Shifting in arrayList

Question

The method public static <T> ArrayList<T> rotate(ArrayList<T> aL, int shift) accepts an Arraylist of String (at least in this example) and a shift which indicated how much the arraylist should shift. If I have, let's say an arayList of

[ A, B, C, D, E, F, G]

and the shift is 2, so the method returns

[ F, G, A, B, C, D, E]

or another example,

[ A, B, C, D, E, F, G]

and shift is 4, then the method returns

[ D, E, F, G, A, B, C]

I did the method completely wrong and have no clue how to approach this problem. Could smb help me with that ?

import java.util.ArrayList;
public class Rotation{
  // Demonstrate rotat(a,shift) method
  public static void main(String arg[]){
    ArrayList<Character> charsL;
    charsL = new ArrayList<Character>();
    char [] chars = { 'A', 'B', 'C',
                      'D', 'E', 'F'};
    for(char c : chars){
      charsL.add(c);
    }
    // charsL = [ A, B, C, D, E, F]

    ArrayList<Character> result1;

    result1 = rotate(charsL, 2);
    // result1== [ E, F, A, B, C, D]
    System.out.println(result1);
    
    result1 = rotate(charsL, 7);
    // result1== [ F, A, B, C, D, E]
    System.out.println(result1);

    // WORKS WITH SRTINGS TOO
    ArrayList<String> stringL;
    stringL = new ArrayList<String>();
    String [] strs = { "A", "B", "C",
                       "D", "E", "F", "G" };
    for(String s : strs){
      stringL.add(s);
    }
    // stringL = [ A, B, C, D, E, F, G]

    ArrayList<String> result2;

    result2 = rotate(stringL, 7);
    // result2== [ A, B, C, D, E, F, G]
    System.out.println(result2);

    result2 = rotate(stringL, 4);
    // result2== [ D, E, F, G, A, B, C]
    System.out.println(result2);
  }

  public static <T>
  ArrayList<T> rotate(ArrayList<T> aL, int shift){
    // YOUR DEFINITION HERE 
      
      ArrayList <T> newValues = new ArrayList<>();
      ArrayList <T> temp = new ArrayList<>();
      
      for(int i = 0; i < aL.size(); i++)
      {
          newValues.remove(aL.get(shift));
          newValues.add(aL.get(i));
         //newValues.add(shift, aL.get(i));
          
      }
      return newValues;

  }

}

Can you explain (or correct) your first example? Why size of list changed? Did you miss G between F and A? — Pshemo
– Pshemo, Commented Apr 9, 2015 at 20:43
I'm a little unclear about the shift rule. Did you intend that a shift of 4 rotates the data, but a shift of 2 both rotates and truncates it? — azurefrog
– azurefrog, Commented Apr 9, 2015 at 20:44
Without giving any though to a rotation algorithm, I can tell you that this line: newValues.remove(aL.get(shift)); is trying to remove values from an empty List, and is going to fail. — azurefrog
– azurefrog, Commented Apr 9, 2015 at 20:48

Pshemo · Accepted Answer · 2022-09-03 12:38:43Z

16

If you can use built-in solution then you can use Collections.rotate(List<?> list, int distance).

BTW when value of distance is

positive it rotates element to the right (example: distance=1 [a, b, c] -> [c, a, b])
negative it rotates elements to the left (example: distance=-1 [a, b, c] -> [b, c, a])

Anyway in your case it looks like your code can be simplified to

public static <T> List<T> rotate(List<T> aL, int shift) {
    List<T> newValues = new ArrayList<>(aL);
    Collections.rotate(newValues, shift);
    return newValues;
}

edited Sep 3, 2022 at 12:38

answered Apr 9, 2015 at 20:54

Pshemo

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S.L. Barth is on codidact.com · Accepted Answer · 2016-04-10 13:42:42Z

6

Try this:

public static <T> ArrayList<T> rotate(ArrayList<T> aL, int shift)
{
    if (aL.size() == 0)
        return aL;

    T element = null;
    for(int i = 0; i < shift; i++)
    {
        // remove last element, add it to front of the ArrayList
        element = aL.remove( aL.size() - 1 );
        aL.add(0, element);
    }

    return aL;
}

edited Apr 10, 2016 at 13:42

S.L. Barth is on codidact.com

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answered Apr 9, 2015 at 20:51

zxgear

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2 Comments

Nikolay Over a year ago

It works but I dont get how it works! Could you please explain ? @JohnH

zxgear Over a year ago

Basically, the method E remove(int index) removes an element from the list and returns the removed element. Then we use the method public void add(int index, E element) to add the element to the very first index of the ArrayList. You may find this link helpful: docs.oracle.com/javase/7/docs/api/java/util/ArrayList.html

Hassnain Jamil · Accepted Answer · 2016-04-10 14:34:47Z

Try this:

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

public class ALUtils {

public static <T> ArrayList<T> rotate(List<T> aL, int shift) {
    ArrayList<T> newValues = new ArrayList<>(aL);
    Collections.rotate(newValues, shift);
    return newValues;
}
}

Test Cases:

// Public tests for ALUtils

import org.junit.*;
import static org.junit.Assert.*;
import java.util.*;
import java.io.*;

public class ALUtilsTests {
  /*Main method runs tests in this file*/ 
  public static void main(String args[]) {
    org.junit.runner.JUnitCore.main("Tests");
  } 

  public <T> void checkRotate(T d[], int shift, String expectS){
    ArrayList<T> a = new ArrayList<T>();
    a.addAll(Arrays.asList(d));
    String sourceBefore = a.toString();

    ArrayList<T> actualA = ALUtils.rotate(a, shift);

    String sourceAfter = a.toString();
    String actualS = actualA.toString();
    String msg =
      String.format("Rotation incorrect\n")+
      String.format("Source  : %s (before rotate)\n",sourceBefore)+
      String.format("Shift   : %d\n",shift)+
      String.format("Expect  : %s\n",expectS)+
      String.format("Actual  : %s\n",actualS)+
      String.format("Source  : %s (after rotate)\n",sourceAfter);
    assertEquals(msg,expectS,actualS);
    assertEquals(msg,sourceBefore,sourceAfter);
  }

  @Test public void test_rotate1() {
    Character [] d = { 'A', 'B', 'C', 'D', 'E', 'F'};
    int shift = 2;
    String expectS = "[E, F, A, B, C, D]";
    checkRotate(d,shift,expectS);
  }

  @Test public void test_rotate2() {
    Character [] d = { 'A', 'B', 'C', 'D', 'E', 'F'};
    int shift = 7;
    String expectS = "[F, A, B, C, D, E]";
    checkRotate(d,shift,expectS);
  }

  @Test public void test_rotate3() {
    String [] d =  { "A", "B", "C", "D", "E", "F", "G" };
    int shift = 7;
    String expectS = "[A, B, C, D, E, F, G]";
    checkRotate(d,shift,expectS);
  }

  @Test public void test_rotate4() {
    String [] d =  { "A", "B", "C", "D", "E", "F", "G" };
    int shift = 4;
    String expectS = "[D, E, F, G, A, B, C]";
    checkRotate(d,shift,expectS);
  }

  @Test public void test_rotate5() {
    Integer [] d =  { 1};
    int shift = 4;
    String expectS = "[1]";
    checkRotate(d,shift,expectS);
  }

  @Test public void test_rotate6() {
    Integer [] d =  {};
    int shift = 2;
    String expectS = "[]";
    checkRotate(d,shift,expectS);
  }

  @Test public void test_rotate7() {
    String [] d =  { "A", "B", "C", "D", "E", "F", "G" };
    int shift = 0;
    String expectS = "[A, B, C, D, E, F, G]";
    checkRotate(d,shift,expectS);
  }

  @Test public void test_rotate8() {
    String [] d =  { "A", "B", "C", "D", "E", "F", "G" };
    int shift = 28;
    String expectS = "[A, B, C, D, E, F, G]";
    checkRotate(d,shift,expectS);
  }

  @Test public void test_rotate9() {
    String [] d =  { "A", "B", "C", "D", "E", "F", "G" };
    int shift = 39;
    String expectS = "[D, E, F, G, A, B, C]";
    checkRotate(d,shift,expectS);
  }
}

Jean Logeart · Accepted Answer · 2015-04-09 20:50:39Z

1

It is very unclear why you keep calling aL.get(shift) and why you keep removing from the list you should only be adding to.

Try:

public static <T> ArrayList<T> rotate(ArrayList<T> l, int shift) {
    List<T> left = l.subList(0, aL.size() - shift);
    List<T> right = l.subList(aL.size() - shift, l.size());
    List<T> res = new ArrayList<T>(right);
    res.addAll(left);
    return res;
}

answered Apr 9, 2015 at 20:50

Jean Logeart

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