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Update classes and basic syntax pages (#4117)
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* update classes page to mention variables, and update variables section in basic syntax page to mention initialization and more.

* SME review: improved code comments

* Replace "." for ":" the lines that were missing it before code blocks.

* Daniel review

* chore: remove two code comments
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@AlejandraPedroza
AlejandraPedroza committed Apr 16, 2024
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125 changes: 83 additions & 42 deletions docs/topics/basic-syntax.md
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Original file line number Diff line number Diff line change
Expand Up @@ -8,7 +8,7 @@ by JetBrains Academy.

## Package definition and imports

Package specification should be at the top of the source file.
Package specification should be at the top of the source file:

```kotlin
package my.demo
Expand All @@ -24,7 +24,7 @@ See [Packages](packages.md).

## Program entry point

An entry point of a Kotlin application is the `main` function.
An entry point of a Kotlin application is the `main` function:

```kotlin
fun main() {
Expand All @@ -33,7 +33,7 @@ fun main() {
```
{kotlin-runnable="true" kotlin-min-compiler-version="1.3" id="kotlin-basic-syntax-hello-world"}

Another form of `main` accepts a variable number of `String` arguments.
Another form of `main` accepts a variable number of `String` arguments:

```kotlin
fun main(args: Array<String>) {
Expand All @@ -44,7 +44,7 @@ fun main(args: Array<String>) {

## Print to the standard output

`print` prints its argument to the standard output.
`print` prints its argument to the standard output:

```kotlin
fun main() {
Expand All @@ -56,7 +56,7 @@ fun main() {
```
{kotlin-runnable="true" kotlin-min-compiler-version="1.3" id="kotlin-basic-syntax-print"}

`println` prints its arguments and adds a line break, so that the next thing you print appears on the next line.
`println` prints its arguments and adds a line break, so that the next thing you print appears on the next line:

```kotlin
fun main() {
Expand All @@ -70,7 +70,7 @@ fun main() {

## Functions

A function with two `Int` parameters and `Int` return type.
A function with two `Int` parameters and `Int` return type:

```kotlin
//sampleStart
Expand All @@ -86,7 +86,7 @@ fun main() {
```
{kotlin-runnable="true" kotlin-min-compiler-version="1.3" id="kotlin-basic-syntax-return-int"}

A function body can be an expression. Its return type is inferred.
A function body can be an expression. Its return type is inferred:

```kotlin
//sampleStart
Expand All @@ -99,7 +99,7 @@ fun main() {
```
{kotlin-runnable="true" kotlin-min-compiler-version="1.3" id="kotlin-basic-syntax-function-expression"}

A function that returns no meaningful value.
A function that returns no meaningful value:

```kotlin
//sampleStart
Expand All @@ -114,7 +114,7 @@ fun main() {
```
{kotlin-runnable="true" kotlin-min-compiler-version="1.3" id="kotlin-basic-syntax-return-unit"}

`Unit` return type can be omitted.
`Unit` return type can be omitted:

```kotlin
//sampleStart
Expand All @@ -133,44 +133,87 @@ See [Functions](functions.md).

## Variables

Read-only local variables are defined using the keyword `val`. They can be assigned a value only once.
In Kotlin, you declare a variable starting with a keyword, `val` or `var`, followed by the name of the variable.

Use the `val` keyword to declare variables that are assigned a value only once. These are immutable, read-only local variables that can’t be reassigned a different value
after initialization:

```kotlin
fun main() {
//sampleStart
val a: Int = 1 // immediate assignment
val b = 2 // `Int` type is inferred
val c: Int // Type required when no initializer is provided
c = 3 // deferred assignment
// Declares the variable x and initializes it with the value of 5
val x: Int = 5
// 5
//sampleEnd
println("a = $a, b = $b, c = $c")
println(x)
}
```
{kotlin-runnable="true" kotlin-min-compiler-version="1.3" id="kotlin-basic-syntax-val"}

Variables that can be reassigned use the `var` keyword.
Use the `var` keyword to declare variables that can be reassigned. These are mutable variables, and you can change their values after initialization:

```kotlin
fun main() {
//sampleStart
var x = 5 // `Int` type is inferred
// Declares the variable x and initializes it with the value of 5
var x: Int = 5
// Reassigns a new value of 6 to the variable x
x += 1
// 6
//sampleEnd
println("x = $x")
println(x)
}
```
{kotlin-runnable="true" kotlin-min-compiler-version="1.3" id="kotlin-basic-syntax-var"}

You can declare variables at the top level.
Kotlin supports type inference and automatically identifies the data type of a declared variable. When declaring a variable, you can omit the type after the variable name:

```kotlin
fun main() {
//sampleStart
// Declares the variable x with the value of 5;`Int` type is inferred
val x = 5
// 5
//sampleEnd
println(x)
}
```
{kotlin-runnable="true" kotlin-min-compiler-version="1.3" id="kotlin-basic-syntax-inference"}

You can use variables only after initializing them. You can either initialize a variable at the moment of declaration or declare a variable first and initialize it later.
In the second case, you must specify the data type:

```kotlin
fun main() {
//sampleStart
// Initializes the variable x at the moment of declaration; type is not required
val x = 5
// Declares the variable c without initialization; type is required
val c: Int
// Initializes the variable c after declaration
c = 3
// 5
// 3
//sampleEnd
println(x)
println(c)
}
```
{kotlin-runnable="true" kotlin-min-compiler-version="1.3" id="kotlin-basic-syntax-initialize"}

You can declare variables at the top level:

```kotlin
//sampleStart
val PI = 3.14
var x = 0

fun incrementX() {
x += 1
fun incrementX() {
x += 1
}
// x = 0; PI = 3.14
// incrementX()
// x = 1; PI = 3.14
//sampleEnd

fun main() {
Expand All @@ -182,40 +225,38 @@ fun main() {
```
{kotlin-runnable="true" kotlin-min-compiler-version="1.3" id="kotlin-basic-syntax-variable-top-level"}

See also [Properties](properties.md).
For information about declaring properties, see [Properties](properties.md).

## Creating classes and instances

To define a class, use the `class` keyword.
To define a class, use the `class` keyword:
```kotlin
class Shape
```

Properties of a class can be listed in its declaration or body.
Properties of a class can be listed in its declaration or body:

```kotlin
class Rectangle(val height: Double, val length: Double) {
val perimeter = (height + length) * 2
}
```

The default constructor with parameters listed in the class declaration is available automatically.
The default constructor with parameters listed in the class declaration is available automatically:

```kotlin
class Rectangle(val height: Double, val length: Double) {
val perimeter = (height + length) * 2
}
fun main() {
//sampleStart
val rectangle = Rectangle(5.0, 2.0)
println("The perimeter is ${rectangle.perimeter}")
//sampleEnd
}
```
{kotlin-runnable="true" kotlin-min-compiler-version="1.3" id="kotlin-basic-syntax-class-constructor"}

Inheritance between classes is declared by a colon (`:`). Classes are final by default; to make a class inheritable,
mark it as `open`.
Inheritance between classes is declared by a colon (`:`). Classes are `final` by default; to make a class inheritable,
mark it as `open`:

```kotlin
open class Shape
Expand All @@ -225,11 +266,11 @@ class Rectangle(val height: Double, val length: Double): Shape() {
}
```

See [classes](classes.md) and [objects and instances](object-declarations.md).
For more information about constructors and inheritance, see [Classes](classes.md) and [Objects and instances](object-declarations.md).

## Comments

Just like most modern languages, Kotlin supports single-line (or _end-of-line_) and multi-line (_block_) comments.
Just like most modern languages, Kotlin supports single-line (or _end-of-line_) and multi-line (_block_) comments:

```kotlin
// This is an end-of-line comment
Expand All @@ -238,7 +279,7 @@ Just like most modern languages, Kotlin supports single-line (or _end-of-line_)
on multiple lines. */
```

Block comments in Kotlin can be nested.
Block comments in Kotlin can be nested:

```kotlin
/* The comment starts here
Expand Down Expand Up @@ -287,7 +328,7 @@ fun main() {
```
{kotlin-runnable="true" kotlin-min-compiler-version="1.3" id="kotlin-basic-syntax-conditional-expressions"}

In Kotlin, `if` can also be used as an expression.
In Kotlin, `if` can also be used as an expression:

```kotlin
//sampleStart
Expand Down Expand Up @@ -316,7 +357,7 @@ fun main() {
```
{kotlin-runnable="true" kotlin-min-compiler-version="1.3" id="kotlin-basic-syntax-for-loop"}

or
or:

```kotlin
fun main() {
Expand Down Expand Up @@ -378,7 +419,7 @@ See [when expression](control-flow.md#when-expression).

## Ranges

Check if a number is within a range using `in` operator.
Check if a number is within a range using `in` operator:

```kotlin
fun main() {
Expand All @@ -393,7 +434,7 @@ fun main() {
```
{kotlin-runnable="true" kotlin-min-compiler-version="1.3" id="kotlin-basic-syntax-range-in"}

Check if a number is out of range.
Check if a number is out of range:

```kotlin
fun main() {
Expand All @@ -411,7 +452,7 @@ fun main() {
```
{kotlin-runnable="true" kotlin-min-compiler-version="1.3" id="kotlin-basic-syntax-out-of-range"}

Iterate over a range.
Iterate over a range:

```kotlin
fun main() {
Expand All @@ -424,7 +465,7 @@ fun main() {
```
{kotlin-runnable="true" kotlin-min-compiler-version="1.3" id="kotlin-basic-syntax-iterate-range"}

Or over a progression.
Or over a progression:

```kotlin
fun main() {
Expand All @@ -445,7 +486,7 @@ See [Ranges and progressions](ranges.md).

## Collections

Iterate over a collection.
Iterate over a collection:

```kotlin
fun main() {
Expand Down Expand Up @@ -535,7 +576,7 @@ fun main() {
```
{kotlin-runnable="true" kotlin-min-compiler-version="1.3" id="kotlin-basic-syntax-function-nullable-value"}

or
or:

```kotlin
fun parseInt(str: String): Int? {
Expand Down Expand Up @@ -601,7 +642,7 @@ fun main() {
```
{kotlin-runnable="true" kotlin-min-compiler-version="1.3" id="kotlin-basic-syntax-is-operator"}

or
or:

```kotlin
//sampleStart
Expand All @@ -624,7 +665,7 @@ fun main() {
```
{kotlin-runnable="true" kotlin-min-compiler-version="1.3" id="kotlin-basic-syntax-is-operator-expression"}

or even
or even:

```kotlin
//sampleStart
Expand Down
2 changes: 1 addition & 1 deletion docs/topics/classes.md
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Expand Up @@ -173,7 +173,7 @@ class DontCreateMe private constructor() { /*...*/ }

## Creating instances of classes

To create an instance of a class, call the constructor as if it were a regular function:
To create an instance of a class, call the constructor as if it were a regular function. You can assign the created instance to a [variable](basic-syntax.md#variables):

```kotlin
val invoice = Invoice()
Expand Down

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