递归 flatMap

Question

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.

Answer 1

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));

Answer 2

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.)

Answer 3

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
    });
  })
}

Answer 4

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));