Recursive flatMap

user_17318051 0
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I'm trying to figure out how to convert the following javascript function into a dynamic function that will perform the flapMap recursively.

function getPermutations(object) {

  let array1 = object[0].options,
    array2 = object[1].options,
    array3 = object[2].options;

  return array1.flatMap(function(array1_item) {
    return array2.flatMap(function(array2_item) {
      return array3.flatMap(function(array3_item) {
        return array1_item + ' ' + array2_item + ' ' + array3_item;
      });
    });
  });
}

let object = [{
  "options": ['blue', 'gray', 'green']
}, {
  "options": ['large', 'medium', 'small']
}, {
  "options": ['wood', 'steel', 'pastic']
}];


console.log('Permutations', getPermutations(object));

In the example, I'm sending 3 arrays into the function which is why it has 3 iterations of flapMap. Works fine, but I am trying to make it dynamic, so I can pass a dynamic array and the function would do the flapMap recursively depending on the array.

javascript
4 Answers

In your example you kept track of array1_item, array2_item and co in their individual variables. You can move them to an array (having dynamic size; I called it _prevItems) and pass them as an parameter to the recursive call.

function getPermutations(objects, _prevItems = []) {
  // join the items at the end of the recursion
  if (objects.length === 0)
    return _prevItems.join(' ')
  
  // call again with all but the first element, and add the current item to _prevItems
  return objects[0].flatMap(item => getPermutations(objects.slice(1), [..._prevItems, item]))
}

let objects = [['blue', 'gray', 'green'], ['large', 'medium', 'small'], ['wood', 'steel', 'pastic']];


console.log('Permutations', getPermutations(objects));

user_65481540

One way to do this is with reduce.

You want to reduce the list of options to one list of permutations.

  

function getPermutations(list) {
  return (
    list
      // First map to list of options (list of list of strings)
      .map((item) => item.options)
      // Then reduce. Do not set any initial value.
      // Then the initial value will be the first list of options (in
      // our example, ["blue", "gray", "green"])
      .reduce((permutations, options) => {
        return permutations.flatMap((permutation) =>
          options.map((option) => permutation + " " + option)
        );
      })
  );
}

// Renamed this to list, since it is an array and not an object
const list = [
  { options: ["blue", "gray", "green"] },
  { options: ["large", "medium", "small"] },
  { options: ["wood", "steel", "pastic"] },
];

console.log("Permutations", getPermutations(list));

Edit: I know you asked for recursion. If this is a school task, then perhaps you have to use recursion, but otherwise I would recommend avoiding recursion when possible, since it tends to make things more complicated. (Of course, this is a general rule, and like all rules it has some exceptions.)

user_20547310

you can flatMap two arrays at a time using the recursive bottom up approach and build your string from the end

const getPermutations = (array) => {
  if(array.length === 1)
     return array[0].options;

  const prefixItems = array[0].options;
  const suffixItems = getPermutations(array.slice(1));

  return prefixItems.flatMap(prefix => {
    return suffixItems.flatMap(suffix => {
      return prefix + ' ' + suffix
    });
  })
}
user_58381280

You can use recursion to make your function dynamic. my solution is that, you need to make a function with three argument (array, index, result), call you function over and over again until you finished iterating all you array object. for the example:

function getPermutations(array, index = 0, prev = "") {
  // if the current index is lower than array length, keep calling the function
  if (index < array.length - 1) {
    return array[index].options.flatMap(item => {
      // if the previous value is empty, send the current item
      const current = prev ? `${prev} ${item}` : item;
      // resend the current value for the next iteration
      return getPermutations(array, index + 1, current);
    });
  } else {
    // end of the iteration
    return array[index].options.flatMap(item => `${prev} ${item}`);
  }
}

let object = [{
  "options": ['blue', 'gray', 'green']
}, {
  "options": ['large', 'medium', 'small']
}, {
  "options": ['wood', 'steel', 'pastic']
}];


console.log('Permutations', getPermutations(object));

user_115618880
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