Java dynamic nested loops

Question

I have a map which contains variable length string list. e.g

Map<String, List<String> map = new HashMap<>();
map.put("key", "key1, key2);
map.put("name", "name1, name2, name3");
map.put("code", "code1, code2");

This will give 12 different permutations. The below code does that job

List<String> values = new ArrayList<>();
for (int i = 0; i < map.get("key").size(); i++) {
     values.add(map.get("key").get(i));
     for (int j = 0; j < map.get("name").size(); j++) {
         values.add(map.get("name").get(j));
         for (int k = 0; k < map.get("code").size(); k++) {
             values.add(map.get("code").get(k));
         }
     }
}

Expected output:

"key1", "name1", "code1",
"key1", "name1", "code2",
"key1", "name2", "code1",
"key1", "name2", "code2",
"key1", "name3", "code1",
"key1", "name3", "code2",
"key2", "name1", "code1",
"key2", "name1", "code2",
"key2", "name2", "code1",
"key2", "name2", "code2",
"key2", "name3", "code1",
"key2", "name3", "code2"

But the problem is this is hard coded with 3 for loops, but what I expect is to support for any number of variables. Prompt help is much appreciated.

This code isn't generating permutations. What do you want/expect to be in the values map after it's done? — Willis Blackburn
– Willis Blackburn, Commented Nov 24, 2019 at 19:24

Tharsan Sivakumar · Accepted Answer · 2019-12-02 14:36:58Z

Following snippet solves this problem.

    protected static void traverseParam(Map<String, List<String>> paramLists, int level, Map<String, String> paramMap) {
                if (level == paramLists.size()) {
                   //contains of all combinations
                    System.out.println(StringUtils.join(paramMap.values(), ", "));
                } else {
                    String paramKey = (String) paramLists.keySet().toArray()[level];
                    List<String> paramValues = (List<String>) paramLists.values().toArray()[level];
                    for (String paramValue : paramValues) {
                        paramMap.put(paramKey, paramValue);
                        //Recursively calls until all params are processed
                        traverseParam(paramLists, level + 1, paramMap);
                    }
                }
            }

Now call this method like below.

Map<String, List<String> map = new HashMap<>();
map.put("key", "key1, key2);
map.put("name", "name1, name2, name3");
map.put("code", "code1, code2");
//Start with 1st parameter, and pass 
//an empty LinkedHashMap and all entries will be added to this map.
traverseParam(map, 0, new LinkedHashMap<String, String>());

Daniele · Accepted Answer · 2019-11-24 21:27:42Z

I provided below a sample solution, using recursion.

In the example we have 3 lists, with rep. 3,4,4 elements; the program prints all 3*4*4=48 combinations of elements.

Starting with an empty combination (an empty list), at each step(i) we take N=list(i).size() copies of the current tuple, add value(j)=list(i)[j] to the j-th new tuple (for j=1..N), and recurse. When i=list.size, then the tuple is complete and we can use it (eg. print it out). (lists can be copied using the constructor).

package sample;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.LinkedList;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;
import java.util.TreeMap;
import java.util.function.Consumer;

public class SampleCombinations {

  public static void main(String[] args) {

    Map<String, List<String>> map = new TreeMap<>();
    map.put("x", Arrays.asList("a", "b", "c"));
    map.put("y", Arrays.asList("0", "1", "2", "3"));
    map.put("z", Arrays.asList("!", "-", "=", "%"));

    ArrayList<Entry<String, List<String>>> entries = new ArrayList<>(map.entrySet());

    System.out.println(entries);
    printCombinations(entries);
  }

  private static void printCombinations(List<Entry<String, List<String>>> entries) {
    Consumer<List<String>> cons = list -> System.out.println(list);
    generateComb(entries, cons, 0, new LinkedList<>());
  }

  private static void generateComb(List<Entry<String, List<String>>> entries,
      Consumer<List<String>> consumer, int index, List<String> currentTuple) {

    /*
     * terminal condition: if i==entries.size the tuple is complete
     * consume it and return
     */
    if(index==entries.size()) {
      consumer.accept(currentTuple);
    }
    else {
      /*
       * get all elements from the i-th list, generate N new tuples and recurse
       */
      List<String> elems = entries.get(index).getValue();
      for (String elem:elems) {
        // copy the current tuple
        LinkedList<String> newTuple = new LinkedList<>(currentTuple);
        newTuple.add(elem);
        generateComb(entries, consumer, index+1, newTuple);
      }
    }
  }
}

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