Duda de código

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En esta tarea he conseguido añadir al final de la lista principal (en mí código es cadenas) las primeras M strings, pero lo que no consigo es eliminarlas de la lista principal. He estado leyendo al respecto del método subList() y hay una cosa que me quedo claro pero que no se hace a la hora de ejecutar mi código y es que, se supone que al borrar los elementos del método subList() también se eliminarían de la lista principal (cadenas). Adjunto código para que podáis leerlo y darme alguna explicación, que se que estoy apunto de resolverlo y no quiero mirar la solución. Llevo atascado un par de días. BufferedReader reader = new BufferedReader(new InputStreamReader(System.in)); //write your code here ArrayList<String> cadenas = new ArrayList<>(); int N = Integer.parseInt(reader.readLine()); // N es el numero de strings que se almacenarán en el ArrayList int M = Integer.parseInt(reader.readLine()); // M es el número de cadenas que voy a mover al final del ArrayList for(int i = 0; i < N; i++){ cadenas.add(reader.readLine()); } ArrayList<String> subCadenas = new ArrayList<>(cadenas.subList(0, M)); cadenas.addAll(subCadenas); subCadenas.clear(); // aquí se borran los elementos de la subList pero no de la lista principal (cadenas) for(String s: cadenas){ System.out.println(s); }

package com.codegym.task.task07.task0720; import java.io.BufferedReader; import java.io.IOException; import java.io.InputStreamReader; import java.lang.reflect.Array; import java.util.ArrayList; import java.util.Iterator; /* Shuffled just in time */ public class Solution { public static void main(String[] args) throws IOException { BufferedReader reader = new BufferedReader(new InputStreamReader(System.in)); //write your code here ArrayList<String> cadenas = new ArrayList<>(); int N = Integer.parseInt(reader.readLine()); // N es el numero de strings que se almacenarán en el ArrayList int M = Integer.parseInt(reader.readLine()); // M es el número de cadenas que voy a mover al final del ArrayList for(int i = 0; i < N; i++){ cadenas.add(reader.readLine()); } ArrayList<String> subCadenas = new ArrayList<>(cadenas.subList(0, M)); for(int i = 0; i < subCadenas.size(); i++){ cadenas.add(subCadenas.get(i)); } subCadenas.clear(); for(String s: cadenas){ System.out.println(s); } } }

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Guadalupe Gagnon

Nivel 37 , Tampa, United States

16 enero, 14:38

Your first problem is that this code will not work if M is larger than N, which the task doesn't say that M has to be less than or equal to N. When M is the larger number then line 29 will throw an exception and crash the program. Line 29 fails the task because the task says that "you must not create more than one list.". As far as moving from the front to the back: You simply need to pull an item from the front of the list and save it to a variable. This task works with Strings so a String variable would be appropriate. Next remove that same item from the list. Then finally put the item saved in the variable back in the list, since the add() method automatically puts new items at the end of the list you need to do nothing special.

M.A.P.M.

Nivel 8 , Spain

17 enero, 18:23

Ok, I understand what you have told me and I am going to address your answers in several parts: 1. I think that I have based my code on the idea that M cannot be greater than N, since an error will occur when trying to pass more strings than there are from the beginning of the list to the end of it. 2. I understand the logic of the task, I have to store in a variable of the same type the element that I want to move to the end of the list, that is, I store in a variable, I delete that element from the list and add the element to the end of the list . I address this by creating a sublist (subStrings) with the first M elements to then add them to the main list (strings) and finally delete the M elements from the sublist which in turn will be eliminated from the main list (lines 29 to 34) . When testing it, I notice that the first M items are not deleted from the main list and are deleted from the sublist. 3. It may be that when making the sublist you are creating a new list. So I don't know if I'm using the sublist() method correctly.

Guadalupe Gagnon

Nivel 37 , Tampa, United States

17 enero, 20:07

1) M can be greater than N. Your code should just keep moving the first item in the list to the end M times 2) it specifically says that only one list can be used in this task. Line 29 creates a new second list which violates this 3) the subList() method returns a new list and violates the restriction that only one list can be used

M.A.P.M.

Nivel 8 , Spain

17 enero, 21:13

Ok, since I had so many doubts about the exercise, I looked at the comments that used the sublist() method and I read the theory that said that using that method updates the main list, but I see that is not the case. I have tried to do it using this removeFirst() method but it doesn't validate it either. I'm gonna go crazy. BufferedReader reader = new BufferedReader(new InputStreamReader(System.in)); //write your code here ArrayList<String> cadenas = new ArrayList<>(); int N = Integer.parseInt(reader.readLine()); // N es el numero de strings que se almacenarán en el ArrayList int M = Integer.parseInt(reader.readLine()); // M es el número de cadenas que voy a mover al final del ArrayList for(int i = 0; i < N; i++){ cadenas.add(reader.readLine()); } for(int i = 0; i < M; i++){ String s = cadenas.get(i); cadenas.add(s); } for(int i = 0; i < M; i++){ cadenas.removeFirst(); // This is the method in question } Help Please jajajajajjaa for(String s: cadenas){ System.out.println(s); }

M.A.P.M.

Nivel 8 , Spain

18 enero, 14:46

Okay, I managed to solve the task a few minutes ago. After thinking about it a lot while trying to sleep, I looked for another alternative to the removeFirst() method and at the same time understood why it is better to do the storage and add and delete points separately. It was difficult for me but I solved it. I just had to understand well what you had told me. Thank you for your help, I have realized that I have to read the task statements carefully to know what they are telling me.

Guadalupe Gagnon

Nivel 37 , Tampa, United States

18 enero, 16:07

Glad you figured it out. The last code you shared was really close to solving. Lists have a remove(int index) method that returns the item at the index specified and removes it from the list. You can use this method to get the first item to save to the String and then add it back into the list. So, using the code you shared:

ArrayList<String> cadenas = new ArrayList<>();

int N = Integer.parseInt(reader.readLine()); // N es el numero de strings que se almacenarán en el ArrayList
int M = Integer.parseInt(reader.readLine()); // M es el número de cadenas que voy a mover al final del ArrayList

for(int i = 0; i < N; i++){
   cadenas.add(reader.readLine());
}

for(int i = 0; i < M; i++){
   // change this line to use remove() and pass 0 as the index
   //String s = cadenas.get(i);
   String s = cadenas.remove(0);
   cadenas.add(s);
}

/* dont need this
for(int i = 0;  i < M; i++){
   cadenas.removeFirst(); // This is the method in question
}*/

//Help Please jajajajajjaa
for(String s: cadenas){
   System.out.println(s);
}

M.A.P.M.

Nivel 8 , Spain

19 enero, 10:35

I'm glad to know that I was close to solving the task with the last code I shared. I didn't know that the remove(index) method, apart from removing the element at that position, also returned said element. That's why in my solution I made lines 10 to 15 separately.

M.A.P.M.

Nivel 8 , Spain

16 enero, 11:48

Al ejecutarlo el código en el compilador de la página de CodeGym, me dice que compruebe el código ya que parece que tengo un bucle infinito. Ayuda