# Ranges and progressions

Kotlin lets you easily create ranges of values using the `rangeTo()`

function from the `kotlin.ranges`

package and its operator form `..`

. Usually, `rangeTo()`

is complemented by `in`

or `!in`

functions.

Integral type ranges (`IntRange`

, `LongRange`

, `CharRange`

) have an extra feature: they can be iterated over. These ranges are also progressions of the corresponding integral types.

Such ranges are generally used for iteration in `for`

loops.

To iterate numbers in reverse order, use the `downTo`

function instead of `..`

.

It is also possible to iterate over numbers with an arbitrary step (not necessarily 1). This is done via the `step`

function.

To iterate a number range which does not include its end element, use the `until`

function:

## Range

A range defines a closed interval in the mathematical sense: it is defined by its two endpoint values which are both included in the range. Ranges are defined for comparable types: having an order, you can define whether an arbitrary instance is in the range between two given instances.

The main operation on ranges is `contains`

, which is usually used in the form of `in`

and `!in`

operators.

To create a range for your class, call the `rangeTo()`

function on the range start value and provide the end value as an argument. `rangeTo()`

is often called in its operator form `..`

.

## Progression

As shown in the examples above, the ranges of integral types, such as `Int`

, `Long`

, and `Char`

, can be treated as arithmetic progressions of them. In Kotlin, these progressions are defined by special types: `IntProgression`

, `LongProgression`

, and `CharProgression`

.

Progressions have three essential properties: the `first`

element, the `last`

element, and a non-zero `step`

. The first element is `first`

, subsequent elements are the previous element plus a `step`

. Iteration over a progression with a positive step is equivalent to an indexed `for`

loop in Java/JavaScript.

When you create a progression implicitly by iterating a range, this progression's `first`

and `last`

elements are the range's endpoints, and the `step`

is 1.

To define a custom progression step, use the `step`

function on a range.

The `last`

element of the progression is calculated this way:

For a positive step: the maximum value not greater than the end value such that

`(last - first) % step == 0`

.For a negative step: the minimum value not less than the end value such that

`(last - first) % step == 0`

.

Thus, the `last`

element is not always the same as the specified end value.

To create a progression for iterating in reverse order, use `downTo`

instead of `..`

when defining the range for it.

If you already have a progression, you can iterate it in reverse order with the `reversed`

function:

Progressions implement `Iterable<N>`

, where `N`

is `Int`

, `Long`

, or `Char`

respectively, so you can use them in various collection functions like `map`

, `filter`

, and other.