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LESSON 126 OF 414 · WHILE LOOPS & FOR LOOPS

Printing Numbers with a For Loop in Scala: Steps with by

Lesson Read the full explanation

Let's see an example where we change the iteration statement.

We can print all even numbers from 2 through 10:

for (i <- 2 to 10 by 2) {
	println(i)
}

The output of this code is:

2
4
6
8
10

Let's break down this code:

  • The range starts at 2.

  • to 10 means the range includes 10.

  • by 2 means we increase our number by 2 every time.

Assignment

Let's print all odd numbers from 7 through 23 using a for loop.

Hint
Look at the examples above if you get stuck.

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    SCALA · WHILE LOOPS & FOR LOOPS

    Printing Numbers 4 in Scala, explained

    Changing the step of a range

    A range goes up by 1 unless you say otherwise. To change the step, add by and the amount after the end of the range. This prints the even numbers from 2 through 10:

    for (i <- 2 to 10 by 2) {
    println(i)
}

    The output is 2, 4, 6, 8 and 10, one per line. Read the range from left to right:

    • it starts at 2;
    • to 10 includes 10 as the last possible value;
    • by 2 adds 2 each time, so the values are 2, 4, 6, 8 and 10.

    The step is applied to the start value, so the start decides which numbers you get. Starting at 2 and adding 2 can only give even numbers.

    Using it with until

    by works with until in the same way. The range 2 until 10 by 2 gives 2, 4, 6 and 8, because 10 is excluded. The end value does not need to be one of the values the step produces: 1 to 10 by 3 gives 1, 4, 7 and 10, and 1 to 9 by 3 gives 1, 4 and 7. The range just stops at the last value that doesn't go past the end.

    The assignment: odd numbers from 7 through 23

    The task is to print 7, 9, 11, and so on up to 23.

    1. Pick the start. The first number is 7, which is odd, so adding 2 each time will keep giving odd numbers.
    2. Pick the end and the kind of end. The last number, 23, must be printed, so use to: the range is 7 to 23.
    3. Set the step. Consecutive odd numbers differ by 2, so add by 2. The full range is 7 to 23 by 2.
    4. Print the loop variable. The body is println(i), which puts each value on its own line.
    for (i <- 7 to 23 by 2) {
    println(i)
}

    The output is 7, 9, 11, 13, 15, 17, 19, 21 and 23, each on its own line. That is nine numbers.

    Checking the result

    To see how many values a range has before looping, ask for its size: (7 to 23 by 2).size is 9. The first value is 7 and each of the other eight values adds 2, so the last one is 7 + 8 * 2 = 23, which matches the end of the range.

    Common mistakes

    • Forgetting by 2. 7 to 23 prints every number from 7 to 23, odd and even.
    • Starting on the wrong parity. 8 to 23 by 2 prints even numbers. The start value decides whether the results are odd or even.
    • Using until 23. That leaves out 23 itself.
    • Using a step of 0. 7 to 23 by 0 compiles, but building the range fails at run time with an IllegalArgumentException saying that the step cannot be 0.
    • Using a bigger step than intended. by 3 would skip some of the numbers you want, such as 9.

    Key takeaways

    • range by step changes how far each step moves; the default is 1.
    • The start value and the step together decide which numbers appear, so odd and even sequences differ only in where they begin.
    • The end of the range is a limit, not a promise that the last value will equal it.
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