freeCodeCamp/guide/english/certifications/javascript-algorithms-and-data-structures/intermediate-algorithm-scripting/steamroller/index.md

title
Steamroller

Remember to use Read-Search-Ask if you get stuck. Try to pair program and write your own code

Problem Explanation:

This problem seems simple but you need to make sure to flatten any array, regardless of the level which is what adds a bit of difficulty to the problem.

Relevant Links

• Array.isArray()

Hint: 1

You need to check if an element is an array or not.

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Hint: 2

If you are dealing with an array, then you need flatten it by getting the value inside of the array. This means if you have [[4]] then instead of returning [4] you need to return 4. If you get [[[4]]] then the same, you want the 4. You can access it with arr[index1][index2] to go a level deeper.

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Hint: 3

You will definitely need recursion or another way to go beyond two level arrays to make the code flexible and not hard-coded to the answers needed. Have fun!

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Spoiler Alert!

Solution ahead!

Basic Code Solution:

function steamrollArray(arr) {
  var flattenedArray = [];

  // Create function that adds an element if it is not an array.
  // If it is an array, then loops through it and uses recursion on that array.
  var flatten = function(arg) {
    if (!Array.isArray(arg)) {
      flattenedArray.push(arg);
    } else {
      for (var a in arg) {
        flatten(arg[a]);
      }
    }
  };

  // Call the function for each element in the array
  arr.forEach(flatten);
  return flattenedArray;
}

// test here
steamrollArray([1, [2], [3, [[4]]]]);

Run Code

Code Explanation:

• Create a new variable to keep flattened arrays.
• Create a function that will add non array elements to the new variable, and for the ones that are array it loops through them to get the element.
• It does that by using recursion, if the element is an array then call the function again with a layer of array deeper to check if it is an array or not. if it is not then push that non-array element to the variable that gets returned. Otherwise, keep going deeper.
• Invoke the function, the first time you will always pass it an array, so it always fall in to the isArray branch
• Return the flattened array.

Relevant Links

• Array.push()
• Array.forEach()

Intermediate Code Solution:

function steamrollArray(arr) {
  let flat = [].concat(...arr);
  return flat.some(Array.isArray) ? steamrollArray(flat) : flat;
}

flattenArray([1, [2], [3, [[4]]]]);

Run Code

Code Explanation:

• Use spread operator to concatenate each element of arr with an empty array
• Use Array.some() method to find out if the new array contains an array still
• If it does, use recursion call steamrollArray again, passing in the new array to repeat the process on the arrays that were deeply nested
• If it does not, return the flattened array

Relevant Links

• Array.some
• Array.concat
• Spread operator
• Ternary Operator (condition ? a : b)

Intermediate Code Solution 2:

function steamrollArray(arr) {
  while(arr.some(element => Array.isArray(element))) {
    arr = arr.flat();
  }
  return arr;
}

steamrollArray([1, [2], [3, [[4]]]]);

Run Code

Code Explanation:

• Use Array.some() method to find out if the new array contains an array still, if it does flatten the array
• Repeats, until there is no more arrays inside arr.

Relevant Links

• Array.some
• Array.flat

Advanced Code Solution:

function steamrollArray(arr) {
  return arr.toString()
    .replace(',,', ',')       // "1,2,,3" => "1,2,3"
    .split(',')               // ['1','2','3']
    .map(function(v) {
      if (v == '[object Object]') { // bring back empty objects
        return {};
      } else if (isNaN(v)) {        // if not a number (string)
        return v;
      } else {
        return parseInt(v);         // if a number in a string, convert it
      }
    });
}

Run Code

Code Explanation:

• First we turn the array to a string, which will give us a string of numbers seperated by a comma, double comma if there was an empty array and literal object text if there was an object, which we can fix it later in our if statement.
• We replace the double comma with one, then split it back into an array.
• map through the array and fix object values and convert string numbers to regular numbers.

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See Wiki Challenge Solution Template for reference.