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Assignment

Overview

Assignment is to set a value into a variable. This expression is evaluated right to left.

a = b = 10;

You can use a variable without declaration, but the scope depends on the situation. If there is a declaration in outer scope, compiler will refer it. Otherwise, compiler will use it as declared there.

Destructuring assignment

You can use the style of an array or object in assignment. The following three styles are available.

  • Array Style ... each item in the array will be assigned to a variable in the order.
  • Object Key Style ... the value according to a key name will be assigned to the variable of the same name as the key.
  • Object Style ... each value will be assigned to the variable bound to each key.

Here is an example.

[a, b, , ...c] = [1, 2, 3, 4, 5, 6];  // 3rd parameter is skipped.
{ x, y } = { x: 20, y: { a: 30, b: 300 } };
{ x: d, y: { a: e, b: f } } = { x: 20, y: { a: 30, b: 300 } };

System.println("a = ", a);
System.println("b = ", b);
System.println("c = ", c);
System.println("d = ", d);
System.println("e = ", e);
System.println("f = ", f);
System.println("x = ", x);
System.println("y = ", y);

Here is the result.

a = 1
b = 2
c = [4, 5, 6]
d = 20
e = 30
f = 300
x = 20
y = {"a":30,"b":300}

Pattern Matching

When it is an assignment expresison, the pattern matching is available. If a part of variables is a literal, it will be checked if the same value. And if matching a pattern is failed, the exdeption of NoMatchingPatternException will be raised.

Here is an example.

[a, b, , ...c] = [1, 2, 3, 4, 5, 6];
{ x, y } = { x: 20, y: { a: 30, b: 300 } };
{ x: d, y: { a: e, b: 300 } } = { x: 20, y: { a: 30, b: 300 } };

System.println("a = ", a);
System.println("b = ", b);
System.println("c = ", c);
System.println("d = ", d);
System.println("e = ", e);
System.println("x = ", x);
System.println("y = ", y);

// => .y.b requires 300, but it is 3 in actual.
{ x: d, y: { a: e, b: 300 } } = { x: 20, y: { a: 30, b: 3 } };

Here is the result.

a = 1
b = 2
c = [4, 5, 6]
d = 20
e = 30
x = 20
y = {"a":30,"b":300}
Uncaught exception: No one catch the exception.
NoMatchingPatternException: Pattern not matched
Stack Trace Information:
        at <main-block>(test.kx:14)
Range in Pattern Matching

If you write a Range in a pattern, it will check inside that Range.

Here is an example.

[1..10, b] = [1, 100];  // b => 100
[1..10, b] = [2, 100];  // b => 100
[1..10, b] = [9, 100];  // b => 100
[1..10, b] = [11, 100]; // NoMatchingPatternException will occur.

Examples

Example 1. Scope (1)

Code

var a = 10;
var b;
{
    var a;
    a = b = 100;
    System.println(a);  // => 100
    System.println(b);  // => 100
}
System.println(a);  // => 10
System.println(b);  // => 100

Result

100
100
10
100

Example 2. Scope (2)

Code

var a = 10;
{
    a = b = 100;
    System.println(a);  // => 100
    System.println(b);  // => 100
}
System.println(a);  // => 10
System.println(b);  // => ???

Result

Error: Symbol(b) is not found near the <test.kx>:8

Example 3. Array & Object Style

Code

[a, b, , ...c] = [1, 2, 3, 4, 5, 6];  // 3rd parameter is skipped.
{ x, y } = { x: 20, y: { a: 30, b: 300 } };
{ x: d, y: { a: e, b: f } } = { x: 20, y: { a: 30, b: 300 } };

System.println("a = ", a);
System.println("b = ", b);
System.println("c = ", c);
System.println("d = ", d);
System.println("e = ", e);
System.println("f = ", f);
System.println("x = ", x);
System.println("y = ", y);

Result

a = 1
b = 2
c = [4, 5, 6]
d = 20
e = 30
f = 300
x = 20
y = {"a":30,"b":300}

Example 4. Pattern Matching

Code

[a, b, , ...c] = [1, 2, 3, 4, 5, 6];
{ x, y } = { x: 20, y: { a: 30, b: 300 } };
{ x: d, y: { a: e, b: 300 } } = { x: 20, y: { a: 30, b: 300 } };

System.println("a = ", a);
System.println("b = ", b);
System.println("c = ", c);
System.println("d = ", d);
System.println("e = ", e);
System.println("x = ", x);
System.println("y = ", y);

// => .y.b requires 300, but it is 3 in actual.
{ x: d, y: { a: e, b: 300 } } = { x: 20, y: { a: 30, b: 3 } };

Result

a = 1
b = 2
c = [4, 5, 6]
d = 20
e = 30
x = 20
y = {"a":30,"b":300}
Uncaught exception: No one catch the exception.
NoMatchingPatternException: Pattern not matched
Stack Trace Information:
        at <main-block>(test.kx:14)

Example 5. Range in Pattern

Code

[1..10, b] = [1, 100];  // b => 100
[1..10, c] = [2, 100];  // c => 100
[1..10, d] = [9, 100];  // d => 100
System.println("b = ", b);
System.println("c = ", c);
System.println("d = ", d);

[1..10, b] = [11, 100]; // NoMatchingPatternException will occur.

Result

b = 100
c = 100
d = 100
Uncaught exception: No one catch the exception.
NoMatchingPatternException: Pattern not matched
Stack Trace Information:
        at <main-block>(test.kx:8)