Question

In Java, given n Items, each with weight w, how does one choose a random Item from the collection with a chance equal to w?

Assume each weight is a double from 0.0 to 1.0, and that the weights in the collection sum to 1. Item.getWeight() returns the Item’s weight.

Answer 1

2020 Update (interesting how this got 37 upvotes with a glaring bug in the 2011 version below):

• Fix the impossibility to select the last item when Math.random() yields a number very close to 1.0, and we are unlucky with floating point precision: random index -1 would be the result, which is obviously wrong.
• Some code compaction
• Less variable names used

Item[] items = ...;

// Compute the total weight of all items together.
// This can be skipped of course if sum is already 1.
double totalWeight = 0.0;
for (Item i : items) {
    totalWeight += i.getWeight();
}

// Now choose a random item.
int idx = 0;
for (double r = Math.random() * totalWeight; idx < items.length - 1; ++idx) {
    r -= items[idx].getWeight();
    if (r <= 0.0) break;
}
Item myRandomItem = items[idx];

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2011 version (for comparison left in):

Item[] items = ...;

// Compute the total weight of all items together
double totalWeight = 0.0d;
for (Item i : items)
{
    totalWeight += i.getWeight();
}
// Now choose a random item
int randomIndex = -1;
double random = Math.random() * totalWeight;
for (int i = 0; i < items.length; ++i)
{
    random -= items[i].getWeight();
    if (random <= 0.0d)
    {
        randomIndex = i;
        break;
    }
}
Item myRandomItem = items[randomIndex];

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