How do i swap 2 particular letters in a particular array list?

Question

As you know the first 6 letters of the English alphabet is a, b, c, d, e, f. I want to swap(i.e. switch) the 2nd letter which is b and the fifth letter which is e. In other words, my goal is that after the code is executed my compiler will print the following: a, e, c, d, b,f.

When I google about swapping things in an array list I keep finding examples that contain collections.swap.. Well in class we did not learn about this. I am asking that you don't include that in your answer. The following 2 lines of codes are very simple examples of the kind of thing that is supposed to be in the code that we submit. The first example is public static void swap(int i, int j, ArrayList<String> myList). The second example is void swap(int i, int j, ArrayList myList)

The professor didn't exactly want to us to write this code from scratch. She wanted us to fix the code she gave us on the hand out the told us what we needed to do for this assignment. This is the incorrect code:

import java.util.ArrayList;

public class abc{


public static void main(String[]args){

  ArrayList<String> myList= new ArrayList();
  myList.add("a");
  myList.add("b");
  myList.add("c");
  myList.add("d");
  myList.add("e");
  myList.add("f");

  swap(1,4,myList); 
  System.out.println(myList);
} 

public static void swap(int i, int j, ArrayList<String> myList) {
  String temp = myList.get(i);
  myList.add(i, myList.get(j));
  myList.add(j, temp);
}

(I tried to give that a gray background like how code is supposed to look but, it didn't work out that way.)

This code prints [a, e, b, c, b, d, e, f] but the first paragraph that I wrote explains what I'm trying to print. I spent such a long time trying to get this to work and I can't. Please help me get Jgrasp to do what I want to it to do (refer to first paragraph.) If possible please make your code look as close to the incorrect code as possible but of course I would like it to be correct.

use ArrayList.set instead of add in your swap method, see https://stackoverflow.com/questions/7452772/java-arraylist-replace-at-specific-index — Peter, Sep 20 '17 at 07:39
Possible duplicate of [Arraylist swap elements](https://stackoverflow.com/questions/15963549/arraylist-swap-elements) — Henrik Aasted Sørensen, Sep 20 '17 at 08:05

Peter · Answer 1 · 2017-09-20T07:52:42.323

2

use List.set instead of add in your swap method, see: Java ArrayList replace at specific index

public static void swap(int i, int j, ArrayList<String> myList) {
  String temp = myList.get(i);
  myList.set(i, myList.get(j));
  myList.set(j, temp);
}

edited Sep 20 '17 at 07:52

answered Sep 20 '17 at 07:41

Peter

1,006
7
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Yes, but please explain your answer: why was what OP doing wrong, what does this do instead? – Andy Turner Sep 20 '17 at 07:47

score 1 · Answer 2 · answered Sep 20 '17 at 07:42

1

Use the built-in Collections.swap

Replace

String temp = myList.get(i);
myList.add(i, myList.get(j));
myList.add(j, temp);

with

Collections.swap(myList, i , j);

answered Sep 20 '17 at 07:42

Naman

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353

score 0 · Answer 3 · answered Sep 20 '17 at 07:45

add(E e)

Appends the specified element to the end of this list (optional operation).

add(int index, E element)

Inserts the specified element at the specified position in this list (optional operation).

set(int index, E element)

Replaces the element at the specified position in this list with the specified element (optional operation).

    import java.util.ArrayList;

    public class abc{


    public static void main(String[]args){

      ArrayList<String> myList= new ArrayList();
      myList.add("a");
      myList.add("b");
      myList.add("c");
      myList.add("d");
      myList.add("e");
      myList.add("f");

      swap(1,4,myList); 
      System.out.println(myList);
    } 

    public static void swap(int i, int j, ArrayList<String> myList) {
      String temp = myList.get(i);
      myList.set(i, myList.get(j));
      myList.set(j, temp);
    }
}

score 0 · Answer 4 · answered Sep 20 '17 at 07:48

If you want to swap using positions, you can use the following:

import java.util.Collections;

ArrayList<String> list = new ArrayList<String>();
list.add("a");
list.add("b");
list.add("c");
list.add("d");
list.add("e");


// swapping
Collections.swap(list, 1, 3);

This will swap 1st element (b) with 3rd (d) to give [ a, d, c, b, e ]

How do i swap 2 particular letters in a particular array list?

4 Answers4