# selectRandomWeighted

## Category
Random

## Arity
**Unary** — `selectRandomWeighted <array>`.

```
selectRandomWeighted <array>   // [[val1, weight1], [val2, weight2], ...]
```

## Description
Picks and returns a random element based on weighted probabilities. The right operand is an array of `[value, weight]` pairs. Higher weight = more likely to be chosen. Returns `nil` if the array is empty.

## How It Works
1. Sums all weights.
2. Generates a random number in `[0, totalWeight)`.
3. Walks the array, subtracting each weight from the random value until it drops below zero.
4. Returns the corresponding value.

## Usage

### Basic weighted pick
```sqf
_loot = selectRandomWeighted [
    ["common", 70],
    ["rare", 25],
    ["epic", 5]
];
// 70% common, 25% rare, 5% epic
```

### AI skill randomization
```sqf
_skill = selectRandomWeighted [
    ["novice", 40],
    ["regular", 35],
    ["veteran", 20],
    ["elite", 5]
];
```

### Loot drop table
```sqf
_drop = selectRandomWeighted [
    [nil, 50],            // 50% nothing
    ["gold", 30],         // 30% gold
    ["potion", 15],       // 15% potion
    ["sword", 5]          // 5% sword
];
if (!isNil "_drop") then {
    giveItem(_drop);
};
```

### Weather selection
```sqf
_weather = selectRandomWeighted [
    ["clear", 60],
    ["cloudy", 25],
    ["rain", 10],
    ["storm", 5]
];
```

## Thread Safety
**Not fully thread-safe for mutation** — reads the array. The internal RNG is thread-safe. Array must be accessible from current scheduler.

## See Also
- [selectRandom](selectRandom.md) — uniform random pick
- [random](random.md) — random float
- [select](select.md) — deterministic element access