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

Question

How can I find an index of a certain value in a Java array of type int   I tried using Arrays binarySearch on my unsorted array  it only sometimes gives the correct answer

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You can use modern Java to solve this problem  Please use the code below  static int findIndexOf int V  int   arr        return IntStream range 0  arr length                       filter i- gt arr i   V                       findFirst                        getAsInt

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Binary search  Binary search can also be used to find the index of the array element in an array  But the binary search can only be used if the array is sorted  Java provides us with an inbuilt function which can be found in the Arrays library of Java which will rreturn the index if the element is present  else it returns -1  The complexity will be O log n   Below is the implementation of Binary search   public static int findIndex int arr    int t        int index   Arrays binarySearch arr  t       return  index  lt  0    -1   index

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The easiest way is to iterate  For example we want to find the minimum value of array and it s index  public static Pair lt Integer  Integer gt  getMinimumAndIndex int   array            int min   array 0           int index   0          for  int i   1  i  lt  array length  i                  if  array i   lt  min                    min   array i                   index   i                             return new Pair lt min  index gt    This way you test all array values and if some of them is minimum you also know minimums index  It can work the same with searching some value  public static int indexOfNumber int   array            int index   0          for  int i   0  i  lt  array length  i                  if  array i     77              here you pass some value for example 77                 index   i                                  return index

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You could convert it to a list  then use the indexOf method   Array asList array  indexOf 1      http   download oracle com javase 1 5 0 docs api java util Arrays html asList T     http   download oracle com javase 1 5 0 docs api java util List html indexOf java lang Object

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Integer   array    1  2  3  4  5  6    for  int i   0  i  lt  array length  i          if  array i     4            system out println i           break

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Another option if you are using Guava Collections is Ints indexOf     Perfect storm  final int needle   42  final int   haystack    1  2  3  42       Spoiler alert  index    3 final int index   Ints indexOf haystack  needle     This is a great choice when space  time and code reuse are at a premium  It is also very terse

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Integer   arr     0  1  1  2  3  5  8  13  21        List lt Integer gt  arrlst   Arrays asList arr       System out println arrlst lastIndexOf 1

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Simple   public int getArrayIndex int   arr int value        for int i 0 i lt arr length i            if arr i   value  return i      return -1

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static int   getIndex int   data  int number        int   positions   new int data length       if  data length  gt  0            int counter   0          for int i  0  i  lt  data length  i                  if data i     number                   positions counter    i                  counter                                      return positions

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ArrayUtils indexOf array  value   Ints indexOf array  value   Arrays asList array  indexOf value

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Copy this method into your class    public int getArrayIndex int   arr int value             int k 0          for int i 0 i lt arr length i                  if arr i   value                   k i                  break                              return k      Call this method with pass two perameters Array and value and store its return value in a integer variable   int indexNum   getArrayIndex array value     Thank you

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You can either walk through the array until you find the index you re looking for  or use a List instead  Note that you can transform the array into a list with asList

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Integer   array    1 2 3 4 5 6    Arrays asList array  indexOf 4     Note that this solution is threadsafe because it creates a new object of type List    Also you don t want to invoke this in a loop or something like that since you would be creating a new object every time

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In the main method using for loops  -the third for loop in my example is the answer to this question  -in my example I made an array of 20 random integers  assigned a variable the smallest number  and stopped the loop when the location of the array reached the smallest value while counting the number of loops   import java util Random  public class scratch       public static void main String   args           Random rnd   new Random            int randomIntegers     new int 20           double smallest   randomIntegers 0           int location   0           for int i   0  i  lt  randomIntegers length  i                    fills array with random integers             randomIntegers i    rnd nextInt 99    1              System out println   --    i    --     randomIntegers i                       for  int i   0  i  lt  randomIntegers length  i                   get the location of smallest number in the array              if randomIntegers i   lt  smallest                   smallest   randomIntegers i                                                     for  int i   0  i  lt  randomIntegers length  i                                 if randomIntegers i     smallest                          break the loop when array location value     lt smallest gt                  break                            location                       System out println  location      location     nsmallest      smallest             Code outputs all the numbers and their locations  and the location of the smallest number followed by the smallest number

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You can do it like this    public class Test    public static int Tab       33 44 55 66 7 88 44 11 23 45 32 12 95   public static int search   23   public static void main String   args        long stop   0      long time   0      long start   0      start   System nanoTime        int index   getIndexOf search Tab       stop   System nanoTime        time   stop - start      System out println  equal to took in nano seconds    time       System out println  Index  of searched value is    index       System out println  De value of Tab with searched index is    Tab index        System out println                                                                    start   System nanoTime        int Bindex   bitSearch search Tab       stop   System nanoTime        time   stop - start      System out println  Binary search took nano seconds    time       System out println  Index  of searched value is    Bindex       System out println  De value of Tab with searched index is    Tab Bindex         public static int getIndexOf  int toSearch  int   tab         int i   0       while   tab i     toSearch            i             return i     or return tab i        public static int bitSearch int toSearch  int   tab       int i   0      for   toSearch tab i    0 i               return i          Added a XOR

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A look at the API and it says you have to sort the array first   So   Arrays sort array   Arrays binarySearch array  value     If you don t want to sort the array   public int find double   array  double value        for int i 0  i lt array length  i              if array i     value               return i

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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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You need to sort values before using binary search  Otherwise  the manual way is to try all ints in your tab   public int getIndexOf  int toSearch  int   tab       for  int i 0  i lt  tab length   i          if  tab  i      toSearch       return i     return -1     met   An alternative method could be to map all index for each value in a map    tab  index     value  if  map get  value     null    map get  value   gt  index       map put  value  index      and then map get value  to get the index   Regards  St  phane   pst  thanks for your comments  Can you post an other alternative method

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In case anyone is still looking for the answer-   You can use ArrayUtils indexOf   from the  Apache Commons Library  1   If you are using Java 8 you can also use the Strean API   public static int indexOf int   array  int valueToFind        if  array    null            return -1            return IntStream range 0  array length               filter i - gt  valueToFind    array i                findFirst                orElse -1        1   https   commons apache org proper commons-lang javadocs api-3 1 org apache commons lang3 ArrayUtils html indexOf int    20int

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

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