Arrays

An array is an ordered list of values of any type. xs[i] reads element i, counting from 0, and xs.len() is the number of elements.

let langs = ["go", "rust", "rune"];
print(langs[0]);     // go
print(langs.len());  // 3

let row = [1, 2.5, "three", true, nil, [4]];
print(row);          // [1, 2.5, "three", true, nil, [4]]

let grid = [[1, 2], [3, 4]];
print(grid[1][0]);   // 3

An index must be an integer from -xs.len() to xs.len() - 1; a negative one counts from the end, as in strings, bytes, slice and remove. One out of bounds is an IndexError, for reading and writing alike. To grow an array, push to it.

let langs = ["go", "rust", "rune"];
print(langs[-1]); // rune
print(langs[3]);  // IndexError: Index out of bounds: 3 of 3

Writing an element needs a mutable variable (see Variables):

let mut scores = [70, 85, 90];
scores[0] = 75;
scores[1] += 5;
scores.push(100);
print(scores); // [75, 90, 90, 100]

for x in xs walks the elements, for i, x in xs the indices too. The elements are taken when the loop starts, so pushing inside the loop does not extend it.

let todo = ["write docs", "fix bug"];
for i, task in todo {
    print("{i + 1}. {task}");
}
// 1. write docs
// 2. fix bug

== compares arrays element by element:

print([1, [2, 3]] == [1, [2, 3]]); // true
print([1, 2] == [2, 1]);           // false

Arrays are references: assigning an array or passing it to a function does not copy it, so push, pop and index assignment are visible through every reference. [...xs] or xs.slice(0) makes a copy.

let a = [1, 2];
let mut b = a;
b.push(3);
print(a); // [1, 2, 3]

let mut c = [...a];
c.push(4);
print(a); // [1, 2, 3]
print(c); // [1, 2, 3, 4]

slice, map, filter, sort, join and the rest are array methods.

let prices = [12, 5, 30, 8];
print(prices.slice(1, 3));               // [5, 30]
print(prices.filter(p => p > 10));       // [12, 30]
print(prices.reduce((a, b) => a + b));   // 55