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101-07: counting loops

This is lesson 101-07 of CU-101.2, checked against cint 0.2.0.

You will repeat lines a set number of times, put one loop inside another and line up numbers in rows.

the program

// Times tables from 1 to 9.
for row in 1..=9 {
    for col in 1..=9 {
        "{row * col:>3}";
    }
    "\n";
}

Run this program in ced

Run it:

  1  2  3  4  5  6  7  8  9
  2  4  6  8 10 12 14 16 18
  3  6  9 12 15 18 21 24 27
  4  8 12 16 20 24 28 32 36
  5 10 15 20 25 30 35 40 45
  6 12 18 24 30 36 42 48 54
  7 14 21 28 35 42 49 56 63
  8 16 24 32 40 48 56 64 72
  9 18 27 36 45 54 63 72 81

what happened

A for loop counts. Lesson 101-05 met the loop: for row in 1..=9 { ... } runs the lines between the braces once for each number from 1 to 9, and row holds that number. Each time through the lines is called a pass.

A range says where to start and stop. There are two ways to write one:

• 1..=9 means 1 up to 9, including 9.
• 1..9 means 1 up to 9, leaving 9 out: 1 to 8.

The second kind is useful when counting starts at 0, as it does for lists in lesson 101-12. 0..7 gives seven numbers, 0 to 6.

A loop can hold another loop. For each row, the inner loop runs through col from 1 to 9. The line "{row * col:>3}"; runs 9 times 9, which is 81 times, once for each number in the table.

A print without \n stays on the same line. The inner loop prints nine numbers side by side. After it ends, "\n"; ends the line, and the outer loop goes on to the next row.

:>3 sets a width. {row * col:>3} prints the number in 3 characters, with spaces in front to fill them, so 4 prints as two spaces and a 4. This keeps the numbers lined up under each other. The > means the number goes at the right of its space.

change it

1. Change 1..=9 to 1..9 in both loops. Which row disappears, and which number disappears from each row?
2. Make the table go up to 12 in both loops. Look at the last rows: what goes wrong, and what width fixes it?
3. Print only the squares, 1, 4, 9 and so on up to 81, one on each line. You need only one loop.

  9 18 27 36 45 54 63 72 81 90 99108
 10 20 30 40 50 60 70 80 90100110120
 11 22 33 44 55 66 77 88 99110121132
 12 24 36 48 60 72 84 96108120132144

when it goes wrong

A for loop's counter belongs to the loop, and the lines inside cannot change it:

for row in 1..=3 {
    row = 5;
}

Run this program in ced

error[C2052]: loop variable assigned
  --> loopvar.ci:2:5
   |
 2 |     row = 5;
   |     ^
   |

A range also counts only upward. This loop never runs its lines, because 10 is past 1, so the program prints only done:

for i in 10..1 {
    "{i}\n";
}
"done\n";

Run this program in ced

Lesson 101-08 shows a loop that can count down.

new words

width, range, pass

Next: lesson 101-08, loops that wait.