Console.WriteLine(5);
Console.WriteLine(4);
Console.WriteLine(3);
Console.WriteLine(2);
Console.WriteLine(1);

int count = 5;

while (count > 0)
{
    Console.WriteLine(count);
    count = count - 1;
}

Console.WriteLine("Liftoff!");

5
4
3
2
1
Liftoff!

int count = 5;

while (count > 0)
{
    Console.WriteLine(count);
}

5
5
5
5
5
... and it never stops

int total = 0;
int n = 1;

while (n <= 4)
{
    total = total + n;
    n = n + 1;
}

Console.WriteLine($"Total: {total}");

Total: 10

int fuel = 0;

while (fuel < 500)
{
    fuel = fuel + 120;
}

Console.WriteLine(fuel);

600

'F' → Fullscreen

Build

Everything today goes in a new program named 1-17-lunar-lander.

Task 1: Free Fall

• Your lander starts 100 meters up with its engine off. Every turn, gravity adds 4 to its speed, and it drops by that speed.
• Before you run it, predict how many turns it lasts and how fast it hits. Write both numbers down, then run it and compare.
• Look at the last turn printed. Its altitude is still above 0 — so why did the loop stop anyway? Be ready to say it.

1
Console.WriteLine("=== LUNAR LANDER ===");
2
Console.WriteLine("Touch down at a speed of 5 or less.");
3
Console.WriteLine();
4

5
int altitude = 100;
6
int speed = 0;
7
int turn = 0;
8

9
while (altitude > 0)
10
{
11
    turn = turn + 1;
12
    Console.WriteLine($"Turn {turn}: altitude {altitude}, speed {speed}");
13

14
    speed = speed + 4;
15
    altitude = altitude - speed;
16
}
17

18
Console.WriteLine();
19
Console.WriteLine($"Touchdown at speed {speed}.");
20

21
if (speed <= 5)
22
{
23
    Console.WriteLine("The Eagle has landed.");
24
}
25
else
26
{
27
    Console.WriteLine("CRASH. The lander is in pieces.");
28
}

Task 2: Take the Controls

• Add fuel and an engine. Each turn you choose a burn, and every point of burn takes 1 off your speed and 1 out of your fuel.
• The if stops you burning fuel you do not have. With an empty tank, you are back in free fall.
• Land it. Play until you touch down at speed 5 or less, and write down the burns that worked. Burn too early and you run out of fuel. Burn too late and you cannot stop in time.
• Notice the loop does not know how many turns there will be — your flying decides that. That is exactly the job a while loop is for.

1
Console.WriteLine("=== LUNAR LANDER ===");
2
Console.WriteLine("Touch down at a speed of 5 or less.");
3
Console.WriteLine();
4

5
int altitude = 100;
6
int speed = 0;
7
int fuel = 40;
8
int turn = 0;
9

10
while (altitude > 0)
11
{
12
    turn = turn + 1;
13
    Console.WriteLine($"Turn {turn}: altitude {altitude}, speed {speed}, fuel {fuel}");
14

15
    Console.Write("Burn? ");
16
    int burn = int.Parse(Console.ReadLine());
17

18
    if (burn > fuel)
19
    {
20
        burn = fuel;
21
    }
22

23
    fuel = fuel - burn;
24
    speed = speed + 4 - burn;
25
    altitude = altitude - speed;
26
}
27

28
Console.WriteLine();
29
Console.WriteLine($"Touchdown at speed {speed}.");
30

31
if (speed <= 5)
32
{
33
    Console.WriteLine("The Eagle has landed.");
34
}
35
else
36
{
37
    Console.WriteLine("CRASH. The lander is in pieces.");
38
}

Task 3: Choose a Difficulty

• The comments describe a loop that refuses to start the game until the player picks 1, 2, or 3. Write the code under them, leaving the comments where they are.
• The question has to be asked twice in your code — once before the loop, and once inside it. Work out why, because it is the part everybody gets wrong first.
• Test 9, then 0, then 2. The first two should both be refused, and the game should only start after the 2.
• Then try to land on Hard. It is possible, and it is not easy.

1
Console.WriteLine("=== LUNAR LANDER ===");
2
Console.WriteLine("Touch down at a speed of 5 or less.");
3
Console.WriteLine();
4

5
int altitude = 100;
6
int speed = 0;
7
int fuel = 40;
8
int turn = 0;
9

10
// Ask for a difficulty from 1 to 3 and convert it
11
// While it is below 1 or above 3, say it is invalid and ask again
12
// Easy (1) gets 60 fuel, Normal (2) keeps 40, Hard (3) gets 30
13

14
while (altitude > 0)
15
{
16
    turn = turn + 1;
17
    Console.WriteLine($"Turn {turn}: altitude {altitude}, speed {speed}, fuel {fuel}");
18

19
    Console.Write("Burn? ");
20
    int burn = int.Parse(Console.ReadLine());
21

22
    if (burn > fuel)
23
    {
24
        burn = fuel;
25
    }
26

27
    fuel = fuel - burn;
28
    speed = speed + 4 - burn;
29
    altitude = altitude - speed;
30
}
31

32
Console.WriteLine();
33
Console.WriteLine($"Touchdown at speed {speed}.");
34

35
if (speed <= 5)
36
{
37
    Console.WriteLine("The Eagle has landed.");
38
}
39
else
40
{
41
    Console.WriteLine("CRASH. The lander is in pieces.");
42
}

Task 4: Break It on Purpose

Two changes, one at a time. Write down exactly what happens, and put each back before the next.

1. In the game loop, delete the line that changes altitude. Run it and keep entering burns for a few turns, then press Ctrl + C.
2. Put it back. Now run the program and, at the difficulty prompt, type abc instead of a number.

For number 1, record what the turns showed and how you stopped it. For number 2, record the exception and the line it names.

Number 2 is not a loop bug, and your difficulty loop did not catch it. Say why a loop that checks whether the number is between 1 and 3 never gets the chance to look at abc.

Challenge (Optional): Survive the Nonsense

• Activity 1.9 left this one open on purpose. There is a version of int.Parse that reports failure instead of crashing: int.TryParse(Console.ReadLine(), out difficulty) is true when the text was a number and false when it was not.
• Rebuild your Task 3 loop around it so that abc is refused exactly like 9 and 0 were, instead of ending the program.
• You will need int difficulty; on its own line first, with no value.
• Write a comment saying what your program now does with abc, and why that is better than the exception even though both stop the wrong value getting through.
• Then find the cheat. There is a burn that gives you fuel instead of costing it. Find it, and add a check that stops it.

Same Idea, Exam Notation

int count = 5;

while (count > 0)
{
    Console.WriteLine(count);
    count = count - 1;
}

count ← 5

REPEAT UNTIL(count ≤ 0)
{
  DISPLAY(count)
  count ← count - 1
}

count > 0 and count ≤ 0 describe the same loop from the two ends. Translating one into the other means flipping the comparison, not copying it across.

The reference sheet also has a counted loop, REPEAT n TIMES, which runs a block a fixed number of times with no condition at all. C# writes that one with for, and that is Activity 1.19.

Checkpoint

Your finished program should run like this.

=== LUNAR LANDER ===
Touch down at a speed of 5 or less.

Difficulty (1-3)? 9 [Enter]
Invalid. Enter 1, 2, or 3.
Difficulty (1-3)? 2 [Enter]
Turn 1: altitude 100, speed 0, fuel 40
Burn? 0 [Enter]
Turn 2: altitude 96, speed 4, fuel 40
Burn? 0 [Enter]
Turn 3: altitude 88, speed 8, fuel 40
Burn? 0 [Enter]
Turn 4: altitude 76, speed 12, fuel 40
Burn? 4 [Enter]
Turn 5: altitude 64, speed 12, fuel 36
Burn? 4 [Enter]
Turn 6: altitude 52, speed 12, fuel 32
Burn? 4 [Enter]
Turn 7: altitude 40, speed 12, fuel 28
Burn? 4 [Enter]
Turn 8: altitude 28, speed 12, fuel 24
Burn? 4 [Enter]
Turn 9: altitude 16, speed 12, fuel 20
Burn? 4 [Enter]
Turn 10: altitude 4, speed 12, fuel 16
Burn? 11 [Enter]

Touchdown at speed 5.
The Eagle has landed.

• You wrote down your free-fall prediction before running Task 1, and can say why the loop stopped with altitude still above 0 on the last turn printed
• You landed on Normal at speed 5 or less, and wrote down the burns that did it
• The difficulty prompt refuses 9 and 0 and accepts 2, asking again each time
• Both Task 4 results are recorded — how you stopped the infinite loop, and the exception abc produced

Reflection

Answer the following questions before submitting your work.

1. Every loop needs a start, a condition, and a change. Explain what goes wrong when the change is missing, and how you stopped it.
2. The fuel loop's condition says under 500 and the loop finishes at 600. Explain why that is correct behavior rather than a bug.
3. Activity 1.16 had you run a program twenty times by hand and you still could not prove the die was broken. Describe how a loop would let you settle that question, and roughly how many runs you would ask for.

Submit

Submit four things to the dropbox:

1. Your 1-17-lunar-lander program file.
2. Your free-fall prediction next to what actually happened, and the burns you used to land on Normal.
3. Both Task 4 results — how you stopped the infinite loop, and the exception and line number from abc.
4. Your three reflection answers.

Activity Complete