Java nested loop logic error

Question

I want to know what is wrong with my logic when my output is supposed to be as follows:

There are two arrays of integers and that prints the index of the first occurrence of the first list in the second list.For example, suppose that you have these arrays:

int[] list1 = {1, 3, 6};
int[] list2 = {1, 3, 5, 8, 12, 1, 3, 17, 1, 3, 6, 9, 1, 3, 6};

Then the call indexOf(list1, list2) should return 8 because the sequence of values stored in list1 appears in list2 starting at index 8. The list1 appears twice in list2, starting at position 8 and starting at position 12. The method should return the first such position.

Currently, my code does not print anything...

public static void indexOf(int[] arr1, int[] arr2){

    for(int i = 0; i < arr2.length; i++){
        for(int j = 0; j < arr1.length; j++){
            if(arr1[j] != arr2[i]){
                break; 
            }
            if(j == arr1.length -1){
                System.out.println(i);
                break;
            }
        }
    }
}

Ahmed Khalaf · Accepted Answer · 2015-07-19 23:02:28Z

3

arr1[j] != arr2[i] should be arr1[j] != arr2[i + j]

Why

In each iteration of the inner loop you should be comparing each element of the original subsequence (arr1[j]) with the corresponding element in the current subsequence you have sliced from arr2 (arr2[i + j]). You were comparing to just the first element in the current slice.

Moreover

Your loop termination condition should be i + (arr1.length - 1) < arr2.length to avoid accessing out of bound index if the last element in arr2 was 1 (or in general equal to the first element in arr1).

...also

The second break should be return to print the first occurrence as you stated.

Full Code

public static void indexOf(int[] arr1, int[] arr2) {

    for(int i = 0; i + (arr1.length - 1) < arr2.length; i++) {
        for(int j = 0; j < arr1.length; j++) {
            if(arr1[j] != arr2[i + j]) {
                break; 
            }
            if(j == (arr1.length - 1)){
                System.out.println(i);
                break; // break to print all the occurrences. return to print only the first.
            }
        }
    }

edited Jul 19, 2015 at 23:02

answered Jul 19, 2015 at 1:23

Ahmed Khalaf

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2 Comments

CMSC Over a year ago

Could you elaborate on where I need to ad the loop termination condition? Also why should my second break be return? I thought it doesn't matter whether its a break or return, since I'm printing as I'm going through the code, not somewhere else...

Ahmed Khalaf Over a year ago

In the outer loop change for(int i = 0; i < arr2.length; i++) to for(int i = 0; i + (arr1.length - 1) < arr2.length; i++). I understood from your description that you want to print the index of the first match (You wrote "The method should return the first such position").

Bimal · Accepted Answer · 2015-07-19 01:40:51Z

0

This works (I added i+j instead of j):

   public static void indexOf(int[] arr1, int[] arr2){

    for(int i = 0; i < arr2.length; i++){
        for(int j = 0; j < arr1.length; j++){
            if((i + j) < arr2.length && arr1[j] != arr2[i + j]){
                break; 
            }
            if(j == arr1.length -1){
                System.out.println(i);
                break;
            }
        }
    }
}

answered Jul 19, 2015 at 1:40

Bimal

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