CodeGym/Help with Java Tasks/Please what's the problem here?

Level 26

Port-Harcourt

21.01.2019
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17comments

Please what's the problem here?

Question about the task

Greater than 10? You're not a good fit for us

Java Syntax, Level 8, Lesson 8

Resolved

I ran it and got the right output. What did I do wrong?

package com.codegym.task.task08.task0814; import java.util.HashSet; import java.util.Iterator; import java.util.Set; /* Greater than 10? You're not a good fit for us */ public class Solution { public static HashSet<Integer> createSet() { // write your code here HashSet<Integer> integerHashSet = new HashSet<Integer>(); for (int i = 0; i < 20; i++) { integerHashSet.add(i); } return integerHashSet; } public static HashSet<Integer> removeAllNumbersGreaterThan10(HashSet<Integer> set) { // write your code here int i = 0; while (i < 20) { if (i > 10 && set.contains(i)) { set.remove(i); i -= 1; } else { i++; } } return set; } public static void main(String[] args) { } }

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Guadalupe Gagnon

Level 37 , Tampa, United States

21 January 2019, 17:32

I mentioned that HashSets run faster than ArrayLists. I made two versions of the code, one HashSet and one ArrayList, that are exact duplicates other than the type of data structure, and wrote code in to time each. I will post both versions below so you can run them yourself and see the difference:

Guadalupe Gagnon

Level 37 , Tampa, United States

21 January 2019, 17:33

HashSet:

public class Solution {
    public static HashSet<Integer> createSet() {
        // write your code here
        HashSet<Integer> integerHashSet = new HashSet<Integer>();
        for (int i = 0; i < 100000; i++) {
            integerHashSet.add(i);
        }
        return integerHashSet;
    }

    public static HashSet<Integer> removeAllNumbersGreaterThan10(HashSet<Integer> set) {
        // write your code here
        Iterator i = set.iterator();
        while(i.hasNext()) if((Integer)i.next() > 10) i.remove();
        return set;
    }

    public static void main(String[] args) {
        long start = System.currentTimeMillis();
        HashSet<Integer> temp = createSet();
        removeAllNumbersGreaterThan10(temp);
        System.out.println(temp);
        long end = System.currentTimeMillis();
        System.out.println("Program run time: " + (end-start) +"ms");

    }
}

Guadalupe Gagnon

Level 37 , Tampa, United States

21 January 2019, 17:34

ArrayList:

public class Solution {
    public static ArrayList<Integer> createSet() {
        // write your code here
        ArrayList<Integer> integerHashSet = new ArrayList<Integer>();
        for (int i = 0; i < 100000; i++) {
            integerHashSet.add(i);
        }
        return integerHashSet;
    }

    public static ArrayList<Integer> removeAllNumbersGreaterThan10(ArrayList<Integer> set) {
        // write your code here
        Iterator i = set.iterator();
        while(i.hasNext()) if((Integer)i.next() > 10) i.remove();
        return set;
    }

    public static void main(String[] args) {
        long start = System.currentTimeMillis();
        ArrayList<Integer> temp = createSet();
        removeAllNumbersGreaterThan10(temp);
        System.out.println(temp);
        long end = System.currentTimeMillis();
        System.out.println("Program run time: " + (end-start) +"ms");

    }
}

Guadalupe Gagnon

Level 37 , Tampa, United States

21 January 2019, 17:36

on my computer the HashSet version ran in 63ms while the ArrayList version ran in 4329ms

Hashirama

Level 26 , Port-Harcourt, Nigeria

21 January 2019, 18:00

Cool. This HashSet really is fast!!! ArrayList took 3226ms in my computer. HashSet took 139ms. It took less but i returned the highest time value based on how many times I ran it. This goes for both of them. Is HashSet faster than ArrayList because it returns values randomly?

Guadalupe Gagnon

Level 37 , Tampa, United States

21 January 2019, 18:07

useful

I don't know specifically why lol..... im still learning. I just know that when the order is not important, I always use hashset now to keep my code running fast.

Guadalupe Gagnon

Level 37 , Tampa, United States

21 January 2019, 18:09

useful

This is what i do now that I have completed all the tasks. I help people in the forums and time code to try to make it faster. I can't wait until levels 21-40 are completed

Hashirama

Level 26 , Port-Harcourt, Nigeria

21 January 2019, 19:31

That's cool.

Guadalupe Gagnon

Level 37 , Tampa, United States

21 January 2019, 16:33

solution

Your code never only removes numbers between 1 and 19. The method should remove all numbers greater than 10. A simple and effective way to do this is to create an iterator and go through each value. If the value is greater than 10 then iterator.remove() that value:

if (iterator.next() > 10) iterator.remove(); // just quick example code, probably won't compile

This way it works with any list passed to it instead of just your list from createSet.

Hashirama

Level 26 , Port-Harcourt, Nigeria

21 January 2019, 16:49

Thanks. Using the iterator() method worked. Could you please explain how my code up there won't work for any list passed to it apart from the list in createSet()?

Hashirama

Level 26 , Port-Harcourt, Nigeria

21 January 2019, 16:49

Your code never only removes numbers between 1 and 19? Please I don't understand

Guadalupe Gagnon

Level 37 , Tampa, United States

21 January 2019, 16:53

useful

paste this in as your main:

public static void main(String[] args) {
        HashSet<Integer> temp = createSet();
        removeAllNumbersGreaterThan10(temp);
        System.out.println(temp);
}

Run this with your old code a couple of time, but each time change the createSet method, for example change it to this:

public static HashSet<Integer> createSet() {
        // write your code here
        HashSet<Integer> integerHashSet = new HashSet<Integer>();
        for (int i = 0; i <= 20; i++) {
            integerHashSet.add(i*2);
        }

        return integerHashSet;
    }

You will see that it only removes numbers 11-19 and leaves everything else.

Hashirama

Level 26 , Port-Harcourt, Nigeria

21 January 2019, 17:04

Woa.....! What happened? I got this output when I tried it : [0, 32, 2, 34, 4, 36, 6, 38, 8, 10, 20, 22, 24, 26, 28, 30] Is there an explanation why? Perhaps my algorithm isn't good enough!!!

Guadalupe Gagnon

Level 37 , Tampa, United States

21 January 2019, 17:11

Hash sets (and maps) return values randomly. They actually run a lot faster than Lists (like ArrayList), and I use them a lot because of this. Now you can see that your code removed numbers between 11-19, as these are not in that list.

Hashirama

Level 26 , Port-Harcourt, Nigeria

21 January 2019, 17:22

I get it now. My first code will only remove values in the range of [11,19] and anything above or below will remain. Thanks

Hashirama

Level 26 , Port-Harcourt, Nigeria

21 January 2019, 17:31

If I make i run through from 0 to Integer.MAX_VALUE then I will get. That's if the iterator() wasn't there!!

Hashirama

Level 26 , Port-Harcourt, Nigeria

21 January 2019, 17:38

Dang it. It worked... but it took quite some time: 15 seconds to complete. the iterator() method is better. It took 0 second with the iterator().