CodeGym/Help with Java Tasks/Really struggling

Level 17

Amsterdam

13.02.2020
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9comments

Really struggling

Question about the task

R or L

Java Syntax, Level 7, Lesson 9

Resolved

I have no idea why this is not working?

1. Create a list of words and populate it yourself.
2. The fix method should:
2.1. remove all words containing the letter "r" from the list
2.2. duplicate all words containing the letter "l".
2.3. if a word contains both "r" and "l", then leave it unchanged.
2.4. don't do anything to other words.

For example:
rose lyre love

Output:
lyre love love

Requirements:

The program must not read data from the keyboard.
The fix method should remove all words containing the letter "r" from the list. There is an exception: words containing both "r" and "l" should be left alone.
The fix method must duplicate words containing the letter "l" (add another instance of this word to the list). There is an exception: words containing both "l" and "r" should not be duplicated.
The fix method should not do anything with words that contain both "l" and "r".
The fix method should not do anything with words that don't contain either "l" or "r".

package com.codegym.task.task07.task0716; import java.util.ArrayList; /* R or L */ public class Solution { public static void main(String[] args) throws Exception { ArrayList<String> list = new ArrayList<String>(); list.add("rose"); // 0 list.add("love"); // 1 list.add("lyre"); // 2 list.add("dole"); list.add("nice"); list.add("four"); list = fix(list); for (String s : list) { System.out.println(s); } } public static ArrayList<String> fix(ArrayList<String> list) { // write your code here for(int i = 0; i < list.size(); i++){//loops over the list if(list.get(i).contains("r")) {// if string contains the letter r remove it list.remove(i); } if(list.get(i).contains("l")) {// if string contains the letter l duplicate it list.add(list.get(i)); } if((list.get(i).contains("r")) && (list.get(i).contains("l"))) //|| { return list;// if string contain r & l don't do anything } else {//if string does not contain r || l don't do anything return list; } } return list; } }

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Jakub Góźdź

Level 31 , Katowice, Poland

13 February 2020, 12:57

1. You are breaking the loop in lines 41 and 45.

return list; // terminates the method and returns a variable, so the loop will also be broken.

2. You are changing the size of the ArrayList during you iterate it. You remove the elements of the list but don't care about how many times the loop will be executed. You declared that the loop should be executed as many times as many elements are in the list. but you change the quantity of the elements inside (by adding and removing them) You should consider this issue.

Liliane Top

Level 17 , Amsterdam, Netherlands

13 February 2020, 13:05

Hi Jakub, 1. I removed the return statements. 2. I am rather confused about the for loop. I thought that I was iterating over the entire loop and for each item something will be done to that item. But that is not the case, why?

Jakub Góźdź

Level 31 , Katowice, Poland

13 February 2020, 13:42

Ad.2. You are right. My mistake But there is an issue with indexing. When you remove an element all subsequent elements are moved to fill the hole. Their indexes are changing. For example, let's say that you have your list: [0] rose [1] love [2] lyre [3] dole ... In first iteration you are removing element [0] rose In next iteration the list looks like this: [0] love [1] lyre [2] dole ... but the i variable's value is now 1. You are checking if [1] lyre contains r and/or l letter and act respectively, but you skipped [0] love element.... You have to handle this issue. I suggest to decrease the i value by 1, when you delete element, and increase it by 1 when you duplicate an element.

if(list.get(i).contains("r"))
        {// if string contains the letter r remove it
            list.remove(i);
            i--;
        }
if(list.get(i).contains("l"))
        {// if string contains the letter l duplicate it
            list.add(i, list.get(i));
            i++;
        }

Liliane Top

Level 17 , Amsterdam, Netherlands

13 February 2020, 14:32

This really makes sense!!! Now I have a much clearer picture of how it actually works. Very much appreciated!!

Liliane Top

Level 17 , Amsterdam, Netherlands

13 February 2020, 14:47

Unfortunately I don't get it at all. What you wrote made sense but does not work. More confused than ever...

Jakub Góźdź

Level 31 , Katowice, Poland

13 February 2020, 19:22

solution

Ah because you have to insert a new element in the proper place. Probably you have to change: list.add(list.get(i)); // this command add a new element on the end of the list to list.add(i, list.get(i)); // this command insert new element at index i, the current element that is located there will be moved to the next position and all subsequent element will move to next position as well Here you can find some visualizations: https://codegym.cc/groups/posts/21-arraylist-in-pictures-- Here you have complete solution:

public class Solution {
    public static void main(String[] args) throws Exception {
        ArrayList<String> list = new ArrayList<String>();
        list.add("rose");
        list.add("love");
        list.add("lyre");
        list = fix(list);

        for (String s : list) {
            System.out.println(s);
        }
    }

    public static ArrayList<String> fix(ArrayList<String> list) {
        for(int i = 0; i < list.size(); i++){

            if(list.get(i).indexOf("r") != -1 && list.get(i).indexOf("l") != -1){
                // do nothing
            }
            else if(list.get(i).indexOf("r") != -1 && list.get(i).indexOf("l") == -1){
                list.remove(i);
                i--;
            }
            else if(list.get(i).indexOf("r") == -1 && list.get(i).indexOf("l") != -1){
                list.add(i, list.get(i));
                i++;
            }
        }

        return list;
    }
}

Liliane Top

Level 17 , Amsterdam, Netherlands

14 February 2020, 01:46

Thanks again for taking the time to help me. Very much appreciated. I learned 3 things from your answer: 1. I create an infinite loop by not taking into account that by adding a new word you have to increase the 'i' otherwise it will keep duplicating that same word. 2. I skipped a word when I did not reduced the 'i' with one as I deleted a word. 3. And this is the MOST important one. My code didn't work as I used two if statements in the for-loop as soon as I changed the second if statement to an else-if statement everything worked properly. I'm not sure what the difference is between two though. It's not almost 3am in the morning it is time to get some sleep now I have the solution. haha Jakub, thanks again and are you really only on level 8 and so incredible capable already??

Jakub Góźdź

Level 31 , Katowice, Poland

14 February 2020, 08:05

if(i == 0) {...} // if i = 0 this will work and skip following statements
else if(i == 1) {...} // if i not equal to 0 and if i = 1 this will work and skip following statement
else if(i == 2) {...} // if i not equal to 0 or 1 and if i = 2 the statement will execute
else {...}// if i not equal to 0, 1 or 2 the statement will execute

if(i == 0) {...} // if i = 0 this will work and check the following conditions also
if(i == 1) {...} // regardless of the i == 0 check, this if condition is checked
if(i == 2) {...} // regardless of the i == 0 and i == 1 check, this if condition is checked
else {...} // if i not equal to 2 the statement will execute

if(this condition is fulfilled) {
      // do this
}
else {
      // do something else
}

Can be read as: "If this condition is fulfilled do this, otherwise do something else"

if(this condition is full filed) {
      // do this
}
else if(second condition is full filed) {
      // do something else
}
else{
      // do something totally different
}

Can be read as: "If this condition is fulfilled do this, otherwise if second condition is full filed do something else, in all other cases do something totally different" So when you use ELSE it means you are continuing the sentence you started earlier. When you use just IF that means you are starting a new sentence.

Liliane Top

Level 17 , Amsterdam, Netherlands

14 February 2020, 08:09

Clear!! Wauh you're very good in explaining.