for (int row = 1; row <= 3; row++)
{
    Console.WriteLine($"Row {row}");
}

Row 1
Row 2
Row 3

for (int row = 1; row <= 3; row++)
{
    for (int seat = 1; seat <= 4; seat++)
    {
        Console.WriteLine($"Row {row} Seat {seat}");
    }
}

Row 1 Seat 1
Row 1 Seat 2
Row 1 Seat 3
Row 1 Seat 4
Row 2 Seat 1
Row 2 Seat 2
Row 2 Seat 3
Row 2 Seat 4
Row 3 Seat 1
Row 3 Seat 2
Row 3 Seat 3
Row 3 Seat 4

for (int a = 1; a <= 3; a++)
{
    for (int b = 1; b <= 4; b++)
    {
        // this line runs 12 times
    }
}

Console.Write("Crew size? ");

for (int col = 1; col <= 5; col++)
{
    Console.Write("* ");
}

Console.WriteLine();

* * * * *

'F' → Fullscreen

Build

Everything today goes in a new program named 1-21-nested-loops.

Task 1: Rows and Seats

• Predict how many lines appear before you run it. Write the number down, then run it and count.
• Change the inner limit from 4 to 2 and predict again. Run it.
• Put that back, then change the outer limit from 3 to 2 instead and predict once more. Run it.
• Write down both predictions and both real answers. They are a graded line on your sheet, and being wrong costs nothing.
• Put both limits back to 3 and 4 before you continue.

1
for (int row = 1; row <= 3; row++)
2
{
3
    for (int seat = 1; seat <= 4; seat++)
4
    {
5
        Console.WriteLine($"Row {row} Seat {seat}");
6
    }
7
}

Task 2: An Actual Grid

• Add a block that prints a rectangle of stars, four rows of six.
• Look hard at the empty Console.WriteLine(). It is inside the outer loop and below the inner loop, and it is the only thing that ends a row.
• The inner loop uses Console.Write, which does not move to a new line. Six of those in a row put six stars on one line.
• Count the stars. There should be twenty-four, and you should be able to say where that number came from without counting.

1
for (int row = 1; row <= 3; row++)
2
{
3
    for (int seat = 1; seat <= 4; seat++)
4
    {
5
        Console.WriteLine($"Row {row} Seat {seat}");
6
    }
7
}
8

9
Console.WriteLine();
10

11
for (int row = 1; row <= 4; row++)
12
{
13
    for (int col = 1; col <= 6; col++)
14
    {
15
        Console.Write("* ");
16
    }
17

18
    Console.WriteLine();
19
}

Task 3: Build a Triangle

• The comments describe a triangle where each row is one star longer than the one above it.
• The inner loop's limit is the outer loop's variable. That is the whole trick, and it is the first time in this course that the two loops have depended on each other.
• Convert the input, or the loop header will not compile.
• Test with 1, then 4, then 7. Then try 0 and say what you expected before you ran it.

1
for (int row = 1; row <= 3; row++)
2
{
3
    for (int seat = 1; seat <= 4; seat++)
4
    {
5
        Console.WriteLine($"Row {row} Seat {seat}");
6
    }
7
}
8

9
Console.WriteLine();
10

11
for (int row = 1; row <= 4; row++)
12
{
13
    for (int col = 1; col <= 6; col++)
14
    {
15
        Console.Write("* ");
16
    }
17

18
    Console.WriteLine();
19
}
20

21
Console.WriteLine();
22

23
// Ask the user for a size and convert it
24
// Print a triangle that many rows tall:
25
//   row 1 has one star, row 2 has two, and so on
26
//   the inner loop's limit is the outer loop's variable

Task 4: Break It on Purpose

Both changes are to the same line — the empty Console.WriteLine() in your grid. Write down exactly what happens, and put it back before the next one.

1. Move it inside the inner loop, right after the Console.Write. Run it and count the lines.
2. Put it back, then delete it entirely. Run it again.

For each one, record how many lines the grid section printed and how many stars were on each. The star total does not change; only its shape does.

Neither version is an error. Both compile, both run, and neither warns you about anything. Say what that one line actually does, now that you have seen the grid, the column, and the single line it sits between.

Challenge (Optional): Upside Down

• Print a triangle that starts wide and narrows to a single star.
• There are two ways. Count the outer loop downward, or keep counting upward and subtract. Write whichever occurs to you first.
• Then work out the other one as well. Both produce identical output, and the second is harder to arrive at than to read.
• Add a comment saying which reads more clearly and why.

Checkpoint

Your finished program, with 4 as the triangle size.

Row 1 Seat 1
Row 1 Seat 2
Row 1 Seat 3
Row 1 Seat 4
Row 2 Seat 1
Row 2 Seat 2
Row 2 Seat 3
Row 2 Seat 4
Row 3 Seat 1
Row 3 Seat 2
Row 3 Seat 3
Row 3 Seat 4

* * * * * *
* * * * * *
* * * * * *
* * * * * *

Size? 4 [Enter]
*
* *
* * *
* * * *

• Twelve seat lines, and you can say why twelve without counting
• Four rows of six stars, twenty-four in total
• The triangle grows by one star a row, and works for 1, 4, and 7
• You recorded both Task 1 predictions and both real answers
• Both Task 4 shapes are recorded, with the line and star counts for each

Reflection

Answer the following questions before submitting your work.

1. Explain why nested loops multiply rather than add, using your rows and seats as the example.
2. Moving one empty Console.WriteLine() changed your grid into a column, and deleting it made a single long line. Describe what that line does and why its position matters so much.
3. In the triangle, the inner loop's limit came from the outer loop's variable. Describe something that lets you build which a fixed limit could not.

Submit

Submit four things to the dropbox:

1. Your 1-21-nested-loops program file.
2. Both Task 1 predictions with both real answers, and what the triangle did with 0.
3. Both Task 4 results — line counts and stars per line for each.
4. Your three reflection answers.

Activity Complete