java - recursion code explanation (check if ascending)

Question

I am recently finished creating a program that checks if a user-inputted int array is arranged in ascending order. I have my function isAsc here:

public static boolean isAsc(int[] arr, int index){
    if (index==1 || arr.length==1){
        return true; //Base case
    }
    else if (arr[index-2] >= arr[index-1]){
        return false; 
    }
    else
        return isAsc(arr, index-1); //recursive step
}

And the logic seems to be correct. However, what I don't get is when I call the function: System.out.print(isAsc(arr, arrayLength-1));

the output is wrong. This System.out.print(isAsc(arr, arrayLength)); yields the correct answer. Why? Thanks!

Your logic returns false when there are duplicate numbers, even though they are sorted correctly (adjacent). Change >= to >. — Bohemian
– Bohemian ♦, Commented Jan 29, 2017 at 10:05
@Malvolio 1 2 4 3 and it returns true. anyways, i got it figured out now thanks — Paul Steven Fantonalgo Nadera
– Paul Steven Fantonalgo Nadera, Commented Jan 29, 2017 at 10:10
@PaulStevenFantonalgoNadera -- your error is so common, it has its own Wikipedia page. — Michael Lorton
– Michael Lorton, Commented Jan 29, 2017 at 18:32

Eran · Accepted Answer · 2017-01-29 10:00:58Z

3

If you are calling the method with

isAsc(arr, arr.length-1)

your method never checks the last element of the array in the if (arr[index-2] >= arr[index-1]) check, so the output may be incorrect.

On the other hand, when you call it with

isAsc(arr, arr.length)

the first execution of your method checks if (arr[index-2] >= arr[index-1]), or in other words if (arr[arr.length-2] >= arr[arr.length-1]) (assuming your array has multiple elements), so arr[arr.length-1] is not skipped.

answered Jan 29, 2017 at 10:00

Eran

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2 Comments

Paul Steven Fantonalgo Nadera Over a year ago

Ohhhh, I got it now. I had to write everything with pen and paper to understand, lol. Thank you very much!

Paul Steven Fantonalgo Nadera Over a year ago

I should've used index-1 >= index instead lol I'm used to declaring arr.length-1. thanks again

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