Difference between for loop and for each loop when typing an array

Question

public class WeightedQuickUnionUF {
    private int[] id;
    private int[] sz;

    public WeightedQuickUnionUF(int N){
        id = new int[N];
        sz = new int[N];
        for(int i=0;i<N;i++)
            id[i] = i;
        for(int j=0;j<N;j++)
            sz[j] = 1;
    } 

    // find the root of that number
    public int root(int p){
        while(p != id[p])
            // // Follow links to find a root.
            p = id[p];
        return p;
    }

    public int getid(int p){
        return id[p];
    }
    public int getsize(int p){
        return sz[p];
    }

    // print the array
    // I use 2 ways to print the final array, but the results are different
    public void show(){
        for(int ele:id)
            System.out.print(id[ele]+ " ");
        for(int j=0;j<id.length;j++)
            System.out.print(id[j]+ " ");
        System.out.print("\n");
    }
    // link two trees, the root of the smaller one will be linked to the
    // root of the larger one 
    public void union(int p, int q){
        int rootp = root(p);
        int rootq = root(q);
        if(sz[rootp] < sz[rootq]){
            id[rootp] = rootq;
            sz[rootq] += sz[rootp];
        }
        else{
            id[rootq] = rootp;
            sz[rootp] += sz[rootq];
        }
    }
    // test the class I have defined
    public static void main(String args[]){
        WeightedQuickUnionUF test1 = new WeightedQuickUnionUF(10);
        test1.union(6, 0);
        test1.union(1, 7);
        test1.union(7, 9);
        test1.union(8, 9);
        test1.union(8, 5);
        test1.union(4, 2);
        test1.union(9, 3);
        test1.union(4, 0);
        test1.union(3, 0);
        test1.show();
    }
}

My Problem is about the performance of the function show(). I use 2 methods to print the same array but the results are different. The correct output of this code should be 6 1 4 1 1 1 4 1 1 1, which means that the for loop can give me the right answer. But the for each loop just can't do the right thing.

Use just ele instead of id[ele]

Mathias Begert
– Mathias Begert

2015-08-16 02:41:15 +00:00
Commented Aug 16, 2015 at 2:41 — Mathias Begert
– Mathias Begert, Commented Aug 16, 2015 at 2:41

Elliott Frisch · Accepted Answer · 2015-08-16 02:40:53Z

3

The for-each loop doesn't behave like you seem to expect. That is,

for(int ele : id)
    System.out.print(id[ele]+ " ");

should be something like (and I recommend using braces)

for (int ele : id) {
    System.out.print(ele + " ");
}

answered Aug 16, 2015 at 2:40

Elliott Frisch

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