Find item in array using weighed probability and a

问题:

Last week I had some problems with a simple program I am doing and somebody here helped me. Now I have run into another problem. I currently have this code:

var findItem = function(desiredItem) {
    var items = [
        { item: "rusty nail", probability: 0.25 },
        { item: "stone", probability: 0.23 },
        { item: "banana", probability: 0.20 },
        { item: "leaf", probability: 0.17 },
        { item: "mushroom", probability: 0.10 },
        { item: "diamond", probability: 0.05 }
    ];
    var possible = items.some( ({item, probability}) => 
          item === desiredItem && probability > 0 );
    if (!possible) {
        console.log('There is no chance you\'ll ever find a ' + desiredItem);
        return;
    }
    var sum = items.reduce( (sum, {item, probability}) => sum+probability, 0 );
    while (true) {
        var value = Math.random() * sum;
        var lootedItem = items.find( 
                ({item, probability}) => (value -= probability) <= 0 ).item;
        if (lootedItem === 'diamond') break;
        console.log("Dang! A " + lootedItem + " was found...");
    }
    console.log("Lucky! A " + desiredItem + " was found!");
}

findItem('diamond');

Now I would like to expand on this by adding another value called category to the items array. I want the categories to have a value of either 2, 5 or 10. So let's say the diamond item would belong to category: 10, and when findItem is executed only items that belong to the same category can be found. I have been trying for a couple of days now but can't seem to get my head around it. Maybe someone can help push me in the right direction? Thanks in advance

回答1:

You could use this update to that code:

// Pass the item list and the desired category as arguments:
var findItem = function(items, category, desiredItem) {
    // apply filter to items, so only those of the given category remain:
    items = items.filter( item => item.category == category );
    // rest of code remains the same:
    var possible = items.some( ({item, probability}) => 
          item === desiredItem && probability > 0 );
    if (!possible) {
        console.log('There is no chance you\'ll ever find a ' + desiredItem);
        return;
    }
    var sum = items.reduce( (sum, {item, probability}) => sum+probability, 0 );
    var t = 10;
    while (true) {
        var value = Math.random() * sum;
        var lootedItem = items.find( 
                ({item, probability}) => (value -= probability) <= 0 ).item;
        if (lootedItem === desiredItem) break; // fixed this condition!
        console.log("Dang! A " + lootedItem + " was found...");
        t--; if (t <= 0) throw "loop";
    }
    console.log("Lucky! A " + desiredItem + " was found!");
}

// Define items here with their category
var items = [
    { item: "rusty nail", probability: 0.25, category:  2 },
    { item: "stone",      probability: 0.23, category:  2 },
    { item: "banana",     probability: 0.20, category:  2 },
    { item: "leaf",       probability: 0.17, category:  5 },
    { item: "mushroom",   probability: 0.10, category:  5 },
    { item: "diamond",    probability: 0.05, category: 10 }
];

// Call function with extra arguments:
findItem(items, 5, 'mushroom');

console.log('second run:');
// This will obviously give a hit immediately, as there is only one possible item:
findItem(items, 10, 'diamond');

The changes are:

• Pass more arguments to your function: the items list and the desired category
• Apply a filter on the items list as first action in the function
• Fix an issue concerning the lootedItem test -- it had "diamond" hard-coded.
• Define the items list outside of the function and add category values to each element.
• Adapt the call of the function to pass the extra arguments.

回答2:

Do you want something like this?

var items = [ { item: "rusty nail", probability: 0.25, category: 10 }
            , { item: "stone",      probability: 0.23, category:  5 }
            , { item: "banana",     probability: 0.20, category:  2 }
            , { item: "leaf",       probability: 0.17, category:  5 }
            , { item: "mushroom",   probability: 0.10, category:  2 }
            , { item: "diamond",    probability: 0.05, category: 10 }
            ];

findItem("diamond", items);

function findItem(needle, haystack) {
    var item = haystack.find(thing => thing.item === needle &&
                                       thing.probability > 0);

    if (item) {
        var category = item.category;
        var items = haystack.filter(thing => thing.category === category);
        var sum = items.reduce((sum, thing) => sum + thing.probability, 0);

        var value = sum * Math.random();
        var loot = items.find(thing => (value -= thing.probability) <= 0).item;

        while (loot !== needle) {
            value = sum * Math.random();
            console.log("Dang! A " + loot + " was found...");
            loot = items.find(thing => (value -= thing.probability) <= 0).item;
        }

        return console.log("Lucky! A " + needle + " was found!");
    }

    console.log("There's no chance that you'll ever find a " + needle);
}

The only major difference from your code is that I used filter to limit the search.