C find highest array value and index

Question

So I have an unsorted numeric array int   anArray     1  5  2  7    and I need to get both the value and the index of the largest value in the array which would be 7 and 3  how would I do this

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Here is a LINQ solution which is O n  with decent constant factors   int   anArray     1  5  2  7  1     int index   0  int maxIndex   0   var max   anArray Aggregate       oldMax  element    gt              index          if  element  lt   oldMax              return oldMax          maxIndex   index          return element            Console WriteLine  max    0   maxIndex    1    max  maxIndex     But you should really write an explicit for lop if you care about performance

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public static class ArrayExtensions       public static int MaxIndexOf lt T gt  this T   input                var max   input Max            int index   Array IndexOf input  max           return index            This works for all variable types     var array   new int   1  2  4  10  0  2   var index   array MaxIndexOf      var array   new double   1 0  2 0  4 0  10 0  0 0  2 0   var index   array MaxIndexOf

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Here are two approaches  You may want to add handling for when the array is empty   public static void FindMax            Advantages            Functional approach          Compact code        Cons            We are indexing into the array twice at each step          The Range and IEnumerable add a bit of overhead          Many people will find this code harder to understand      int   array     1  5  2  7         int maxIndex   Enumerable Range 0  array Length  Aggregate  max  i    gt  array max   gt  array i    max   i       int maxInt   array maxIndex        Console WriteLine   Maximum int  maxInt  is found at index  maxIndex        public static void FindMax2            Advantages            Near-optimal performance      int   array     1  5  2  7        int maxIndex   -1      int maxInt   Int32 MinValue          Modern C  compilers optimize the case where we put array Length in the condition     for  int i   0  i  lt  array Length  i                  int value   array i           if  value  gt  maxInt                        maxInt   value              maxIndex   i                       Console WriteLine   Maximum int  maxInt  is found at index  maxIndex

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Another answer in this long list  but I think it s worth it  because it provides some benefits that most  or all   other answers don t   The method below loops only once through the collection  therefore the order is O N   The method finds ALL indices of the maximum values  The method can be used to find the indices of any comparison  min  max  equals  not equals  etc  The method can look into objects via a LINQ selector   Method     -------------------------------------------------------------------      lt summary gt      Get the indices of all values that meet the condition that is defined by the comparer       lt  summary gt       lt typeparam name  quot TSource quot  gt The type of the values in the source collection  lt  typeparam gt       lt typeparam name  quot TCompare quot  gt The type of the values that are compared  lt  typeparam gt       lt param name  quot i collection quot  gt The collection of values that is analysed  lt  param gt       lt param name  quot i selector quot  gt The selector to retrieve the compare-values from the source-values  lt  param gt       lt param name  quot i comparer quot  gt The comparer that is used to compare the values of the collection  lt  param gt       lt returns gt The indices of all values that meet the condition that is defined by the comparer  lt  returns gt      Create  lt see cref  quot IComparer T  quot   gt  from comparison function        Comparer T  Create   comparison       Comparison examples      - max        a  b    gt  a CompareTo  b      - min        a  b    gt  - a CompareTo  b       -    x       a  b    gt  a    4   0   -1     -    x       a  b    gt  a    4   0   -1    ------------------------------------------------------------------- public static IEnumerable lt int gt  GetIndices lt TSource  TCompare gt   this IEnumerable lt TSource gt  i collection                                                                     Func lt TSource  TCompare gt  i selector                                                                     IComparer lt TCompare gt  i comparer      if  i collection    null      throw new ArgumentNullException  nameof  i collection      if   i collection Any         return new int 0      int index   0    var indices   new List lt int gt        TCompare reference   i selector  i collection First         foreach  var value in i collection          var compare   i selector  value       int result   i comparer Compare  compare  reference       if  result  gt  0              reference   compare        indices Clear           indices Add  index             else if  result    0        indices Add  index        index           return indices     If you don t need the selector  then change the method to public static IEnumerable lt int gt  GetIndices lt TCompare gt   this IEnumerable lt TCompare gt  i collection                                                            IComparer lt TCompare gt  i comparer   and remove all occurences of i selector  Proof of concept               test  1  int array            int   test     1  5  4  9  2  7  4  6  5  9  4        get indices of maximum  var indices   test GetIndices  t   gt  t  Comparer lt int gt  Create   a  b    gt  a CompareTo  b        indices    3  9       get indices of all  4   indices   test GetIndices  t   gt  t  Comparer lt int gt  Create   a  b    gt  a    4   0   -1       indices    2  6  10       get indices of all except  4   indices   test GetIndices  t   gt  t  Comparer lt int gt  Create   a  b    gt  a    4   0   -1       indices    0  1  3  4  5  7  8  9       get indices of all  15   indices   test GetIndices  t   gt  t  Comparer lt int gt  Create   a  b    gt  a    15   0   -1       indices                    test  2  named tuple array            var datas   new  object anything  double score         999                0 1      new object         0 42       quot hello quot             0 3      new Exception      0 16       quot abcde quot             0 42        get indices of highest score  indices   datas GetIndices  data   gt  data score  Comparer lt double gt  Create   a  b    gt  a CompareTo  b        indices    1  4    Enjoy   -

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This is not the most glamorous way but works    must have using System Linq     int maxValue   anArray Max     int maxIndex   anArray ToList   IndexOf maxValue

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public static void Main         int a b 0      int   arr  1  2  2  3  3  4  5  6  5  7  7  7  100  8  1        for int i arr Length-1   i gt -1   i--                        a   arr i                if a  gt  b                                b a                                  Console WriteLine b

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The obligatory LINQ one 1 -liner   var max   anArray Select  value  index    gt  new  value  index                     OrderByDescending vi   gt  vi value                    First       The sorting is probably a performance hit over the other solutions     1   For given values of  one

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If the index is not sorted  you have to iterate through the array at least once to find the highest value   I d use a simple for loop   int  maxVal   null    nullable so this works even if you have all super-low negatives int index   -1  for  int i   0  i  lt  anArray Length  i        int thisNum   anArray i     if   maxVal HasValue    thisNum  gt  maxVal Value          maxVal   thisNum      index   i          This is more verbose than something using LINQ or other one-line solutions  but it s probably a little faster   There s really no way to make this faster than O N

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If you know max index accessing the max value is immediate  So all you need is max index   int max 0   for int i   1  i  lt  arr Length  i        if  arr i   gt  arr max   max   i

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Output for bellow code   00 00 00 3279270 - max1 00 00 00 2615935 - max2 00 00 00 6010360 - max3  arr Max     With 100000000 ints in array not very big difference but still     class Program               static void Main string   args                        int   arr   new int 100000000                Random randNum   new Random                for  int i   0  i  lt  arr Length  i                                  arr i    randNum Next -100000000  100000000                             Stopwatch stopwatch1   new Stopwatch                Stopwatch stopwatch2   new Stopwatch                Stopwatch stopwatch3   new Stopwatch                stopwatch1 Start                 var max   GetMaxFullIterate arr                Debug WriteLine  stopwatch1 Elapsed ToString                   stopwatch2 Start                var max2   GetMaxPartialIterate arr                Debug WriteLine  stopwatch2 Elapsed ToString                  stopwatch3 Start                var max3   arr Max                Debug WriteLine stopwatch3 Elapsed ToString                    private static int GetMaxPartialIterate int   arr                        var max   arr 0               var idx   0              for  int i   arr Length   2  i  lt  arr Length  i                                  if  arr i   gt  max                                        max   arr i                                      if  arr idx   gt  max                                        max   arr idx                                     idx                              return max                      private static int GetMaxFullIterate int   arr                        var max   arr 0               for  int i   0  i  lt  arr Length  i                                  if  arr i   gt  max                                        max   arr i                                               return max

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A succinct one-liner  var max   anArray Select  n  i    gt   Number  n  Index  i   Max     Test case  var anArray   new int     1  5  7  4  2    var max   anArray Select  n  i    gt   Number  n  Index  i   Max    Console WriteLine   quot Maximum number    max Number   on index  max Index   quot       Maximum number   7  on index 2   Features   Uses Linq  not as optimized as vanilla  but the trade-off is less code   Does not need to sort  Computational complexity  O n   Space complexity  O n    Remarks   Make sure the number  and not the index  is the first element in the tuple because tuple sorting is done by comparing tuple items from left to right

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This is a C  Version  It s based on the idea of sort the array   public int solution int   A            write your code in C  6 0 with  NET 4 5  Mono      Array Sort A       var max   A Max        if max  lt  0          return 1      else         for  int i   1  i  lt  max  i                          if  A Contains i                     return i                              return max   1

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Consider following           lt summary gt          Returns max value          lt  summary gt           lt param name  quot arr quot  gt array to search in lt  param gt           lt param name  quot index quot  gt index of the max value lt  param gt           lt returns gt max value lt  returns gt      public static int MaxAt int   arr  out int index                index   -1          int max   Int32 MinValue           for  int i   0  i  lt  arr Length  i                          if  arr i   gt  max                                 max   arr i                   index   i                                   return max         Usage  int m  at  m   MaxAt new int   1 2 7 3 4 5 6   out at   Console WriteLine  quot Max   0   found at   1  quot   m  at

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anArray Select  n  i    gt  new   Value   n  Index   i         Where s   gt  s Value    anArray Max

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This can be done with a bodiless for loop  if we re heading towards golf      a is the array   int mi   a Length - 1  for  int i -1    i lt a Length-1  mi a mi  lt a i  i mi     The check of   i lt a Length-1 omits checking the last index  We don t mind this if we set it up as if the max index is the last index to start with   When the loop runs for the other elements it will finish and one or the other thing is true   we found a new max value and hence a new max index mi the last index was the max value all along  so we didn t find a new mi  and we stuck with the initial mi  The real work is done by the post-loop modifiers   is the max value  a mi  i e  array indexed by mi  we found so far  less than the current item   yes  then store a new mi by remembering i  no then store the existing mi  no-op     At the end of the operation you have the index at which the max is to be found  Logically then the max value is a mi  I couldn t quite see how the  quot find max and index of max quot  really needed to track the max value too  given that if you have an array  and you know the index of the max value  the actual value of the max value is a trivial case of using the index to index the array

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int   numbers   new int 7  45 67 23 45 19 85 64    int smallest   numbers 0    for  int index   0  index  lt  numbers Length  index         if  numbers index   lt  smallest  smallest   numbers index       Console WriteLine smallest

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int   anArray     1  5  2  7       Finding max int m   anArray Max        Positioning max int p   Array IndexOf anArray  m

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Finds the biggest and the smallest number in the array   int   arr   new int    35 28 20 89 63 45 12   int big   0  int little   0   for  int i   0  i  lt  arr Length  i          Console WriteLine arr i         if  arr i   gt  arr 0                 big   arr i             else               little   arr i             Console WriteLine  most big number inside of array is     big   Console WriteLine  most little number inside of array is     little

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Just another perspective using DataTable  Declare a DataTable with 2 columns called index and val  Add an AutoIncrement option and both AutoIncrementSeed and AutoIncrementStep values 1 to the index column  Then use a foreach loop and insert each array item into the datatable as a row  Then by using Select method  select the row having the maximum value   Code  int   anArray     1  5  2  7    DataTable dt   new DataTable    dt Columns AddRange new DataColumn 2    new DataColumn  index    new DataColumn  val      dt Columns  index   AutoIncrement   true  dt Columns  index   AutoIncrementSeed   1  dt Columns  index   AutoIncrementStep   1  foreach int i in anArray      dt Rows Add null  i    DataRow   dr   dt Select   val    MAX  val      Console WriteLine  Max Value    0   Index    1    dr 0  1   dr 0  0      Output  Max Value   7  Index   4   Find a demo here

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int   Data    1  212  333 2 12 3311 122 23    int large   Data Max    Console WriteLine large

[c#] C# find highest array value and index

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