground, the sensor spider to detect spiders, etc.

PIC

Figure 4.9: *

Collectible objects - part 2.

4.8. Collecting bananas

For illustration, let’s look at the following maze where several bananas lie at random positions between Karel and his home square. The home square is 12 steps ahead. The robot’s task is to collect them all:

PIC

Figure 4.10: *

Karel is collecting bananas.

The robot has a sensor banana which helps him detect bananas on the ground. Here is a program that solves this task:

Program 4.3: Collect all the bananas!
1repeat 12 
2  if banana 
3    get 
4  go

Notice that the condition is part of the body of a repeat loop - therefore, the command get on line 3 is indented twice.

4.9. Containers

The third type of objects one can find in Karel’s mazes are containers. Containers are useful when one wants Karel to place collectible objects at a designated place in the maze which he does not know in advance.

PIC

Figure 4.11: *

Karel can place objects in these containers.

When installing a container into the maze, one can define its capacity (number of objects the container can store) as well as the type of objects which can be stored in it. picture

The process of building mazes and creating games will be explained in detail in Appendices A.2 and A.3.

4.10. Collecting coconuts

You already saw Karel collect bananas, now let’s enlist his help collecting coconuts! There are four coconuts placed randomly between the robot and his home square. The home square is 12 steps away. Also, there is a box somewhere between the last coconut and the home square. The blue number 4 located close to the box indicates that the box can store up to four bananas. Karel’s task is to collect all the coconuts and put them into the box:

PIC

Figure 4.12: *

Karel is collecting coconuts.

The program to solve this task is shown below. Notice that it contains a loop which contains a condition which contains another loop. Hence, the command put on line 6 is indented three times:

Program 4.4: Put all the coconuts in the box!
1repeat 12 
2  if coconut 
3    get 
4  if box 
5    repeat 4 
6      put 
7  go

4.11. The else branch

So far we have used the if statement to just skip some code when the condition was not satisfied. But sometimes one needs to also execute alternative code. This is exactly what the else branch does! picture

Imagine that Karel wants to move by one square to the East. If there is a wall in front of him, he needs to go around it. If not, he can just make one step forward:

PIC

Figure 4.13: *

Karel needs to move by one square East, bypassing a wall if necessary.

Here is the corresponding code:

Program 4.5: Move by one square East, bypassing a wall if necessary!
1if wall 
2  left 
3  go 
4  repeat 2 
5    right 
6    go 
7  left 
8else 
9  go

Since the condition on line 1 was satisfied, lines 2-7 were executed and line 9 was skipped. This is Karel’s position after the program ends:

PIC

Figure 4.14: *

Karel went around the wall successfully.

Now let’s see what the program will do when there is no wall:

PIC

Figure 4.15: *

This time Karel’s way is clear.

Since there is no wall in front of the robot, the condition on line 1 is not satisfied. Therefore lines 2-7 are skipped, line 9 is executed, and Karel just makes one step forward:

PIC

Figure 4.16: *

Karel’s position after the program ends.

4.12. Hurdle race

Today Karel participates in a hurdle race! He must get to his home square, jumping over (i.e., going around) several randomly placed walls. His home square is 12 steps ahead:

PIC

Figure 4.17: *

Hurdle race.

The solution program is based on Program 4.5:

Program 4.6: Jump over all hurdles and enter the home square!
1repeat 12 
2  if wall 
3    left 
4    go 
5    repeat 2 
6      right 
7      go 
8    left 
9  else 
10    go

4.13. Function print

As you already know, Karel shares significant functionality with Python. One such function is the function print which Karel can use to display text messages. For example, typing

Program 4.7: Display a greeting!
1print(~Hello world, I am Karel!~)

will generate the following text output:

Hello world, I am Karel!

Notice: picture

4.14. Sensor north

Karel has a sensor north to detect whether he is facing North (top of your screen). picture

Let’s try this! We will execute the code below with Karel facing various directions on the compass:

Program 4.8: Tell if you are facing North!
1if north 
2  print(~I am facing North!~) 
3else 
4  print(~I am not facing North.~)

Here are the outcomes:

PIC *

"I am facing North!"

PIC *

"I am not facing North."

PIC *

"I am not facing North."

PIC *

"I am not facing North."

4.15. Command pass

Another command which is the same in Karel and in Python is the command pass. picture

This command is often used as a placeholder for a future code. Or to really do nothing, as we will show in the next section.

4.16. Finding North

Imagine that Karel is facing a random direction - he does not know whether he faces North, South, East or West. This program will make sure that the robot turns to face North:

Program 4.9: Turn to face North!
1repeat 3 
2  if north 
3    pass 
4  else 
5    left

But why does the program begin with repeat3 when there are four major directions on the compass? Well, if the robot already faces North, then he does not have to turn at all. If he faces East, then he must turn left once to face North. If he faces South, then he must turn left two times. And when he faces West, he must turn left three times. So he never needs to turn four times!

Imagine that Karel faces North, and step through the above program in your mind: The condition on line 2 is always satisfied, so the program executes pass three times. The left command is skipped every time.

Now let’s say that the robot faces East. Then the condition is not satisfied in the first cycle, and therefore Karel turns left. But then he faces North, so in the remaining two cycles the condition is satisfied, and so the program executes pass in the second and third cycles of the loop.

Analogously, when the robot faces South, then the condition is not satisfied in the first cycle, and Karel turns left. Then he faces East. So, the condition is not satisfied in the second cycle either, and he turns left again. Then he faces North, so in the third cycle the condition is satisfied and the robot does not turn anymore.

When Karel faces West, then the condition will never be satisfied. In the first cycle he turns left to face South, in the second cycle he turns left to face East, and finally in the third cycle he turns left to face North.

As a conclusion - no matter what direction he faces initially, Karel will always face North after the program finishes. Below is a graphical breakdown of the analysis. Let’s begin with the case when Karel initially faces North. The four images in the row show where he is facing initially and then after the first, second, and third cycle of the loop:

PIC PIC PIC PIC

Figure 4.19: *

Initial direction: North

PIC PIC PIC PIC

Figure 4.20: *

Initial direction: East

PIC PIC PIC PIC

Figure 4.21: *

Initial direction: South

PIC PIC PIC PIC

Figure 4.22: *

Initial direction: West

4.17. Keyword not

Karel has a keyword not which can be used to reverse the outcome of conditions. For example, ifnotwall is satisfied when ifwall isn’t, and vice versa. picture

The behavior of the keyword not can be summarized in the following table:

Table 4.1: *
Keyword not

condition

not condition

Satisfied

Not satisfied

Not satisfied

Satisfied

Let’s look again at Program 4.9:

1repeat 3 
2  if north 
3    pass 
4  else 
5    left

Using the keyword not, it can be simplified to:

Program 4.10: Shorter version of Program 4.9
1repeat 3 
2  if not north 
3    left

Isn’t this neat? But hold on, in Chapter 5 we will show you how this program can be simplified further to only two lines!

4.18. Sensor empty

Karel has a sensor empty to check whether his bag is empty. When in doubt whether the robot is carrying any objects, checking this sensor prior to executing the put command will prevent the program from crashing:

Program 4.11: Check the bag before removing an object!
1if not empty 
2  put

The empty sensor is useful in many other situations as well - for example, when the robot needs to remove all objects from his bag:

PIC

Figure 4.23: *

Karel has an unknown number of nuggets in his bag but no more than 10.

To be clear - the robot cannot know the number of objects in his bag. He does not have a sensor that would count the objects or anything like that. Here is the corresponding code:

Program 4.12: Remove all objects from the bag!
1repeat 10 
2  if not empty 
3    put

Here is the outcome of the program (there were seven nuggets in the bag):

PIC

Figure 4.24: *

When the program finishes, seven nuggets lie on the ground.

The empty sensor also can be used to have Karel collect only the first of several objects:

PIC

Figure 4.25: *

Karel only needs to collect the first water bottle.

When Karel’s bag is initially empty, the following program will do it:

Program 4.13: Only collect the first water bottle!
1repeat 10 
2  if empty 
3    if bottle 
4      get 
5  go

When the program finishes, Karel brings home one water bottle only:

PIC

Figure 4.26: *

Karel arrives home with one water bottle only.

4.19. Keyword and

As you could see, Program 4.13 had two nested conditions: ifbottle inside of ifempty. As a result, the command get on line 4 was only executed when both these conditions were satisfied. But Karel has a keyword and which can do the same in a much simpler way! picture

The single condition ifemptyandbottle is identical to the two nested conditions. So, Program 4.13 can be simplified:

Program 4.14: Program 4.13 simplified using the keyword and
1repeat 10 
2  if empty and bottle 
3    get 
4  go

This table summarizes how the keyword and works:

Table 4.2: *
Keyword and

condition1

condition2

condition1 and condition2

Satisfied

Satisfied

Satisfied

Satisfied

Not satisfied

Not satisfied

Not satisfied

Satisfied

Not satisfied

Not satisfied

Not satisfied

Not satisfied

4.20. Bounty

Let’s show one more example which uses the keyword and. This time Karel is in a mine, and he found an unknown number of gold nuggets. Now he needs to put them on mining carts (one nugget per cart). The number and positions of the carts are unknown too:

PIC

Figure 4.27: *

Karel is in an abandoned gold mine.

If he knew that he has enough nuggets to put one on each cart, then he could use the following program:

1repeat 10 
2  if cart 
3    put 
4  go

But he does not know that, and the program will fail if there aren’t enough nuggets in Karel’s bag. Namely, at some point the robot will attempt to put a nugget on a cart when his bag is empty. The solution to this problem is very simple though - all he needs to do is also check that his bag is not empty before placing a nugget:

Program 4.15: Place gold nuggets on the carts!
1repeat 10 
2  if cart and (not empty) 
3    put 
4  go

Notice that the expression notempty is enclosed in parentheses. picture

It turns out that Karel had four nuggets in his bag, so when the program finishes, only four carts are loaded:

PIC

Figure 4.28: *

When the program finishes, four carts are loaded.

4.21. Keyword or

Today Karel needs to collect nuggets and gems which are placed randomly between him and his home square.

PIC

Figure 4.29: *

Karel is collecting nuggets and gems.

This could be done with the following program:

1repeat 10 
2  if nugget 
3    get 
4  if gem 
5    get 
6  go

However, as you can see, there are two conditions with the same body which are checked separately, one after another. In a situation like this, the code can be simplified using the keyword or:

Program 4.16: Collect all nuggets and gems!
1repeat 10 
2  if nugget or gem 
3    get 
4  go

Importantly - Karel needs to collect nuggets and gems, but typing ifnuggetandgem on line 2 would be a mistake. Namely, Karel would be checking for squares which contain both a nugget and a gem. Such a condition would never be satisfied, so Karel would not collect anything. With the current line 2, using the keyword or, the condition will be satisfied when Karel detects any of the two objects - a nugget or a gem. picture

The following table summarizes how the keyword or works (for two conditions). It can be used to combine three, four, or even more conditions as well.

Table 4.3: *
Keyword or

condition1

condition2

condition1 or condition2

Satisfied

Satisfied

Satisfied

Satisfied

Not satisfied

Satisfied

Not satisfied

Satisfied

Satisfied

Not satisfied

Not satisfied

Not satisfied

4.22. Danger

This time Karel has a difficult task to find his home square in a wall of fire, water, and acid! The position of his home, as well as the positions of the fire, water, and acid are unknown:

PIC

Figure 4.30: *

Karel is looking for his home square.

The following program will solve the task:

Program 4.17: Find your home square!
1repeat 10 
2  if fire or water or acid 
3    # Move one square to the right: 
4    right 
5    go 
6    left 
7  else 
8    # You are in front of the home square - do nothing: 
9    pass 
10# Make one final step home: 
11go

In Chapter 5 we will show you how this program can be simplified using a different type of loop.

4.23. Sensor home

Karel has a sensor home to check whether he is in his home square. For illustration, let’s write a simple program for the robot to walk straight ahead until he reaches his home square, and to stay there. He does not know how far ahead the home square is:

PIC

Figure 4.31: *

Karel is still in the mines.

Karel knows that the width of the maze is 15 steps, so he uses a repeat loop with 15 cycles:

Program 4.18: Find your home square in the tunnel!
1repeat 15 
2  if home 
3    pass 
4  else 
5    go

In Chapter 5 we will show you a different type of loop which will make this task easier to solve.

4.24. The full if-elif-else statement

The Karel language provides the full Python-style if-elif-else statement which is useful when one has more than two options to choose from. The keyword elif is an abbreviation of "else if", which makes its meaning self-explanatory. The general form of the if-elif-else statement is

if condition_1 
  action_1 
elif condition_2 
  action_2 
... 
else 
  last_action

In place of the ellipsis (the three dots) can be one of more additional elif branches.

Today, Karel needs to sort the treasures he collected in the mines. Located randomly between him and his home square lie on the ground light bulbs, gold nuggets, and gems. He needs to collect the bulbs, move the nuggets one square up, and the gems one square down:

PIC

Figure 4.32: *

Karel is sorting his treasures.

The solution program looks as follows:

1repeat 11 
2  go 
3  if nugget # move nuggets one square up 
4    get 
5    left 
6    go 
7    put 
8    right 
9    right 
10    go 
11    left 
12  elif gem # move gems one square down 
13    get 
14    right 
15    go 
16    put 
17    left 
18    left 
19    go 
20    right 
21  else # it’s a bulb 
22    get 
23go

When the program finishes, Karel is at his home square, and the treasures are sorted!

PIC

Figure 4.33: *

Karel’s treasures are sorted.

4.25. More is coming!

In this chapter we have only shown you the basic use of Karel’s sensors and the if, if-else and if-elif-else statements. There is much more that can be done with them. In the following chapters we will introduce the Boolean values True and False, as well as Boolean expressions, variables and functions. You will learn that Karel’s sensors are in fact Boolean functions. So, keep reading!

4.26. Review questions

Friendly reminder - for every question either none, one, or several answers may be correct.

QUESTION 4.1. Why are conditions used in computer programming?

A
To repeat commands or sequences of commands.
B
To check user data.
C
To make runtime decisions.
D
To correct syntax errors.

QUESTION 4.2. Why does Karel need conditions?

A
To detect and avoid obstacles.
B
To execute commands.
C
To find out which way is North.
D
To check whether his bag is empty.

QUESTION 4.3. When is the body of a condition executed?

A
Always.
B
When the condition is satisfied.
C
When the condition is not satisfied.
D
Never.

QUESTION 4.4. When can Karel detect an obstacle?

A
When it is one step ahead.
B
When it is less than two steps ahead.
C
When it is is an adjacent grid square.
D
When it is right in front of the robot.

QUESTION 4.5. When can Karel detect a collectible object?

A
When it is right in front of the robot.
B
When it is in an adjacent grid square.
C
When it is beneath the robot.
D
When it is above the robot.

QUESTION 4.6. Which of the following are collectible objects?

A
Scorpion
B
Snake
C
Plant
D
Spider

QUESTION 4.7. Which of the following are obstacles?

A
Fishing net
B
Vault
C
Fire
D
Acid

QUESTION 4.8. Which of the following are containers?

A
Box
B
Wall
C
Basket
D
Tomb

QUESTION 4.9. What is the else branch used for?

A
For code to be executed when the condition is not satisfied.
B
For code to be executed after the body of the condition.
C
For code to be executed before the body of the condition.
D
For code to be never executed.

QUESTION 4.10. What is the correct way to display Helloworld! in Karel?

A
print(Hello world!)
B
print(~Hello world!~)
C
print Hello world!
D
print[~Hello world!~]

QUESTION 4.11. When is the condition ifnorth satisfied?

A
When the robot faces North.
B
When the robot does not face North.
C
When the robot is in the top (North) row of the maze.
D
When the robot is in the bottom row of the maze, facing North.

QUESTION 4.12. What does the command pass do?

A
It bypasses a condition.
B
It bypasses a loop.
C
It does nothing.
D
It bypasses the else branch.

QUESTION 4.13. Karel is facing South. What direction will he face when the following program finishes?

1if north 
2  repeat 3 
3    left

A
North
B
South
C
West
D
East

QUESTION 4.14. When is the condition ifempty satisfied?

A
When all collectible objects in the maze have been collected.
B
When Karel’s bag is empty.
C
When all containers in the maze are empty.
D
When at least one container in the maze is empty.

QUESTION 4.15. What line of code is equivalent to the following nested conditions?

1if north 
2  if not wall

A
if north and (not wall)
B
if north or (not wall)
C
if north (and not) wall
D
if north (or not) wall

QUESTION 4.16. Condition1 and Condition2 is satisfied when:

A
Condition1 is satified, Condition2 is satisfied.
B
Condition1 is not satified, Condition2 is satisfied.
C
Condition1 is satified, Condition2 is not satisfied.
D
Condition1 is not satified, Condition2 is not satisfied.

QUESTION 4.17. Condition1 or Condition2 is satisfied when:

A
Condition1 is satified, Condition2 is satisfied.
B
Condition1 is not satified, Condition2 is satisfied.
C
Condition1 is satified, Condition2 is not satisfied.
D
Condition1 is not satified, Condition2 is not satisfied.

QUESTION 4.18. Karel stands above a nugget (there is no gem beneath him). Which of the following conditions will be satisfied?

A
if gem and nugget
B
if gem or nugget
C
if gem and (not nugget)
D
if (not gem) or (not nugget)

QUESTION 4.19. Karel stands above a beeper and faces a wall. Which of the following conditions will be satisfied?

A
if beeper and wall
B
if beeper and (not wall)
C
if (not beeper) or wall
D
if beeper or (not wall)

QUESTION 4.20. Karel stands above a spider, away from walls, looks West, and his bag is empty. What will the following code print?

1if north 
2  print(~I am looking West!~) 
3elif wall 
4  print(~I don’t see any walls!~) 
5elif not empty 
6  print(~My bag is empty!~) 
7else 
8  print(~There is a spider!~)

A
I am looking West!
B
I don’t see any walls!
C
My bag is empty!
D
There is a spider!

Table of Contents

Created on September 27, 2018 in Karel.
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