How do I reverse an int array in Java

Question

I am trying to reverse an int array in Java   This method does not reverse the array     for int i   0  i  lt  validData length  i          int temp   validData i       validData i    validData validData length - i - 1       validData validData length - i - 1    temp      What is wrong with it

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Simple for loop   for  int start   0  end   array length - 1  start  lt   end  start    end--        int aux   array start       array start  array end       array end  aux

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Collections reverse Arrays asList yourArray      java util Collections reverse   can reverse java util Lists and java util Arrays asList   returns a list that wraps the the specific array you pass to it  therefore yourArray is reversed after the invocation of Collections reverse     The cost is just the creation of one List-object and no additional libraries are required   A similar solution has been presented in the answer of Tarik and their commentors  but I think this answer would be more concise and more easily parsable

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public class TryReverse       public static void main String   args                    int    array    2 3 4 5 6 7 8 9                  reverse array           for int i 0  i lt array length    i              System out print array i                    public static void reverse  int    array           for int start 0  end array length-1  start lt  end  start    end--               int aux   array start               array start  array end               array end  aux

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If working with data that is more primitive  i e  char  byte  int  etc  then you can do some fun XOR operations   public static void reverseArray4 int   array        int len   array length      for  int i   0  i  lt  len 2  i              array i    array i    array len - i  - 1           array len - i  - 1    array i    array len - i  - 1           array i    array i    array len - i  - 1

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static int   reverseArray int   a         int ret     new int a length        for int i 0  j a length-1  i lt a length  amp  amp  j gt  0  i    j--           ret i    a j        return ret

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Try this program in JAVA  -  import java util Scanner   public class Rev one D        static int row       static int   trans arr   new int row        public static void main String   args            Scanner sc   new Scanner System in           int n   sc nextInt            row   n           int   arr   new int row           for  int i   0  i  lt  row  i                   arr i    sc nextInt                System out print arr i                       System out println                       for  int i   0  i  lt  arr length   2  i                  int temp   arr i               arr i    arr arr length - i - 1               arr arr length - i - 1    temp                      for  int i   0  i  lt  row  i                  System out print arr i                      System out println

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As I intended to keep my original Array as it was  I solved this problem in the following manner   List lt Integer gt  normalArray  new ArrayList lt  gt     List lt Integer gt  reversedArray   new ArrayList lt  gt         Fill up array here  for  int i   1  i  lt   normalArray size    i        reversedArray  add normalArray get normalArray size  -i        So basically loop through the initial array and add all the values in reversed order to the new  reversed  array  The type of the list can be anything  I work my way through this code multiple times  this causes some of the other solutions not to work

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public void display      String x   new String  5     for int i   4   i  gt    0   i--     runs backwards        i is the nums running backwards therefore its printing from              highest element to the lowest ie the back of the array to the front  as i decrements      System out println x i

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import java util Scanner  class ReverseArray        public static void main String   args                 int   arra   new int 10           Scanner sc   new Scanner System in           System out println  Enter Array Elements               for int i   0   i  lt arra length i                          arra i    sc nextInt                       System out println  Printing  Array               for int i   0  i  lt arra length i                          System out print arra i                             System out println            System out println  Printing  Reverse Array               for int i   arra length-1  i  gt  0 i--                        System out print arra i

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I think it s a little bit easier to follow the logic of the algorithm if you declare explicit variables to keep track of the indices that you re swapping at each iteration of  the loop   public static void reverse int   data        for  int left   0  right   data length - 1  left  lt  right  left    right--               swap the values at the left and right indices         int temp   data left           data left     data right           data right    temp            I also think it s more readable to do this in a while loop   public static void reverse int   data        int left   0      int right   data length - 1       while  left  lt  right                swap the values at the left and right indices         int temp   data left           data left    data right           data right    temp              move the left and right index pointers in toward the center         left            right--

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public class ArrayHandle       public static Object   reverse Object   arr            List lt Object gt  list   Arrays asList arr           Collections reverse list           return list toArray

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You can use this  public final class ReverseComparator lt T extends Comparable lt T gt  gt  implements  Comparator lt T gt       Override   public int compare T o1  T o2              return o2 compareTo o1           An simply  Integer   a    1 6 23 4 6 8 2  Arrays sort a  new ReverseComparator lt Integer gt

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private static int   reverse int   array       int   reversedArray   new int array length       for int i   0  i  lt  array length  i             reversedArray i    array array length - i - 1             return reversedArray

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It is most efficient to simply iterate the array backwards   I m not sure if Aaron s solution does this vi this call Collections reverse list   Does anyone know

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below is the complete program to run in your machine   public class ReverseArray       public static void main String   args            int arr     new int     10 20 30 50 70            System out println  reversing an array             for int i   0  i  lt  arr length   2  i                 int temp   arr i               arr i    arr arr length - i - 1               arr arr length - i - 1    temp                    for  int i   0  i  lt  arr length  i                  System out println arr i                           For programs on matrix using arrays this will be the good source Go through the link

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public static int   reverse int   array         int j   array length-1         swap the values at the left and right indices                for int i 0  i lt  j  i                           int temp   array i                   array i    array j                   array j    temp             j--                      return array               public static void main String   args           int   data    1 2 3 4 5 6 7 8 9           reverse data

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There are already a lot of answers here  mostly focused on modifying the array in-place  But for the sake of completeness  here is another approach using Java streams to preserve the original array and create a new reversed array       int   a    8  6  7  5  3  0  9       int   b   IntStream rangeClosed 1  a length  map i - gt  a a length-i   toArray

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This is how I would personally solve it  The reason behind creating the parametrized method is to allow any array to be sorted    not just your integers   I hope you glean something from it    Test public void reverseTest       Integer   ints     1  2  3  4       Integer   reversedInts   reverse ints       assert ints 0  equals reversedInts 3       assert ints 1  equals reversedInts 2       assert ints 2  equals reversedInts 1       assert ints 3  equals reversedInts 0        reverseInPlace reversedInts      assert ints 0  equals reversedInts 0        SuppressWarnings  unchecked   private static  lt T gt  T   reverse T   array        if  array    null            return  T    new ArrayList lt T gt    toArray              List lt T gt  copyOfArray   Arrays asList Arrays copyOf array  array length        Collections reverse copyOfArray       return copyOfArray toArray array      private static  lt T gt  T   reverseInPlace T   array        if array    null               didn t want two unchecked suppressions         return reverse array              Collections reverse Arrays asList array        return array

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2 ways to reverse an Array     Using For loop and swap the elements till the mid point with time complexity of O n 2    private static void reverseArray     int   array   new int     1  2  3  4  5  6     for  int i   0  i  lt  array length   2  i          int temp   array i       int index   array length - i - 1      array i    array index       array index    temp    System out println Arrays toString array        Using built in function  Collections reverse     private static void reverseArrayUsingBuiltInFun     int   array   new int     1  2  3  4  5  6     Collections reverse Ints asList array    System out println Arrays toString array         Output    6  5  4  3  2  1

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Just for the sake of it  People often do only need a  view  on an array or list in reversed order instead of a completely   do not need a reversed array when working with streams and collections but a  reversed  view on the original array collection    it is best to create a toolkit that has a reverse view on a list   array  So create your Iterator implementation that takes an array or list and provide the input      Reverse Iterator public class ReverseIterator lt T gt  implements Iterator lt T gt      private int index    private final List lt T gt  list    public ReverseIterator List lt T gt  list         this list   list       this index   list size   - 1        public boolean hasNext         return index  gt   0   true   false        public T next         if index  gt   0         return list get index--       else        throw new NoSuchElementException           An implementation for the array situation is quite similar  Of cause an iterator can be source for a stream or a collection as well  So not always it is best to create a new array just to provide a reverse view when all you want to do is iterating over the array   list or feed it into a stream or a new collection   array

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There are some great answers above  but this is how I did it   public static int   test int   arr         int   output   arr clone        for  int i   arr length - 1  i  gt  -1  i--            output i    arr arr length - i - 1             return output

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To reverse an int array  you swap items up until you reach the midpoint  like this   for int i   0  i  lt  validData length   2  i          int temp   validData i       validData i    validData validData length - i - 1       validData validData length - i - 1    temp      The way you are doing it  you swap each element twice  so the result is the same as the initial list

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Here is a condensed version   My solution creates a new array reversed With each iteration of i the for loop inserts the last index  array length - 1  into the current index  i  Then continues the same process by subtracting the current iteration array  array length - 1  - i  from the last index and inserting the element into the next index of the reverse array   private static void reverse int   array        int   reversed   new int array length        for  int i   0  i  lt  array length  i              reversed i    array  array length - 1  - i             System out println Arrays toString reversed

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public static void main  String args            create  array     String   stuff    eggs   lasers   hats   pie   apples           print out  array     for String x  stuff          System out printf   s    x               System out println                   print out array in reverse order             for int i stuff length-1  i  gt   0  i--                  System out printf   s   stuff i

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public void getDSCSort int   data           for  int left   0  right   data length - 1  left  lt  right  left    right--                  swap the values at the left and right indices             int temp   data left               data left     data right               data right    temp

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Here is a simple implementation  to reverse array of any type  plus full partial support   import java util logging Logger   public final class ArrayReverser    private static final Logger LOGGER   Logger getLogger ArrayReverser class getName       private ArrayReverser            public static  lt T gt  void reverse T   seed        reverse seed  0  seed length        public static  lt T gt  void reverse T   seed  int startIndexInclusive  int endIndexExclusive        if  seed    null    seed length    0            LOGGER warning  Nothing to rotate              int start   startIndexInclusive  lt  0   0   startIndexInclusive      int end   Math min seed length  endIndexExclusive  - 1      while  start  lt  end            swap seed  start  end           start            end--            private static  lt T gt  void swap T   seed  int start  int end        T temp    seed start       seed start    seed end       seed end    temp            Here is the corresponding Unit Test   import static org hamcrest CoreMatchers is  import static org junit Assert assertThat   import org junit Before  import org junit Test   public class ArrayReverserTest   private Integer   seed    Before public void doBeforeEachTestCase         this seed   new Integer   1 2 3 4 5 6 7 8       Test public void wholeArrayReverse         ArrayReverser  lt Integer gt reverse seed       assertThat seed 0   is 8         Test  public void partialArrayReverse         ArrayReverser  lt Integer gt reverse seed  1  5       assertThat seed 1   is 5

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Your program will work for only length   0  1  You can try    int i   0  j   validData length-1    while i  lt  j         swap validData  i    j--       code for swap not shown  but easy enough

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public static void main String args                 int    arr    10  20  30  40  50            reverse arr  arr length              private static void reverse int   arr     int length                for int i length i gt 0 i--                    System out println arr i-1

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Using the XOR solution to avoid the temp variable your code should look like  for int i   0  i  lt  validData length  i         validData i    validData i    validData validData length - i - 1       validData validData length - i - 1    validData i    validData validData length - i - 1       validData i    validData i    validData validData length - i - 1       See this link for a better explanation   http   betterexplained com articles swap-two-variables-using-xor

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a piece of cake  i did it for string but  it s not much different

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int   arrTwo    5  8  18  6  20  50  6        for  int i   arrTwo length-1  i  gt  0  i--                System out print arrTwo i

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There are two ways to have a solution for the problem   1  Reverse an array in space   Step 1  Swap the elements at the start and the end index   Step 2  Increment the start index decrement the end index   Step 3  Iterate Step 1 and Step 2 till start index  lt  end index  For this  the time complexity will be O n  and the space complexity will be O 1   Sample code for reversing an array in space is like   public static int   reverseAnArrayInSpace int   array        int startIndex   0      int endIndex   array length - 1      while startIndex  lt  endIndex            int temp   array endIndex           array endIndex    array startIndex           array startIndex    temp          startIndex            endIndex--            return array      2  Reverse an array using an auxiliary array   Step 1  Create a new array of size equal to the given array   Step 2  Insert elements to the new array starting from the start index  from the  given array starting from end index   For this  the time complexity will be O n  and the space complexity will be O n   Sample code for reversing an array with auxiliary array is like   public static int   reverseAnArrayWithAuxiliaryArray int   array        int   reversedArray   new int array length       for int index   0  index  lt  array length  index              reversedArray index    array array length - index -1              return reversedArray      Also  we can use the Collections API from Java to do this       The Collections API internally uses the same reverse in space approach   Sample code for using the Collections API is like   public static Integer   reverseAnArrayWithCollections Integer   array        List lt Integer gt  arrayList   Arrays asList array       Collections reverse arrayList       return arrayList toArray array

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With Guava   Collections reverse Ints asList array

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Solution with o n  time complexity and o 1  space complexity   void reverse int   array        int start   0      int end   array length - 1      while  start  lt  end            int temp   array start           array start    array end           array end    temp          start            end--

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Wouldn t doing it this way be much more unlikely for mistakes       int   intArray    1  2  3  4  5  6  7  8  9  10       int   temp   new int intArray length       for int i   intArray length - 1  i  gt  -1  i --               temp intArray length - i -1    intArray i             intArray   temp

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for int i validData length-1  i gt  0  i--     System out println validData i

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A short way to reverse without additional libraries  imports  or static references  int   a    1 2 3 4 5 6 7 23 9   b    compound declaration var j   a length  b   new int j   for  var i   a      b --j    i    --j so you don t have to subtract 1 from j  Otherwise you would get ArrayIndexOutOfBoundsException  System out println Arrays toString b     Of course if you actually need a to be the reversed array just use a   b    after the loop

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In case of Java 8 we can also use IntStream to reverse the array of integers  as   int   sample   new int   1 2 3 4 5   int size   sample length  int   reverseSample   IntStream range 0 size  map i - gt  sample size-i-1                          toArray      Output   5  4  3  2  1

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Here is what I ve come up with      solution 1 - boiler plated  Integer   original    100  200  300  400   Integer   reverse   new Integer original length    int lastIdx   original length -1  int startIdx   0   for  int endIdx   lastIdx  endIdx  gt   0  endIdx--  startIdx       reverse startIdx    original endIdx    System out printf  reverse form   s   Arrays toString reverse        solution 2 - abstracted     convert to list then use Collections static reverse   List lt Integer gt  l   Arrays asList original   Collections reverse l   System out printf  reverse form   s   l

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With Commons Lang  you could simply use   ArrayUtils reverse int   array    Most of the time  it s quicker and more bug-safe to stick with easily available libraries already unit-tested and user-tested when they take care of your problem

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Try this code        int arr     new int   1 2 3 4 5 6 7       for int i 0 i lt arr length 2 i             int temp   arr i           arr i    arr  arr length-1 -i           arr  arr length-1 -i    temp              System out println Arrays toString arr

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Here s a simple an quick solution  Hope it helps    public int   reverse int   arr        for int i   arr length  i  gt  0   i--           System out print arr i-1                    return arr

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Another way to reverse array   public static int   reversing int   array       int arraysize   array length      int   reverse   new int  arraysize 1       for int i 1  i  lt   arraysize   i             int dec  arraysize -i          reverse i    array dec             return reverse

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This will help you  int a      1 2 3 4 5   for  int k   0  k  lt  a length 2  k          int temp   a k       a k    a a length- 1 k        a a length- 1 k     temp

[java] How do I reverse an int array in Java?

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