How to find the index of an element in an array in Java

Question

I am looking to find the index of a given element  knowing its contents  in Java   I tried the following example  which does not work   class masi        public static void main  String   args              char   list     m    e    y                should print 1         System out println list       e                                   Can anyone please explain what is wrong with this and what I need to do to fix it

User · Answer

Now it does print 1  class Masi       public static void main  String    args              char    list      m    e    y                  Prints 1          System out println  indexOf   e   list                 private static int indexOf  char c   char    arr             for  int i   0   i  lt  arr length   i                   if  arr i     c                      return i                                    return -1              Bear in mind that    e    is an string object literal   which represents an string object that is     While    e   Is a character literal   which represents a character primitive datatype      Even when   list     Would be valid Java   which is not    comparing the a character element with a string element would return false anyway    Just use that indexOf string function and you could find any character within any alphabet   or array of characters

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This way you can use upto  N  letters   char   a    a    b    c    d    e     int indexOf 0  for int i 0 i lt a length i       if a i      b        indexOf         else      System out println  index of given    indexOf

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I am providing the proper method to do this one             Method to get the index of the given item from the list         param stringArray         param name         return index of the item if item exists else return -1             public static int getIndexOfItemInArray String   stringArray  String name            if  stringArray    null  amp  amp  stringArray length  gt  0                ArrayList lt String gt  list   new ArrayList lt String gt  Arrays asList stringArray                int index   list indexOf name               list clear                return index                    return -1

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Very Heavily Edited   I think either you want this   class CharStorage           set offset to 1 if you want b 1  o 2  y 3 instead of b 0           private final int offset 0      private int array   new int 26            Call this with up to 26 characters to initialize the array   For          instance  if you pass  boy  it will init to b 0 o 1 y 2               void init String s            for int i 0 i lt s length i                store s charAt i - a    offset i               void store char ch  int value            if ch  lt   a     ch  gt   z   throw new IllegalArgumentException            array ch- a   value             int get char ch            if ch  lt   a     ch  gt   z   throw new IllegalArgumentException            return array ch- a               Note that you may have to adjust the init method if you want to use 1-26 instead of 0-25   or you want this   int getLetterPossitionInAlphabet char c        return c -  a    1     The second is if you always want a 1  z 26   The first will let you put in a string like  qwerty  and assign q 0  w 1  e 2  r 3

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Alternatively  you can use Commons Lang ArrayUtils class   int   arr   new int 3  5  1  4  2   int indexOfTwo   ArrayUtils indexOf arr  2     There are overloaded variants of indexOf   method for different array types

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Simplest solution       Convert array to list and you can get position of an element    List lt String gt  abcd    Arrays asList yourArray   int i   abcd indexOf  abcd      Other solution  you can iterator array   int position   0  for  String obj   yourArray        if  obj equals  abcd         return position            position    1        OR for  int i   0  i  lt  yourArray length  i          if  obj equals  abcd            return i

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In this case  you could create e new String from your array of chars and then do an indeoxOf  e   on that String   System out println new String list  indexOf  e        But in other cases of primitive data types  you ll have to iterate over it

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The problem with your code is that when you do    list       e    you re asking if the array object  not the contents  is equal to the string  e   which is clearly not the case   You ll want to iterate over the contents in order to do the check you want    for String element   list          if  element equals  e                    do something here

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For primitive arrays  Starting with Java 8  the general purpose solution for a primitive array arr  and a value to search val  is   public static int indexOf char   arr  char val        return IntStream range 0  arr length  filter i - gt  arr i     val  findFirst   orElse -1       This code creates a stream over the indexes of the array with IntStream range  filters the indexes to keep only those where the array s element at that index is equal to the value searched and finally keeps the first one matched with findFirst  findFirst returns an OptionalInt  as it is possible that no matching indexes were found  So we invoke orElse -1  to either return the value found or -1 if none were found   Overloads can be added for int    long    etc  The body of the method will remain the same   For Object arrays  For object arrays  like String    we could use the same idea and have the filtering step using the equals method  or Objects equals to consider two null elements equal  instead of      But we can do it in a simpler manner with   public static  lt T gt  int indexOf T   arr  T val        return Arrays asList arr  indexOf val       This creates a list wrapper for the input array using Arrays asList and searches the index of the element with indexOf   This solution does not work for primitive arrays  as shown here  a primitive array like int   is not an Object   but an Object  as such  invoking asList on it creates a list of a single element  which is the given array  not a list of the elements of the array

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This way should work  change  char  to  Character    public static void main String   args     Character   list     m    e    y      System out println Arrays asList list  indexOf  e        print  1

User · Answer

That s not even valid syntax  And you re trying to compare to a string  For arrays you would have to walk the array yourself   public class T     public static void main  String args           char   list     m    e    y         int index   -1       for  int i   0   i  lt  list length   amp  amp   index    -1   i              if  list i      e                 index   i                       System out println index           If you are using a collection  such as ArrayList lt Character gt  you can also use the indexOf   method   ArrayList lt Character gt  list   new ArrayList lt Character gt     list add  m    list add  e    list add  y     System out println list indexOf  e       There is also the Arrays class which shortens above code   List list   Arrays asList new Character      m    e    y      System out println list indexOf  e

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I believe the only sanest way to do this is to manually iterate through the array   for  int i   0  i  lt  list length  i        if  list i      e         System out println i       break

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If the initial order of elements isn t really important  you could just sort the array  then binarySearch it   import java util Arrays   class masi        public static void main  String   args              char   list     m    e    y            Arrays sort list               should print 0  as e is now sorted to the beginning            returns negative number if the result isn t found         System out println  Arrays binarySearch list   e

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here is another example and how indexOf is working   public int indexOf Object target        for  int i   0  i  lt  array length  i              if  equals target  array i                  return i                      return -1

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int i   0  int count   1          while   e    list i                  i                count                       System out println count

[java] How to find the index of an element in an array in Java?

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