CodeGym/Help with Java Tasks/Need some help with removeFirstName

Level 18

22.12.2018
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Need some help with removeFirstName

Question about the task

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Any idea or suggestion ??

Create a Map<String, String> and add ten entries that represent (last name, first name) pairs.
Remove people with the same first name.

Requirements:

The program should not display text on the screen.
The program should not read values from the keyboard.
The createMap() method must create and return a HashMap that has (String, String) elements and contains 10 entries.
The removeFirstNameDuplicates() method must remove from the map all people who have the same first name.
The removeFirstNameDuplicates() method must call the removeItemFromMapByValue() method.

package com.codegym.task.task08.task0817; import java.util.*; /* We don't need repeats */ public class Solution { public static HashMap<String, String> createMap() { //write your code here HashMap<String, String> map = new HashMap(); map.put("A", "AAAA"); map.put("B", "SSSS"); map.put("C", "BBBB"); map.put("D", "AAAA"); map.put("E", "CCCC"); map.put("F", "AAAA"); map.put("G", "AAAA"); map.put("H", "CCCC"); map.put("I", "AAAA"); map.put("J", "SAQSS"); return map; } public static void removeFirstNameDuplicates(Map<String, String> map) { //write your code here int count = 0, no = 0; HashMap<String, String> copy = new HashMap<String, String>(map); Iterator<Map.Entry<String, String>> itr = copy.entrySet().iterator(); while (itr.hasNext()) { String value1 = itr.next().getValue(); Iterator<Map.Entry<String, String>> itr2 = copy.entrySet().iterator(); while(itr2.hasNext()) { String value2 = itr.next().getValue(); if (value1.equals(value2)) { count++; } if (count > 1) { removeItemFromMapByValue(map, value2); no = 1; } } if (no == 1) { removeItemFromMapByValue(map, value1); } no = 0; } } public static void removeItemFromMapByValue(Map<String, String> map, String value) { HashMap<String, String> copy = new HashMap<String, String>(map); for (Map.Entry<String, String> pair : copy.entrySet()) { if (pair.getValue().equals(value)) map.remove(pair.getKey()); } } public static void main(String[] args) { //HashMap<String, String> map = createMap(); //System.out.println(map); //removeFirstNameDuplicates(map); //System.out.println(map); } }

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Davedabull

Level 22 , New York City, United States

19 December 2019, 18:28

public static void removeFirstNameDuplicates(Map<String, String> map) {
    //write your code here
    ArrayList<String> list = new ArrayList<String>();
    ArrayList<String> list2 = new ArrayList<String>();
    for (String s : map.values()) {
        if(!list.contains(s)){
            list.add(s);
        }
        else{
            list2.add(s);
        }

    }
    for(String j : list2){
        removeItemFromMapByValue(map, j);
    }


}

Guadalupe Gagnon

Level 37 , Tampa, United States

22 December 2018, 22:34

solution

Though if you want simple, just try this:

public static void removeFirstNameDuplicates(Map<String, String> map) {
    ArrayList<String> test = new ArrayList<>();

    for(Map.Entry<String, String> pair : map.entrySet()){
        if(!test.contains(pair.getValue())) test.add(pair.getValue());
        else{
            removeItemFromMapByValue(map, pair.getValue());
            removeFirstNameDuplicates(map);
            break;
        }
    }
}

+12

Zbigniew

Level 23 , Dabrowa Tarnowska, Poland

14 January 2019, 09:12

Could you explain what happens when removeFirstNameDuplicates method is called from within it's body (in the else block)? I understand the method up to this point.

Guadalupe Gagnon

Level 37 , Tampa, United States

14 January 2019, 15:23

It is a recursive function ( ill post a link below). Read my posts below detailing how the original poster was iterating through a collection while making changes to it (removing items to be specific). Basically my function, by calling itself with 'map' after an item is removed, starts the iterating process over and avoids iterating over a collection while removing from that same collection. This works because collections are passed by reference which means that changes in a function will change value that is being passed in, unlike with primitives (ill post an example). The function will hold at line 8 each time until the function call is returned. The last level will just reach the end without the 'else' block being activated, then when returned up, the next line breaks each level all the way back until the code reaches main. I'll try to illustrate that here: from main removeItems(map) else block 1: removeItems() else block 2: removeItems() else block 3: removeItems() no items removed level 4: else block 3: break else block 2: break else block 1; break next line in main Recursive function link: https://introcs.cs.princeton.edu/java/23recursion/

Guadalupe Gagnon

Level 37 , Tampa, United States

14 January 2019, 15:38

primitive change example, copy this into your IDE and run:

public class Solution {
    public static void main(String[] args) throws Exception {
        int x = 5;
        System.out.println("The value of x in main before function is: " + x);
        addFive(x);
        System.out.println("The value of x in main after function is: " + x);
    }

    static public void addFive(int x){
        x = x+ 5;
        System.out.println("The value of x in function is: " + x);
    }
}

The function does not change the value of the variable x in the main, this is because x is a primitive and is not passed by reference. Change int to an int[] (which is not primitive), like below, and run code and see the difference:

public class Solution {
    public static void main(String[] args) throws Exception {
        int[] x = {5,2};
        System.out.println("The values of x in main before function are: " + Arrays.toString(x));
        addFive(x);
        System.out.println("The values of x in main after function are: " + Arrays.toString(x));
    }

    static public void addFive(int[] x){
        for(int i = 0; i < x.length; i++) x[i] = x[i] + 5;
        System.out.println("The values of x in function are: " + Arrays.toString(x));
    }
}

Pholoxo

Level 8 , Blacksburg, United States

18 July 2019, 02:21

I thought you can only use iterators for these

Guadalupe Gagnon

Level 37 , Tampa, United States

22 December 2018, 21:36

useful

add 4 sys.out statements into your code and re-run it (see below):

int count = 0, no = 0;
        HashMap<String, String> copy = new HashMap<String, String>(map);
        Iterator<Map.Entry<String, String>> itr = copy.entrySet().iterator();
        while (itr.hasNext()) {

            String value1 = itr.next().getValue();
            Iterator<Map.Entry<String, String>> itr2 = copy.entrySet().iterator();
            System.out.println("Case map value: " + value1);    //add
            while(itr2.hasNext()) {
                String value2 = itr.next().getValue();
                System.out.println("Against copy: " + value2);   //add
                if (value1.equals(value2)) {
                    count++;
                }
                if (count > 1) {
                    System.out.println("Value found: removing: " + value1);  //add
                    removeItemFromMapByValue(map, value2);
                    no = 1;
                    System.out.println("Map values now: " + map);  //add
                }
            }
            if (no == 1) {
                removeItemFromMapByValue(map, value1);
            }
            no = 0;


        }

Guadalupe Gagnon

Level 37 , Tampa, United States

22 December 2018, 21:56

useful

#1 if you run it the first occurance will be AAAA, and the first time it finds a match it skips it because count needs to be greater that 1 *if (count > 1)*. Count is never reset either, so once count = 2 (or greater) all subsequent values are removed. This is a problem and needs to be fixed. #2 removeItemFromMapByValue() removes all occurances of the value, so you can actually just break the inner loop when one occurance is found and subsequently removed, you also don't need to remove value 1 afterwards. #3 You are using two iterators but only moving one. *while(itr2.hasNext())* keeps returning true while you are removing from map. This goes on until map has nothing left to iterate through and it errors out. If you fix these three things your code will work.

Peerawas

Level 18

23 December 2018, 02:46

Thanks for your explanation. I really appreciated :)

Guadalupe Gagnon

Level 37 , Tampa, United States

23 December 2018, 04:25

Good luck with your learning, it all makes more sense the more you do it!

Peerawas

Level 18

25 December 2018, 10:30

I tried to continue with my code with 3 things you mentioned before. I still got NoSuchElementException what did I miss.

public static void removeFirstNameDuplicates(Map<String, String> map) {

        int count = 0;
        HashMap<String, String> copy = new HashMap<String, String>(map);
        Iterator<Map.Entry<String, String>> itr = copy.entrySet().iterator();
        while (itr.hasNext()) {

            String value1 = itr.next().getValue();
            Iterator<Map.Entry<String, String>> itr2 = copy.entrySet().iterator();
            System.out.println("Case map value: " + value1);    //add
            while(itr2.hasNext()) {

                String value2 = itr.next().getValue();
                System.out.println("Against copy: " + value2);   //add
                if (value1.equals(value2)) {
                   count++;
                }
                if (count > 0) {
                    System.out.println("Value found: removing: " + value1);  //add
                    removeItemFromMapByValue(map, value2);
                    count = 0;
                    System.out.println("Map values now: " + map);  //add
                    break;
                }
            }
        }
    }

Guadalupe Gagnon

Level 37 , Tampa, United States

26 December 2018, 00:18

useful

#1 The big thing is that line 11 **(while(itr2.hasNext())** is an infinite loop because you never call itr2.next(). It continues pulling values from itr until there are no more then errors out. #2 along with the first point, i think your logic is to compare each value in the copy to value contained in value 1(line8). This is achievable but you have to remember that HashMaps pull values randomly, so you cant say skip the first pull of the copy. Use line 18 so that it doesn't enter that if block until the second time it is hit. #3 when you remove an item, make sure to reset the itr value to the map.entry.iterator() of the returned map after removeItemFromMapByValue. Also make the variable copy equal to the new map. #4 move the line 21, count = 0, to under the first while loop, line 7. I'll leave you with these hint like instructions to see if you can figure it out. Message me if you want a copy of the changes i made (and verify work) to your code.