How to store one value at a time into a hash table.

Question

I've just started learning java. I'm trying to write a program to find three elements from three different arrays such that, a + b + c = sum. (I'm avoiding using three for loops to make it more efficient)

I'm getting the following error.

10: error: cannot find symbol
       HashMap<int> s = new HashMap<int>();
       ^
   class HashMap
 class YesorNo

This is my code:

class YesorNo
    {
         // Function to check if there is an element from
         // each array such that sum of the three elements
         // is equal to given sum.
         boolean findTriplet(int a1[], int a2[], int a3[],
                int n1, int n2, int n3, int sum)
        {
        // Store elements of first array in hash table
           HashMap<int> s = new HashMap<int>();
             //unordered_set <int> s;
             for (int i=0; i<n1; i++)
             s.add(a1[i]);

             // sum last two arrays element one by one
             for (int i=0; i<n2; i++)
             {
                for (int j=0; j<n3; j++)
                {
                // Consider current pair and find if there
                // is an element in a1[] such that these
                // three form a required triplet
                if (s.find(sum - a2[i] - a3[j]) != s.end())
                    return true;
                }
            }
            return false;
        }

        // Driver Code
        public static void main(String[] args) 
        {
            YesorNo check = new YesorNo();
            int a1[] = { 1 , 2 , 3 , 4 , 5 };
            int n1 = a1.length;
            int a2[] = { 2 , 3 , 6 , 1 , 2 };
            int n2 = a2.length;
            int a3[] = { 3 , 2 , 4 , 5 , 6 };
            int n3 = a3.length;
            int sum=9;

            System.out.println(check.findTriplet(a1, a2, a3, n1, n2, n3, sum));
        }
    }

Use this to find the correct Collection : http://www.sergiy.ca/img/doc/java-map-collection-cheat-sheet.gif for a list of value (like an array but without limit) : List -> ArrayList ..., Map -> HashMap.. if for pairs — azro, Feb 11 '17 at 15:39

Elliott Frisch · Answer 1 · 2017-02-11T16:25:27.493

4

Use a HashSet, and it should be an Integer (since you can't use primitives as generic types with collections and prefer the interface).

Set<Integer> s = new HashSet<>();

edited Feb 11 '17 at 16:25

answered Feb 11 '17 at 15:39

Elliott Frisch

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2

"you can't use primitives with collections" you mean we can't use primitives as *generic types*. We can use primitives with collections: `s.add(1)` will fork fine thanks to autobixing :) – Pshemo Feb 11 '17 at 15:45
@Pshemo Yes. Yes I do mean that. :) – Elliott Frisch Feb 11 '17 at 16:25

score 0 · Accepted Answer · edited May 23 '17 at 12:08

HashMap is not a single dimension Collection and it can't hold primitive types like 'int' as @Pshemo mentioned in the comments. You have to specify key and the values mapped to that key. Refer this tutorial for more information.

To insert values into HashMap,

HashMap<Integer, Integer> s = new HashMap<Integer, Integer>();
for (int i=0; i<n1; i++)
         s.put(i, a1[i]);

As an alternative for s.find, you can use

s.containsValue

But you can't jump to the end of HashMap as you have written s.end. You need to follow some tweaks mentioned here.

How to store one value at a time into a hash table.

2 Answers2