CODEWITHSUNDEEP
javascript
DAS
DAS

Reputation: 25

Why does reduce return the number itself and, as in functional programming, replace if and break?

There is a simple function, its essence is to count from a number (n) to 0. But when using reduce, the function just doesn't work, and no matter how I rewrite it, it returns either an empty array, or undefined, or the number itself 2. First, I created an array that will take n, then I created a reduce method in which currentValue will take n and subtract 1 from it, after accumulator it takes the resulting number and using the push method, add it to the list array, but I don’t understand how I should add a condition that if accumulator is equal to 0, then the function must be stopped.

const countBits = (n) => {
    let list = [n];
    let resultReduce = n.reduce((accumulator, currentValue) => {
            accumulator = currentValue - 1;
            list.push(accumulator);
        }); 
    return resultReduce;
};
console.log(countBits([2]));

Why isn't this working the way I intended it to?

Upvotes: 0

Views: 53

Answers (2)

Julian
Julian

Reputation: 4366

The answer by Jack Bashford is correct, but for completeness I would like to point out that generating a range of numbers is a common need. Libraries like Underscore, Lodash and Ramda provide a ready-to-use function for this purpose. You don’t have to write your own implementation every time you need something common and mundane like that; save the time and enjoy the fact that you can spend your time on something more groundbreaking instead.

console.log(_.range(2, -1, -1));
<script src="https://underscorejs.org/underscore-umd-min.js"></script>

Also for the sake of completeness, let’s consider how you might implement a downwards range function using reduce, anyway. reduce expects an input array, though it can also accept an object if using Underscore or Lodash. To make meaningful use of the input collection, we could generate a consecutive number for every element of the collection. For an array, we could of course just do _.range(collection.length - 1, -1, -1) instead, but for an object, or something that you don’t know the length of in advance, such as a generator, using reduce for this purpose might make sense. The mapDownwardsRange function below will do this:

function unshiftNext(array) {
    const front = array.length ? array[0] : -1;
    return [front + 1].concat(array);
}

function mapDownwardsRange(collection) {
    return _.reduce(collection, unshiftNext, []);
}

console.log(mapDownwardsRange(['a', 'b', 'c']));
<script src="https://underscorejs.org/underscore-umd-min.js"></script>

Upvotes: 0

Jack Bashford
Jack Bashford

Reputation: 44125

reduce will run on each of the items in the array, with the accumulator (first argument to the callback function) being the value that is returned from the callback function's previous iteration. So if you don't return anything, accumulator will be undefined for the next iteration.

If you want to count from n to 0, reduce is not the way to go (as well as the fact that in your current implementation, you don't even use list which would contain all of your stored numbers from n to 0). I would advise that instead, you simply loop from n to 0 and push those values into an array like so:

const countBits = (n) => {
    let list = [];
    for (let i = n; i > -1; i--) {
      list.push(i);
    }
    return list;
};
console.log(countBits(2));

Also note I've changed your syntax slightly in the function call - you were passing an array with a single element seemingly unnecessarily, so I just passed the element itself to simplify the code.

Upvotes: 3

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