reorganize module
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Timur Gordon
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_archive/rhai_engine/rhaibook/ref/for.md
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_archive/rhai_engine/rhaibook/ref/for.md
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For Loop
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========
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{{#include ../links.md}}
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Iterating through a numeric [range](ranges.md) or an [array](arrays.md), or any iterable type,
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is provided by the `for` ... `in` loop.
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There are two alternative syntaxes, one including a counter variable:
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> `for` _variable_ `in` _expression_ `{` ... `}`
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>
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> `for (` _variable_ `,` _counter_ `)` `in` _expression_ `{` ... `}`
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Break or Continue
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-----------------
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`continue` can be used to skip to the next iteration, by-passing all following statements;
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`break` can be used to break out of the loop unconditionally.
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For Expression
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--------------
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`for` statements can also be used as _expressions_.
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The `break` statement takes an optional expression that provides the return value.
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The default return value of a `for` expression is `()`.
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```js
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let a = [42, 123, 999, 0, true, "hello", "world!", 987.6543];
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// 'for' can be used just like an expression
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let index = for (item, count) in a {
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// if the 'for' loop breaks here, return a specific value
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switch item.type_of() {
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"i64" if item.is_even => break count,
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"f64" if item.to_int().is_even => break count,
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}
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// ... if the 'for' loop exits here, the return value is ()
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};
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if index == () {
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print("Magic number not found!");
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} else {
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print(`Magic number found at index ${index}!`);
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}
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```
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Counter Variable
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----------------
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The counter variable, if specified, starts from zero, incrementing upwards.
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```js , no_run
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let a = [42, 123, 999, 0, true, "hello", "world!", 987.6543];
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// Loop through the array
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for (item, count) in a {
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if x.type_of() == "string" {
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continue; // skip to the next iteration
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}
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// 'item' contains a copy of each element during each iteration
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// 'count' increments (starting from zero) for each iteration
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print(`Item #${count + 1} = ${item}`);
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if x == 42 { break; } // break out of for loop
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}
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```
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Iterate Through Arrays
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----------------------
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Iterating through an [array](arrays.md) yields cloned _copies_ of each element.
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```rust
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let a = [1, 3, 5, 7, 9, 42];
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// Loop through the array
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for x in a {
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if x > 10 { continue; } // skip to the next iteration
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print(x);
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if x == 42 { break; } // break out of for loop
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}
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```
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Iterate Through Strings
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-----------------------
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Iterating through a [string](strings-chars.md) yields individual [characters](strings-chars.md).
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The `chars` method also allow iterating through characters in a [string](strings-chars.md),
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optionally accepting the character position to start from (counting from the end if negative), as
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well as the number of characters to iterate (defaults to all).
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`char` also accepts a [range](ranges.md) which can be created via the `..` (exclusive) and `..=`
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(inclusive) operators.
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```rust
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let s = "hello, world!";
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// Iterate through all the characters.
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for ch in s {
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print(ch);
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}
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// Iterate starting from the 3rd character and stopping at the 7th.
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for ch in s.chars(2, 5) {
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if ch > 'z' { continue; } // skip to the next iteration
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print(ch);
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if x == '@' { break; } // break out of for loop
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}
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// Iterate starting from the 3rd character and stopping at the end.
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for ch in s.chars(2..s.len) {
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if ch > 'z' { continue; } // skip to the next iteration
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print(ch);
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if x == '@' { break; } // break out of for loop
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}
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```
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Iterate Through Numeric Ranges
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------------------------------
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[Ranges](ranges.md) are created via the `..` (exclusive) and `..=` (inclusive) operators.
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The `range` function similarly creates exclusive [ranges](ranges.md), plus allowing optional step values.
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```rust
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// Iterate starting from 0 and stopping at 49
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// The step is assumed to be 1 when omitted for integers
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for x in 0..50 {
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if x > 10 { continue; } // skip to the next iteration
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print(x);
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if x == 42 { break; } // break out of for loop
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}
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// The 'range' function is just the same
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for x in range(0, 50) {
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if x > 10 { continue; } // skip to the next iteration
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print(x);
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if x == 42 { break; } // break out of for loop
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}
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// The 'range' function also takes a step
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for x in range(0, 50, 3) { // step by 3
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if x > 10 { continue; } // skip to the next iteration
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print(x);
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if x == 42 { break; } // break out of for loop
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}
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// The 'range' function can also step backwards
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for x in range(50..0, -3) { // step down by -3
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if x < 10 { continue; } // skip to the next iteration
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print(x);
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if x == 42 { break; } // break out of for loop
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}
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// It works also for floating-point numbers
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for x in range(5.0, 0.0, -2.0) { // step down by -2.0
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if x < 10 { continue; } // skip to the next iteration
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print(x);
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if x == 4.2 { break; } // break out of for loop
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}
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```
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Iterate Through Bit-Fields
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--------------------------
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The `bits` function allows iterating through an integer as a [bit-field](bit-fields.md).
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`bits` optionally accepts the bit number to start from (counting from the most-significant-bit if
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negative), as well as the number of bits to iterate (defaults all).
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`bits` also accepts a [range](ranges.md) which can be created via the `..` (exclusive) and `..=`
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(inclusive) operators.
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```js , no_run
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let x = 0b_1001110010_1101100010_1100010100;
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let num_on = 0;
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// Iterate through all the bits
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for bit in x.bits() {
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if bit { num_on += 1; }
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}
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print(`There are ${num_on} bits turned on!`);
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const START = 3;
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// Iterate through all the bits from 3 through 12
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for (bit, index) in x.bits(START, 10) {
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print(`Bit #${index} is ${if bit { "ON" } else { "OFF" }}!`);
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if index >= 7 { break; } // break out of for loop
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}
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// Iterate through all the bits from 3 through 12
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for (bit, index) in x.bits(3..=12) {
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print(`Bit #${index} is ${if bit { "ON" } else { "OFF" }}!`);
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if index >= 7 { break; } // break out of for loop
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}
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```
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Iterate Through Object Maps
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---------------------------
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Two methods, `keys` and `values`, return [arrays](arrays.md) containing cloned _copies_
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of all property names and values of an [object map](object-maps.md), respectively.
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These [arrays](arrays.md) can be iterated.
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```rust
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let map = #{a:1, b:3, c:5, d:7, e:9};
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// Property names are returned in unsorted, random order
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for x in map.keys() {
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if x > 10 { continue; } // skip to the next iteration
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print(x);
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if x == 42 { break; } // break out of for loop
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}
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// Property values are returned in unsorted, random order
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for val in map.values() {
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print(val);
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}
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```
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