How to add an element to Array and shift indexes

Question

I need to add an element to Array specifying position and value  For example  I have Array  int   a    1  2  3  4  5  6     after applying addPos int 4  int 87  it should be  int   a    1  2  3  4  87  5     I understand that here should be a shift of Array s indexes  but don t see how to implement it in code

User · Answer

The most simple way of doing this is to use an ArrayList lt Integer gt  and use the add int  T  method   List lt Integer gt  list   new ArrayList lt Integer gt     list add 1   list add 2   list add 3   list add 4   list add 5   list add 6       Now  we will insert the number list add 4  87

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Have a look at commons  It uses arrayCopy    but has nicer syntax  To those answering with the element-by-element code  if this isn t homework  that s trivial and the interesting answer is the one that promotes reuse  To those who propose lists  probably readers know about that too and performance issues should be mentioned

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int   b   new int a length  1   System arraycopy a 0 b 0 4      System arraycopy srcArray  srcPosition  destnArray  destnPosition  length  b 4  87  System arraycopy a 4 b 5 2     b array would be created as  1  2  3  4  87  5 6

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This should do the trick   public static int   addPos int   a  int pos  int num        int   result   new int a length       for int i   0  i  lt  pos  i            result i    a i       result pos    num      for int i   pos   1  i  lt  a length  i            result i    a i - 1       return result      Where a is the original array  pos is the position of insertion  and num is the number to be inserted

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Here is a quasi-oneliner that does it   String   prependedArray   new ArrayList lt String gt              add  newElement        addAll Arrays asList originalArray          toArray new String 0

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You must make a new array  use System arraycopy to copy the prefix and suffix  and set that one slot to the new value

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Following code will insert the element at specified position and shift the existing elements to move next to new element   public class InsertNumInArray    public static void main String   args        int   inputArray   new int     10  20  30  40        int inputArraylength   inputArray length      int tempArrayLength   inputArraylength   1      int num   50  position   2      int   tempArray   new int tempArrayLength        for  int i   0  i  lt  tempArrayLength  i              if  i    position  amp  amp  i  lt  position              tempArray i    inputArray i           else if  i    position              tempArray i    num          else             tempArray i    inputArray i-1                inputArray   tempArray       for  int number   inputArray            System out println  Number is      number

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public class HelloWorld        public static void main String   args           int   LA    1 2 4 5           int k   2          int item   3          int j   LA length          int   LA NEW   new int LA length 1            while j  gt k               LA NEW j    LA j-1               j   j-1                    LA NEW k    item          for int i   0 i lt k i                 LA NEW i    LA i                     for int i   LA NEW               System out println i

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org apache commons lang3 ArrayUtils add T    int  T  is deprecated in newest commons lang3  you can use org apache commons lang3 ArrayUtils insert int  T    T     instead      Deprecated this method has been superseded by insert int  T    T     and may be removed in a future release  Please note the handling of null input arrays differs in the new method  inserting X into a null array results in null not X   Sample code       Assert assertArrayEquals              org apache commons lang3 ArrayUtils insert              4  new int   1  2  3  4  5  6   87   new int   1  2  3  4  87  5  6

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Jrad solution is good but I don t like that he doesn t use array copy  Internally System arraycopy   does a native call so you will a get faster results    public static int   addPos int   a  int index  int num        int   result   new int a length       System arraycopy a  0  result  0  index       System arraycopy a  index  result  index   1  a length - index - 1       result index    num      return result

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System arraycopy is more performant but tricky to get right due to indexes calculations  Better stick with jrad answer or ArrayList if you don t have performance requirements   public static int   insert      int   array  int elementToInsert  int index      int   result   new int array length   1        copies first part of the array from the start up until the index   System arraycopy        array    src           0    srcPos           result    dest           0    destPos           index    length           copies second part from the index up until the end shifting by 1 to the right   System arraycopy        array    src           index    srcPos           result    dest           index   1    destPos           array length - index    length        result index    elementToInsert    return result      And JUnit4 test to check it works as expected    Test public void shouldInsertCorrectly       Assert assertArrayEquals        new int   1  2  3   insert new int   1  3   2  1      Assert assertArrayEquals        new int   1   insert new int      1  0      Assert assertArrayEquals        new int   1  2  3   insert new int   2  3   1  0      Assert assertArrayEquals        new int   1  2  3   insert new int   1  2   3  2

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I smell homework  so probably an ArrayList won t be allowed       Instead of looking for a way to  shift indexes   maybe just build a new array   int   b   new int a length  1     Then   copy indexes form array a counting from zero up to insert position             edit  copy values of course  not indexes

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Unless I m missing something  the question is not about increasing the array size  In the example the array size remains the same   Like a bit shift    In this case  there is really no reason to create a new array or to copy it  This should do the trick   static void addPos int   array  int pos  int value           initially set to value parameter so the first iteration  the value is replaced by it     int prevValue   value          Shift all elements to the right  starting at pos     for  int i   pos  i  lt  array length  i              int tmp   prevValue          prevValue   array i           array i    tmp              int   a    1  2  3  4  5  6   addPos a  4  87      output   1  2  3  4  87  5

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Try this   public static int    insertArry  int inputArray    int index  int value       for int i 0  i lt  inputArray length-1  i               if  i    index                for  int j   inputArray length-1  j  gt   index  j--                    inputArray j   inputArray j-1                              inputArray index  value                       return inputArray

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If you prefer to use Apache Commons instead of reinventing the wheel  the current approach is this   a   ArrayUtils insert 4  a  87     It used to be ArrayUtils add      but that was deprecated a while ago  More info here  1

[java] How to add an element to Array and shift indexes?

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