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LESSON 125 OF 414 · WHILE LOOPS & FOR LOOPS

Printing Numbers with a For Loop in Scala: to and until

Lesson Read the full explanation

We can achieve many different outcomes with for loops.

It's all about tuning the initialization, condition and iteration statements.

For example, we can print all numbers from 2 through 6:

for (i <- 2 to 6) {
	println(i)
}

The output of this code is:

2
3
4
5
6

Let's break down this code:

  • The range starts at 2.

  • to 6 means the range includes 6.

  • The range increases by 1 by default.

Assignment

Let's print all numbers from 3 through 10 using a for loop.

Hint
Look at the examples above if you get stuck.

STEP 1 OF 2
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    SCALA · WHILE LOOPS & FOR LOOPS

    Printing Numbers 3 in Scala, explained

    Choosing where a range starts and stops

    With a for loop over a range, the two ends of the range decide which numbers appear. To print every number from 2 through 6, start the range at 2 and end it at 6 with to:

    for (i <- 2 to 6) {
    println(i)
}

    The output is 2, 3, 4, 5 and 6, each on its own line. The range 2 to 6 starts at 2, includes 6, and goes up by 1 because that is the default step.

    to versus until

    Scala has two ways to write the end of a range:

    RangeValues
    2 to 62, 3, 4, 5, 6
    2 until 62, 3, 4, 5

    Use to when the end value is one of the numbers you want, and until when it is the first number you don't want. Both are ordinary methods written without dots, so 2 to 6 is the same as 2.to(6).

    The assignment: print 3 through 10

    The task is to print every number from 3 through 10.

    1. Write the loop with the right range. Both ends are wanted, so use to: for (i <- 3 to 10). With 3 until 10 the loop would stop at 9.
    2. Print the loop variable. Inside the body, println(i) prints the current value. println ends each output with a newline, so every number gets its own line.
    for (i <- 3 to 10) {
    println(i)
}

    The output is 3, 4, 5, 6, 7, 8, 9 and 10, one per line.

    Why a range-based loop is safe

    The range is fixed before the loop starts, and the loop variable takes each of its values exactly once. There is no condition that could fail to become false, and no update line to forget. That makes this loop a good fit whenever you know the numbers you want in advance.

    Notes on ranges

    • An empty range is fine. 5 to 2 has no values, because ranges count upward by default. The loop body simply never runs, and there is no error. Counting down needs a negative step, which later lessons cover.
    • The loop variable follows the range. You don't declare i as an Int yourself. Its type comes from the range, which here is a range of Int.
    • The same start and end give one value. 4 to 4 runs the body once, while 4 until 4 runs it zero times.

    Common mistakes

    • Using until when the end should be included. This leaves out the last number, which is the most frequent error.
    • Using to when the end should be excluded. This prints one value too many.
    • Starting from the wrong number. Starting at 0 or 1 prints extra numbers before 3.
    • Printing a different variable. The body must print the loop variable, here i, or every line shows the same value.

    Key takeaways

    • a to b includes b; a until b excludes it.
    • A range goes up by 1 unless a step is given.
    • If the start is already past the end, the loop body doesn't run at all.
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