CODEWITHSUNDEEP
javaarrays
ThomasThiebaud
ThomasThiebaud

Reputation: 11969

How to merge several list of arrays based on first element?

I have several lists of string arrays. I would like to merge this lists in a single one based on the first element of each array.

Basically, I would like to merge this two list

ArrayList<String[]> list1 = new ArrayList<String[]>();
list1.add(new String[]{"key1","a","a","a"});
list1.add(new String[]{"key2","a","a","a"});
list1.add(new String[]{"key4","a","a","a"});

ArrayList<String[]> list2 = new ArrayList<String[]>();
list2.add(new String[]{"key1","b","b"});
list2.add(new String[]{"key2","b","b"});
list2.add(new String[]{"key3","c","c"});

in this one

[key4,a,a,a,null,null]
[key3,null,null,null,c,c]
[key2,a,a,a,b,b]
[key1,a,a,a,b,b]

Some more things :

Does someone know an efficient way to do it ?

My current approach

My current approach use an HashMap to store the key (first element of the array)

HashMap<String, String[]> results = new HashMap<String, String[]>();
process(list1); 
process(list2); 

void process(ArrayList<String[]> list) {
    for(String[]s : list) {
        if(!results.containsKey(s[0])) {
            results.put(s[0], Arrays.copyOfRange(s, 1, s.length));
        } else {
            String[] current = results.get(s[0]);
            results.put(s[0], concat(current, Arrays.copyOfRange(s, 1, s.length)));
        }
    }
}

which give me the ouput (incorrect because no "padding")

key4 [a, a, a]
key3 [c, c]
key2 [a, a, a, b, b]
key1 [a, a, a, b, b]

Upvotes: 1

Views: 276

Answers (1)

user1438038
user1438038

Reputation: 6069

Basically, what you need to do, is check list2 if the desired key exists. If it does, simply merge the arrays. However, if the key is not present in list2, you have to add a padded array. This can be done relatively easy by returning a new array with the desired length.

Have a look at my code below:

public void mergeArrays() {

  ArrayList<String[]> list1 = new ArrayList<String[]>();
  list1.add(new String[] { "key1", "a", "a", "a" });
  list1.add(new String[] { "key2", "a", "a", "a" });
  list1.add(new String[] { "key4", "a", "a", "a" });

  ArrayList<String[]> list2 = new ArrayList<String[]>();
  list2.add(new String[] { "key1", "b", "b" });
  list2.add(new String[] { "key2", "b", "b" });
  list2.add(new String[] { "key3", "c", "c" });

  ArrayList<String[]> result = new ArrayList<String[]>();

  // Merge list1 to list2
  for (String[] array1: list1) {
     // Find key from list1 in list2
     int index = this.findIndex(array1[0], list2);

     // Key was found, merge arrays
     if (index > -1) {
        result.add(this.concat(array1, list2.get(index)));
     }
     // Key was not found, use padding
     else {
        // Padding second array with null values
        result.add(this.concat(array1, this.padding(list2.get(0).length, null)));
     }
  }

  // Merge list2 to list1
  for (String[] array2: list2) {
     // Find key from list2 in list1
     int index = this.findIndex(array2[0], list1);

     // Key was not found, use padding
     if (index == -1) {
        // Padding first array with null values
        String[] temp = this.concat(this.padding(list1.get(0).length, null), array2);
        temp[0] = array2[0]; // Set key
        result.add(temp);
     }
  }

  // Debug output
  for (String[] array: result) {
     for (int i = 0; i < array.length; ++i) {
        System.out.print(array[i] + " ");
     }
     System.out.println("");
  }
}

// Return array with padding
private String[] padding(final int size, final String padding) {
  String[] result = new String[size];
  Arrays.fill(result, padding);

  return result;
}

// Find needle in haystack and return index
private int findIndex(final String needle, final ArrayList<String[]> haystack) {
  for (int i = 0; i < haystack.size(); ++i) {
     if (haystack.get(i)[0].equals(needle)) {
        return i;
     }
  }

  return -1;
}

// Merge two arrays, omit first element of second array
private String[] concat(String[] arrayOne, String[] arrayTwo) {
  int arrayOneLength = arrayOne.length;
  int arrayTwoLength = arrayTwo.length;

  String[] result = new String[arrayOneLength + arrayTwoLength - 1];
  System.arraycopy(arrayOne, 0, result, 0, arrayOneLength);
  System.arraycopy(arrayTwo, 1, result, arrayOneLength, arrayTwoLength - 1); // Skip key in second array

  return result;
}

Output:

key1 a a a b b
key2 a a a b b
key4 a a a null null
key3 null null null c c

Upvotes: 1

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