解析JavaScript中的数组数组

Question

I need a function that would return a value from an array that may contain any number of arrays. It should be called like getValueFromArray(array, [2, 4]) - this example should return 4th element of the 2d array of the passed array.

Here is my code:

function getValueFromArray(arr, indexes){

var val,
    currentIndex = indexes[0];

    if(!arr[currentIndex] || arr[currentIndex] === '') return value = '';

    indexes.splice(0, 1);

    if(arr[currentIndex].length) 
        getValueFromArray(arr[currentIndex], indexes);
    else {
        val = arr[currentIndex];
        return val;
    }
 }


      var y = getValueFromArray([[1,2,3,4], [1,2,3,4]], [0, 2]); // should return 3

      var x = getValueFromArray([[1,2,3,4], [1,2,3,4], [5,6,7,8]], [2, 3]); // should return 8

      var z = getValueFromArray(
                           [
                               [[1,2,3,4], [1,2,3,4], [1,2,3,4]],
                               [[1,2,3,4], [1,2,3,4]]
                           ], 
                           [0, 1, 2]
                          ); // should return 3

Such call should return 3, and if I debug the function, it actually returns the correct value but when I assign it to a variable, it returns undefined. I guess it's because of the recursion, variable gets the value that is undefined during the first function call. How can this be fixed?

Thank you!

Answer 1

You are not returning your recursive results.

if(arr[currentIndex].length) 
    getValueFromArray(arr[currentIndex], indexes);

Should be:

if(arr[currentIndex].length) 
    return getValueFromArray(arr[currentIndex], indexes);

Answer 2

you missed one thing in your code(returning the result after if condition) try below given code:-

function getValueFromArray(arr, indexes){

    var val,
        currentIndex = indexes[0];

        if(!arr[currentIndex] || arr[currentIndex] === '') return value = '';

        indexes.splice(0, 1);

        if(arr[currentIndex].length) 
           return getValueFromArray(arr[currentIndex], indexes);
        else {
            val = arr[currentIndex];
            return val;
        }
     }
    var y = getValueFromArray([[1,2,3,4], [1,2,3,4]], [0, 2]);
    console.log(y)

run it and see, now its showing the result into the variable.

Answer 3

Your guess is correct. You simply forgot to return whatever the recursive call returns:

if(arr[currentIndex].length) 
    getValueFromArray(arr[currentIndex], indexes); // <---- here

That being said, I have to agree that you could easily do it more concisely (this will eventually destroy indexes , however):

function getValueFromArray(arr, indexes){
  while(indexes.length) arr=arr[indexes.shift()]
  return arr
}

Answer 4

I think you need to add a return before getValueFromArray(arr[currentIndex], indexes);
To make the final calculated value to recurse up the recursive method call stack as each recursed call returns.

Answer 5

It's because if condition is not returning any value. Try following code

function getValueFromArray(arr, indexes){

var val='',currentIndex = indexes[0];

    if(!arr[currentIndex] || arr[currentIndex] === '') return val;

    indexes.splice(0, 1);

    if(arr[currentIndex].length) {

       // Because if your function walks this way 
       // it does not meet any 'return' statement
       // till the end and returns nothing.
        return getValueFromArray(arr[currentIndex], indexes);
    }   
    else { 
        val = arr[currentIndex];
        return val;
    }
 }

Then console log you variable

var y = getValueFromArray([[1,2,3,4], [1,2,3,4]], [0, 2]);
console.log(y)

Answer 6

I'll post my code too as I feel it provides a simpler solution to your problem.
There is no recursion involved, so it should run a bit faster in theory.

var arr = [
  [
    [
      [12, 5, 6],
      [ 6, 7, 8],
      [11, 0, 9]
    ],
    [
      [-1, 1, 8],
      [ 4, 5, 6]
    ]
  ],
  [
    [
        [7, 8, 9, 10]
    ]
  ]
];

function getValueFromArray(arr, indexes){
    var value = arr, i = 0, len = indexes.length, index = null;

    for (; i < len; i += 1) {
        index = indexes[i];
        // check if the current array contains an {index}-th record
        if ( index in value ) { 
            value = value[index];
        } else {
            // or throw an exception if you want
            return null;
        }
    }

    return value;
 }

getValueFromArray(arr, [0, 1, 1, 2]) // 6

Answer 7

function getValueFromArray(arr, indexes) {
    // exit if arr is not an array
    // exit if arr is empty
    // exit if indexes is not an array
    // exit if indexes is empty
    if (!Array.isArray(arr) || !arr.length || !Array.isArray(indexes) || !indexes.length) {
        return; // may throw exception or return other value
    }
    var currentIndex = indexes.shift();
    // exit if index is not in arr
    // exit if index is negative
    if (arr.length <= currentIndex || currentIndex < 0) {
        return; // may throw exception or return other value
    }
    return Array.isArray(arr[currentIndex]) ? getValueFromArray(arr[currentIndex], indexes) : arr[currentIndex];
}

Answer 8

You are overthinking this:

function getValueFromArray(arr, indexes){
    return arr[indexes[0]][indexes[1]];
}

EDIT for 1 to 3 dimensional array:

function getValueFromArray(arr, indexes){
    if (indexes.length == 1) {
        return arr[indexes[0]];
    } else if (indexes.length == 2) {
        return arr[indexes[0][indexes[1]];
    } else if (indexes.length == 3) {
        return arr[indexes[0][indexes[1][indexes[2]];
    } else {
      // 4 dimensional arrays???
    }
}

Would you have more than 3 dimensional array?

It would be best find a way to append indexes[i] but I can't think of a way at the moment and I don't think it's possible.