Exploring nested arrays with recursive looping

Question

Exploring nested arrays with recursive looping

I have been working on solving this challenge using recursion because I enjoy the challenge. The task at hand involves taking an array of arrays and transforming it into a single array with all the values combined. While I have made good progress, I am encountering an issue where the new array I am appending to keeps resetting after each recursion. I would greatly appreciate any advice or suggestions to help me overcome this obstacle.

var multiArray = [[1, 2],[3, 4, 5], [6, 7, 8, 9]]

const flattenArrays = function (arr) {
  let result = [];

  arr.map(element => {
    if (Array.isArray(element)) {
      console.log('Is Array ---> ', element)
      flattenArrays(element);
    } else {
      console.log('Result ----->', result)
      console.log('Else     --->', element)
      result.push(element);
    }

  });

  return result;

};

console.log('Result ----->', flattenArrays(multiArray)); //[ 1, 2, 3, 4, 5, 6, 7, 8, 9 ]

javascript arrays recursion

Answer 1

Answer №1

To utilize the results from calling flatten(element), it is recommended to employ the spread syntax like this:

output.push(...flatten(element));

Additionally, consider replacing the map call with Array.forEach() since the returned array isn't being used.

For instance:

const flatten = function(arr) {
  const output = [];

  arr.forEach(element => {
    if (Array.isArray(element)) {
      output.push(...flatten(element));
    } else {
      output.push(element);
    }

  });

  return output;

};

var myArray = [[1, 2],[3, 4, 5], [6, 7, 8, 9]]


console.log(flatten(myArray)); // [ 1, 2, 3, 4, 5, 6, 7, 8, 9 ]

Answer 2

To utilize the results from calling flatten(element), it is recommended to employ the spread syntax like this:

output.push(...flatten(element));

Additionally, consider replacing the map call with Array.forEach() since the returned array isn't being used.

For instance:

const flatten = function(arr) {
  const output = [];

  arr.forEach(element => {
    if (Array.isArray(element)) {
      output.push(...flatten(element));
    } else {
      output.push(element);
    }

  });

  return output;

};

var myArray = [[1, 2],[3, 4, 5], [6, 7, 8, 9]]


console.log(flatten(myArray)); // [ 1, 2, 3, 4, 5, 6, 7, 8, 9 ]

Answer 3

Answer №2

When dealing with nested arrays that do not have nested arrays themselves, recursion may not be necessary:

Array.prototype.flatMap provides a simple solution to flatten such arrays:

The flatMap() method first maps each element using a mapping function, then flattens the result into a new array. It is similar to using map() and then flat() with a depth of 1, but flatMap() is more convenient and slightly more efficient.

For example:

[[1, 2],[3, 4, 5], [6, 7, 8, 9]].flatMap(xs => xs);
//=> [1, 2, 3, 4, 5, 6, 7, 8, 9]

If you encounter deeply nested arrays like

[[1, 2, [3, 4, 5, [6, 7, 8, 9]]]]

Then a recursive flatMap function remains a solid choice:

const flatten = xs =>
  xs.flatMap(x =>
    Array.isArray(x)
      ? flatten(x)
      : x);

flatten([[1, 2, [3, 4, 5, [6, 7, 8, 9]]]]);
//=> [1, 2, 3, 4, 5, 6, 7, 8, 9]

Although, the introduction of Array.prototype.flat renders this obsolete as it can flatten deeply nested arrays:

The flat() method creates a new array with all sub-array elements concatenated into it recursively up to the specified depth.

If you are unsure of the depth, simply set it to Infinity:

[[1, 2, [3, 4, 5, [6, 7, 8, 9]]]].flat(Infinity);
//=> [1, 2, 3, 4, 5, 6, 7, 8, 9]

It is important to note that flat is not widely supported in all modern browsers yet. If Babel is an option in your project, using it may be beneficial.

While creating a custom recursive function is possible, exploring the native language options is also recommended.

Answer 4

When dealing with nested arrays that do not have nested arrays themselves, recursion may not be necessary:

Array.prototype.flatMap provides a simple solution to flatten such arrays:

The flatMap() method first maps each element using a mapping function, then flattens the result into a new array. It is similar to using map() and then flat() with a depth of 1, but flatMap() is more convenient and slightly more efficient.

For example:

[[1, 2],[3, 4, 5], [6, 7, 8, 9]].flatMap(xs => xs);
//=> [1, 2, 3, 4, 5, 6, 7, 8, 9]

If you encounter deeply nested arrays like

[[1, 2, [3, 4, 5, [6, 7, 8, 9]]]]

Then a recursive flatMap function remains a solid choice:

const flatten = xs =>
  xs.flatMap(x =>
    Array.isArray(x)
      ? flatten(x)
      : x);

flatten([[1, 2, [3, 4, 5, [6, 7, 8, 9]]]]);
//=> [1, 2, 3, 4, 5, 6, 7, 8, 9]

Although, the introduction of Array.prototype.flat renders this obsolete as it can flatten deeply nested arrays:

The flat() method creates a new array with all sub-array elements concatenated into it recursively up to the specified depth.

If you are unsure of the depth, simply set it to Infinity:

[[1, 2, [3, 4, 5, [6, 7, 8, 9]]]].flat(Infinity);
//=> [1, 2, 3, 4, 5, 6, 7, 8, 9]

It is important to note that flat is not widely supported in all modern browsers yet. If Babel is an option in your project, using it may be beneficial.

While creating a custom recursive function is possible, exploring the native language options is also recommended.

Answer 5

Answer №3

Here is my approach

let numbers = [[10, 20],[30, 40, 50], [60, 70, 80, 90]]
const flattenArray = (arr, result = []) => {
   if (Array.isArray(arr)) {
      arr.forEach(item => {
         flattenArray(item, result)
      })
   } else {
      result.push(arr)
   }
   return result
}
console.log(flattenArray(numbers))

Answer 6

Here is my approach

let numbers = [[10, 20],[30, 40, 50], [60, 70, 80, 90]]
const flattenArray = (arr, result = []) => {
   if (Array.isArray(arr)) {
      arr.forEach(item => {
         flattenArray(item, result)
      })
   } else {
      result.push(arr)
   }
   return result
}
console.log(flattenArray(numbers))

Answer 7

Answer №4

let numbers = [[1, 2],[3, 4, 5], [6, 7, 8, 9]]

function flattenArray (arr) {
  return arr.reduce((acc, element) => {
    if (Array.isArray(element)) return [...acc, ...flattenArray(element)];
    acc.push(element);
    return acc;
  }, []);
}

console.log(flattenArray(numbers))

Answer 8

let numbers = [[1, 2],[3, 4, 5], [6, 7, 8, 9]]

function flattenArray (arr) {
  return arr.reduce((acc, element) => {
    if (Array.isArray(element)) return [...acc, ...flattenArray(element)];
    acc.push(element);
    return acc;
  }, []);
}

console.log(flattenArray(numbers))

Answer 9

Answer №5

If you're looking for a solution without relying on any 'magic':

function flattenArray(input, output){
  if(!output)
    output=[];
  if(!Array.isArray(input))
    output.push(input);
  else
    for(var i=0;i<input.length;i++)
      flattenArray(input[i],output);
  return output;
}

var testArray = [[1, 2],[3, 4, 5], 6, [[7, 8], 9]];
console.log(flattenArray(testArray).join());

This approach follows the traditional recursive method often taught in schools.

The main issue in your code lies in the recursive call: flatten(element);. The function doesn't receive the partial result, output (unlike the code above that includes an 'optional' parameter initialized to [] when absent), and whatever it returns is not appended to the existing output either.

You have two options to fix this:

var myArray = [[1, 2],[3, 4, 5], [6, 7, 8, 9]]

const flatten = function (arr, output) { // added output
  if(!output)                            // initialize if missing
    output = [];

  arr.map(element => {
    if (Array.isArray(element)) {
      console.log('Is Array ---> ', element.join())
      flatten(element, output);          // added output
    } else {
      console.log('Output ----->', output.join())
      console.log('Else     --->', element)
      output.push(element);
    }

  });

  return output;

};

console.log('Return -----> ', flatten(myArray).join()); //[ 1, 2, 3, 4, 5, 6, 7, 8, 9 ]

(Included some .join() methods too, so the log displays horizontally)

Alternatively, the second fix:

var myArray = [[1, 2],[3, 4, 5], [6, 7, 8, 9]]

const flatten = function (arr) {
  let output = [];

  arr.map(element => {
    if (Array.isArray(element)) {
      console.log('Is Array ---> ', element.join())
      output = output.concat(flatten(element));     // concatenate with result
    } else {
      console.log('Output ----->', output.join())
      console.log('Else     --->', element)
      output.push(element);
    }

  });

  return output;

};

console.log('Return -----> ', flatten(myArray).join()); //[ 1, 2, 3, 4, 5, 6, 7, 8, 9 ]

Answer 10

If you're looking for a solution without relying on any 'magic':

function flattenArray(input, output){
  if(!output)
    output=[];
  if(!Array.isArray(input))
    output.push(input);
  else
    for(var i=0;i<input.length;i++)
      flattenArray(input[i],output);
  return output;
}

var testArray = [[1, 2],[3, 4, 5], 6, [[7, 8], 9]];
console.log(flattenArray(testArray).join());

This approach follows the traditional recursive method often taught in schools.

The main issue in your code lies in the recursive call: flatten(element);. The function doesn't receive the partial result, output (unlike the code above that includes an 'optional' parameter initialized to [] when absent), and whatever it returns is not appended to the existing output either.

You have two options to fix this:

var myArray = [[1, 2],[3, 4, 5], [6, 7, 8, 9]]

const flatten = function (arr, output) { // added output
  if(!output)                            // initialize if missing
    output = [];

  arr.map(element => {
    if (Array.isArray(element)) {
      console.log('Is Array ---> ', element.join())
      flatten(element, output);          // added output
    } else {
      console.log('Output ----->', output.join())
      console.log('Else     --->', element)
      output.push(element);
    }

  });

  return output;

};

console.log('Return -----> ', flatten(myArray).join()); //[ 1, 2, 3, 4, 5, 6, 7, 8, 9 ]

(Included some .join() methods too, so the log displays horizontally)

Alternatively, the second fix:

var myArray = [[1, 2],[3, 4, 5], [6, 7, 8, 9]]

const flatten = function (arr) {
  let output = [];

  arr.map(element => {
    if (Array.isArray(element)) {
      console.log('Is Array ---> ', element.join())
      output = output.concat(flatten(element));     // concatenate with result
    } else {
      console.log('Output ----->', output.join())
      console.log('Else     --->', element)
      output.push(element);
    }

  });

  return output;

};

console.log('Return -----> ', flatten(myArray).join()); //[ 1, 2, 3, 4, 5, 6, 7, 8, 9 ]

Exploring nested arrays with recursive looping

Answer №1

Answer №2

Answer №3

Answer №4

Answer №5

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