Loops

What you’ll build

Three number puzzles: a times table, the steps of the Collatz sequence, and a search for the first pair of numbers with a given product. Each uses a different loop, and the last one leaves two loops at once.

Counting with For

For i = a To b counts from a up to b, both included. Until stops before the end instead, and Step counts by another amount, or down. The loop declares its variable, which exists only inside the loop and can’t be assigned there.

Import std.core

For row = 1 To 4
    Local line = ""
    For column = 1 To 4
        line += "{row * column:>4}"
    End For
    Print(line)
End For

For i = 0 Until 10 Step 3
    Print(i)
End For

   1   2   3   4
   2   4   6   8
   3   6   9  12
   4   8  12  16
0
3
6
9

Going over values

For x In values gives each element of an array in turn, and For i, x In values gives each with its index, from 0. A range, 1 To 5 or 0 Until n, is a value too, which a loop can go over, and which has Methods like Sum.

Import std.core

Local primes = [2, 3, 5, 7]
For i, p In primes
    Print("prime {i}: {p}")
End For

For n In 1 To 3
    Print(n * n)
End For
Print((1 To 100).Sum())

prime 0: 2
prime 1: 3
prime 2: 5
prime 3: 7
1
4
9
5050

While and Repeat

While tests its condition before each pass, so its body may not run at all. Repeat ... Until runs its body first and tests after, so the body always runs at least once. The Collatz sequence halves an even number and takes an odd one to three times it plus one, until it reaches 1.

Import std.core

Local n = 27
Local steps = 0
Local highest = n
While n != 1
    If n % 2 == 0
        n /= 2
    Else
        n = 3 * n + 1
    End If
    If n > highest Then highest = n
    steps++
End While
Print("27 reaches 1 in {steps} steps, going as high as {highest}")

Local square = 0
Repeat
    square++
Until square * square > 500
Print("the first square over 500 is {square * square}")

27 reaches 1 in 111 steps, going as high as 9232
the first square over 500 is 529

Leaving early

Break leaves a loop, and Continue skips to its next pass. Loop ... End Loop repeats until a Break or a Return ends it. A label before a loop, like Search:, lets Break and Continue name it, to leave an outer loop from inside an inner one.

Import std.core

Local found = ""
Search: For a = 2 To 20
    For b = a To 20
        If (a + b) % 2 != 0 Then Continue
        If a * b == 96
            found = "{a} x {b}"
            Break Search
        End If
    End For
End For
Print("the first even-sum pair with product 96: {found}")

Local tries = 0
Loop
    tries++
    If tries * 7 % 5 == 1 Then Break
End Loop
Print("7 x {tries} leaves 1 when divided by 5")

the first even-sum pair with product 96: 6 x 16
7 x 3 leaves 1 when divided by 5

What you learned

• For i = a To b counts with both ends included; Until leaves the end out, and Step sets the step.
• For x In values goes over an array or a range, and For i, x In adds the index.
• While tests first, and Repeat ... Until tests after the body.
• Loop repeats until Break; Continue skips to the next pass.
• A label lets Break and Continue name an outer loop.