LED Fantom LED Fantom - 2 months ago 7
Java Question

ArrayIndexOutOfBoundsException for my Comparator Interface in Java

I am learning about Comparator interface to solve a fractional knapsack problem. The two arrays in line a and b indicate 3 value-weight pairs - (60, 20), (100, 50), (120, 30). The Comparator is supposed to sort arr[] based on the ratios of v[i]/w[i]. However, The following code gives me weird error:

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 3


My Comparator seems to be trying to access index=3 of arr[], but I can't limit its access of indices. Any solutions?

Here is my code:

int[] v = {60, 100, 120}; // v: values. line a
int[] w = {20, 50, 30}; // w: weights. line b
int len = v.length;
Integer[] arr = new Integer[len];

for (int i=0; i<len; i++)
arr[i] = v[i] / w[i];

//sort arr[] based on the ratios of v[i]/w[i]
Arrays.sort(arr, new Comparator<Integer>() {
@Override public int compare(Integer o1, Integer o2) {
return Double.compare(v[o1]/w[o1], v[o2]/w[o2]);
}
});

double[] v_sorted = new double[len];
double[] w_sorted = new double[len];
for (int i = 0; i < len; ++i) {
v_sorted[i] = v[arr[i]];
w_sorted[i] = w[arr[i]];
}

Answer

The problem is that you are trying to sort indexes, as represented by o1 and o2 in your comparator code, but your arr contains pre-computed ratios.

Filling the array with numbers 0 through len-1, inclusive, will solve the problem:

for (int i=0; i<len; i++) 
    arr[i] = i;

Now the comparator will get a pair of indexes, and compare ratios of v[...] to w[...] based on them.