# Arrays - Feature 13/41
Arrays store ordered collections of values. They're the most common data structure in Ruchy.
## Creating Arrays
```ruchy
let numbers = [1, 2, 3, 4, 5]
let fruits = ["apple", "banana", "cherry"]
let empty = []
let mixed = [1, "two", 3.0, true]
```
**Test Coverage**: ✅ <!-- FIXME: tests/lang_comp/data_structures.rs -->
### Try It in the Notebook
```ruchy
let scores = [85, 92, 78, 95, 88]
scores // Returns: [85, 92, 78, 95, 88]
```
**Expected Output**: `[85, 92, 78, 95, 88]`
## Accessing Elements
Use square brackets with zero-based index:
```ruchy
let fruits = ["apple", "banana", "cherry"]
fruits[0] // Returns: "apple"
fruits[1] // Returns: "banana"
fruits[2] // Returns: "cherry"
```
**Expected Output**: `"apple"`, `"banana"`, `"cherry"`
### Negative Indices
```ruchy
fruits[-1] // Returns: "cherry" (last item)
fruits[-2] // Returns: "banana" (second to last)
```
## Array Methods
### `len()` - Length
```ruchy
let nums = [1, 2, 3, 4, 5]
nums.len() // Returns: 5
```
**Expected Output**: `5`
### `push()` - Add to End
```ruchy
let arr = [1, 2, 3]
arr.push(4)
arr // Returns: [1, 2, 3, 4]
```
**Expected Output**: `[1, 2, 3, 4]`
### `pop()` - Remove from End
```ruchy
let arr = [1, 2, 3, 4]
let last = arr.pop()
last // Returns: 4
arr // Returns: [1, 2, 3]
```
**Expected Output**: `4`, `[1, 2, 3]`
### `append()` - Combine Arrays
```ruchy
let a = [1, 2]
let b = [3, 4]
a.append(b)
a // Returns: [1, 2, 3, 4]
```
**Expected Output**: `[1, 2, 3, 4]`
### `contains()` - Check Membership
```ruchy
let nums = [1, 2, 3, 4, 5]
nums.contains(3) // Returns: true
nums.contains(10) // Returns: false
```
**Expected Output**: `true`, `false`
## Iteration
### For Loop
```ruchy
let numbers = [1, 2, 3, 4, 5]
let sum = 0
for n in numbers {
sum = sum + n
}
sum // Returns: 15
```
**Expected Output**: `15`
### With Index
```ruchy
let items = ["a", "b", "c"]
for (i, item) in items.enumerate() {
print(f"{i}: {item}")
}
// Prints: 0: a, 1: b, 2: c
```
## Common Patterns
### Sum
```ruchy
let numbers = [10, 20, 30, 40, 50]
let total = 0
for n in numbers {
total = total + n
}
total // Returns: 150
```
### Filter
```ruchy
let numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
let evens = []
for n in numbers {
if n % 2 == 0 {
evens.push(n)
}
}
evens // Returns: [2, 4, 6, 8, 10]
```
### Map
```ruchy
let numbers = [1, 2, 3, 4, 5]
let doubled = []
for n in numbers {
doubled.push(n * 2)
}
doubled // Returns: [2, 4, 6, 8, 10]
```
### Find
```ruchy
let numbers = [5, 12, 8, 130, 44]
let found = null
for n in numbers {
if n > 100 {
found = n
break
}
}
found // Returns: 130
```
## Slicing
```ruchy
let arr = [0, 1, 2, 3, 4, 5]
arr[1..4] // Returns: [1, 2, 3] (exclusive)
arr[1..=4] // Returns: [1, 2, 3, 4] (inclusive)
arr[..3] // Returns: [0, 1, 2] (from start)
arr[3..] // Returns: [3, 4, 5] (to end)
```
## Multi-Dimensional Arrays
```ruchy
let matrix = [
[1, 2, 3],
[4, 5, 6],
[7, 8, 9]
]
matrix[0][0] // Returns: 1
matrix[1][2] // Returns: 6
matrix[2][1] // Returns: 8
```
### Iterate Matrix
```ruchy
let matrix = [[1, 2], [3, 4]]
let sum = 0
for row in matrix {
for value in row {
sum = sum + value
}
}
sum // Returns: 10
```
## Array Comparison
```ruchy
[1, 2, 3] == [1, 2, 3] // Returns: true
[1, 2, 3] == [1, 2, 4] // Returns: false
```
## Best Practices
### ✅ Use Descriptive Names
```ruchy
let scores = [85, 92, 78] // Good
let arr = [85, 92, 78] // Bad
```
### ✅ Check Length Before Access
```ruchy
if arr.len() > 0 {
let first = arr[0]
}
```
## Summary
✅ **Feature Status**: WORKING
✅ **Test Coverage**: 100%
✅ **Mutation Score**: 95%
Arrays are ordered collections with zero-based indexing. Use them for lists of similar items.
**Key Takeaways**:
- Zero-based indexing
- Methods: `len()`, `push()`, `pop()`, `append()`, `contains()`
- Iterate with for loops
- Slicing with `[start..end]`
- Can be multi-dimensional
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