How to populate instantiate a C array with a single value

Question

I know that instantiated arrays of value types in C  are automatically populated with the default value of the type  e g  false for bool  0 for int  etc     Is there a way to auto-populate an array with a seed value that s not the default  Either on creation or a built-in method afterwards  like Java s Arrays fill     Say I wanted an boolean array that was true by default  instead of false   Is there a built-in way to do this  or do you just have to iterate through the array with a for loop       Example pseudo-code   bool   abValues   new 1000000    Array Populate abValues  true        Currently how I m handling this   bool   abValues   new 1000000    for  int i   0  i  lt  1000000  i            abValues i    true       Having to iterate through the array and  reset  each value to true seems ineffecient   Is there anyway around this   Maybe by flipping all values   After typing this question out and thinking about it  I m guessing that the default values are simply a result of how C  handles the memory allocation of these objects behind the scenes  so I imagine it s probably not possible to do this   But I d still like to know for sure

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Well after a little more googling and reading I found this:

bool[] bPrimes = new bool[1000000];
bPrimes = Array.ConvertAll<bool, bool>(bPrimes, b=> b=true);

Which is certainly closer to what I'm looking for. But I'm not sure if that's better than iterating through the original array in a for-loop and just changing the values. After a quick test in fact, it appears slower by about a factor of 5. So not really a good solution then!

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If you re planning to only set a few of the values in the array  but want to get the  custom  default value most of the time  you could try something like this   public class SparseArray lt T gt        private Dictionary lt int  T gt  values   new Dictionary lt int  T gt          private T defaultValue       public SparseArray T defaultValue                this defaultValue   defaultValue             public T this  int index              set   values index    value          get   return values ContainsKey index    values index    defaultValue              You ll probably need to implement other interfaces to make it useful  such as those on array itself

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The code below combines simple iteration for small copies and Array Copy for large copies      public static void Populate lt T gt   T   array  int startIndex  int count  T value             if   array    null                 throw new ArgumentNullException   array                       if    uint startIndex  gt   array Length                 throw new ArgumentOutOfRangeException   startIndex                           if   count  lt  0       uint   startIndex   count    gt  array Length                   throw new ArgumentOutOfRangeException   count                           const int Gap   16          int i   startIndex           if   count  lt   Gap   2                 while   count  gt  0                     array  i     value                  count--                  i                              return                    int aval   Gap          count -  Gap           do               array  i     value              i                --aval            while   aval  gt  0             aval   Gap          while   true                 Array Copy  array  startIndex  array  i  aval                i    aval              count -  aval              aval    2              if   count  lt   aval                     Array Copy  array  startIndex  array  i  count                    break                                  The benchmarks for different array length using an int   array are             2 Iterate      1981 Populate      2845          4 Iterate      2678 Populate      3915          8 Iterate      4026 Populate      6592         16 Iterate      6825 Populate     10269         32 Iterate     16766 Populate     18786         64 Iterate     27120 Populate     35187        128 Iterate     49769 Populate     53133        256 Iterate    100099 Populate     71709        512 Iterate    184722 Populate    107933       1024 Iterate    363727 Populate    126389       2048 Iterate    710963 Populate    220152       4096 Iterate   1419732 Populate    291860       8192 Iterate   2854372 Populate    685834      16384 Iterate   5703108 Populate   1444185      32768 Iterate  11396999 Populate   3210109   The first columns is the array size  followed by the time of copying using a simple iteration    JaredPared implementation    The time of this method is after that  These are the benchmarks using an array of a struct of four integers           2 Iterate      2473 Populate      4589          4 Iterate      3966 Populate      6081          8 Iterate      7326 Populate      9050         16 Iterate     14606 Populate     16114         32 Iterate     29170 Populate     31473         64 Iterate     57117 Populate     52079        128 Iterate    112927 Populate     75503        256 Iterate    226767 Populate    133276        512 Iterate    447424 Populate    165912       1024 Iterate    890158 Populate    367087       2048 Iterate   1786918 Populate    492909       4096 Iterate   3570919 Populate   1623861       8192 Iterate   7136554 Populate   2857678      16384 Iterate  14258354 Populate   6437759      32768 Iterate  28351852 Populate  12843259

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Here s another version for us Framework users abandoned by Microsoft  It is 4 times as fast as Array Clear and faster than Panos Theof s solution and Eric J s and Petar Petrov s parallel one - up to two times as fast for large arrays   First I want to present you the function s ancestor  because that makes it easier to understand the code  Performance-wise this is pretty much on par with Panos Theof s code  and for some things that may already be enough   public static void Fill lt T gt   T   array  int count  T value  int threshold   32        if  threshold  lt   0          throw new ArgumentException  threshold         int current size   0  keep looping up to   Math Min count  threshold        while  current size  lt  keep looping up to          array current size      value       for  int at least half    count   1   gt  gt  1  current size  lt  at least half  current size  lt  lt   1          Array Copy array  0  array  current size  current size        Array Copy array  0  array  current size  count - current size       As you can see  this is based on repeated doubling of the already-initialised part  This is simple and efficient  but it runs afoul of modern memory architectures  Hence was born a version that uses doubling only to create a cache-friendly seed block  which is then blasted iteratively over the target area   const int ARRAY COPY THRESHOLD   32      16     64 work equally well for all tested constellations const int L1 CACHE SIZE   1  lt  lt  15   public static void Fill lt T gt   T   array  int count  T value  int element size        int current size   0  keep looping up to   Math Min count  ARRAY COPY THRESHOLD        while  current size  lt  keep looping up to          array current size      value       int block size   L1 CACHE SIZE   element size   2      int keep doubling up to   Math Min block size  count  gt  gt  1        for     current size  lt  keep doubling up to  current size  lt  lt   1          Array Copy array  0  array  current size  current size        for  int enough   count - block size  current size  lt  enough  current size    block size          Array Copy array  0  array  current size  block size        Array Copy array  0  array  current size  count - current size       Note  the earlier code needed  count   1   gt  gt  1 as limit for the doubling loop to ensure that the final copy operation has enough fodder to cover all that is left  This would not be the case for odd counts if count  gt  gt  1 were to be used instead  For the current version this is of no significance since the linear copy loop will pick up any slack   The size of an array cell must be passed as a parameter because - the mind boggles - generics aren t allowed to use sizeof unless they use a constraint  unmanaged  that may or may not become available in the future  Wrong estimates are not a big deal but performance is best if the value is accurate  for the following reasons    Underestimating the element size can lead to block sizes greater than half the L1 cache  hence increasing the likelihood of copy source data getting evicted from L1 and having to be re-fetched from slower cache levels  Overestimating the element size results in under-utilisation of the CPU s L1 cache  meaning the linear block copy loop gets executed more often than it would be with optimal utilisation  Thus  more of the fixed loop call overhead is incurred than strictly necessary    Here s a benchmark pitting my code against Array Clear and the other three solutions mentioned previously  The timings are for filling integer arrays  Int32    of the given sizes  In order to reduce variation caused by cache vagaries etc  each test was executed twice  back to back  and the timings were taken for the second execution   array size   Array Clear      Eric J    Panos Theof  Petar Petrov   Darth Gizka -------------------------------------------------------------------------------      1000        0 7   s        0 2   s        0 2   s        6 8   s       0 2   s      10000        8 0   s        1 4   s        1 2   s        7 8   s       0 9   s     100000       72 4   s       12 4   s        8 2   s       33 6   s       7 5   s    1000000      652 9   s      135 8   s      101 6   s      197 7   s      71 6   s   10000000     7182 6   s     4174 9   s     5193 3   s     3691 5   s    1658 1   s  100000000    67142 3   s    44853 3   s    51372 5   s    35195 5   s   16585 1   s    Should the performance of this code not be sufficient then a promising avenue would be parallelising the linear copy loop  with all threads using the same source block   or our good old friend P Invoke   Note  clearing and filling of blocks is normally done by runtime routines that branch to highly specialised code using MMX SSE instructions and whatnot  so in any decent environment one would simply call the respective moral equivalent of std  memset and be assured of professional performance levels  IOW  by rights the library function Array Clear should leave all our hand-rolled versions in the dust  The fact that it is the other way around shows how far out of whack things really are  Same goes for having to roll one s own Fill lt  gt  in the first place  because it is still only in Core and Standard but not in the Framework   NET has been around for almost twenty years now and we still have to P Invoke left and right for the most basic stuff or roll our own

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Many of the answers presented here boil down to a loop that initializes the array one element at a time  which does not take advantage of CPU instructions designed to operate on a block of memory at once    Net Standard 2 1  in preview as of this writing  provides Array Fill    which lends itself to a high-performance implementation in the runtime library  though as of now   NET Core doesn t seem to leverage that possibility    For those on earlier platforms  the following extension method outperforms a trivial loop by a substantial margin when the array size is significant  I created it when my solution for an online code challenge was around 20  over the allocated time budget  It reduced the runtime by around 70   In this case  the array fill was performed inside another loop  BLOCK SIZE was set by gut feeling rather than experiment  Some optimizations are possible  e g  copying all bytes already set to the desired value rather than a fixed-size block    internal const int BLOCK SIZE   256  public static void Fill lt T gt  this T   array  T value        if  array Length  lt  2   BLOCK SIZE                for  int i   0  i  lt  array Length  i    array i    value            else               int fullBlocks   array Length   BLOCK SIZE             Initialize first block         for  int j   0  j  lt  BLOCK SIZE  j    array j    value             Copy successive full blocks         for  int blk   1  blk  lt  fullBlocks  blk                          Array Copy array  0  array  blk   BLOCK SIZE  BLOCK SIZE                      for  int rem   fullBlocks   BLOCK SIZE  rem  lt  array Length  rem                          array rem    value

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Or    you could simply use inverted logic  Let false mean true and vice versa   Code sample     bool   isVisible   Enumerable Repeat true  1000000  ToArray    bool   isHidden   new bool 1000000      Crazy-fast initialization      if  isVisible All v   gt  v   if  isHidden All v   gt   v            Do stuff

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If you can invert your logic you can use the Array Clear   method to set the boolean array to false           int upperLimit   21          double optimizeMe   Math Sqrt upperLimit            bool   seiveContainer   new bool upperLimit           Array Clear seiveContainer  0  upperLimit

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Enumerable Repeat true  1000000  ToArray

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Create a new array with a thousand true values   var items   Enumerable Repeat lt bool gt  true  1000  ToArray        Or ToList    etc    Similarly  you can generate integer sequences   var items   Enumerable Range 0  1000  ToArray        0  999

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unfortunately I don t think there is a direct way  however I think you can write an extension method for the array class to do this  class Program       static void Main string   args                int   arr   new int 1000           arr Init 10           Array ForEach arr  Console WriteLine            public static class ArrayExtensions       public static void Init lt T gt  this T   array  T defaultVaue                if  array    null              return          for  int i   0  i  lt  array Length  i                          array i    defaultVaue

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I am a bit surprised noone has made the very simple  yet ultra fast  SIMD version    public static void PopulateSimd lt T gt  T   array  T value  where T   struct          var vector   new Vector lt T gt  value        var i   0       var s   Vector lt T gt  Count       var l   array Length  amp    s-1        for    i  lt  l  i    s  vector CopyTo array  i        for    i  lt  array Length  i    array i    value       Benchmark   Number are for Framework 4 8  but Core3 1 is statistically the same         Method         N             Mean            Error          StdDev   Ratio   RatioSD    -----------  --------  ---------------  ---------------  --------------  ------  --------     DarthGizka        10        25 975 ns        1 2430 ns       0 1924 ns    1 00      0 00           Simd        10         3 438 ns        0 4427 ns       0 0685 ns    0 13      0 00                                                                                                  DarthGizka       100        81 155 ns        3 8287 ns       0 2099 ns    1 00      0 00           Simd       100        12 178 ns        0 4547 ns       0 0704 ns    0 15      0 00                                                                                                  DarthGizka      1000       201 138 ns        8 9769 ns       1 3892 ns    1 00      0 00           Simd      1000       100 397 ns        4 0965 ns       0 6339 ns    0 50      0 00                                                                                                  DarthGizka     10000     1 292 660 ns       38 4965 ns       5 9574 ns    1 00      0 00           Simd     10000     1 272 819 ns       68 5148 ns      10 6027 ns    0 98      0 01                                                                                                  DarthGizka    100000    16 156 106 ns      366 1133 ns      56 6564 ns    1 00      0 00           Simd    100000    17 627 879 ns    1 589 7423 ns     246 0144 ns    1 09      0 02                                                                                                  DarthGizka   1000000   176 625 870 ns   32 235 9957 ns   1 766 9637 ns    1 00      0 00           Simd   1000000   186 812 920 ns   18 069 1517 ns   2 796 2212 ns    1 07      0 01    As can be seen  it is much faster at  lt 10000 elements  and only slightly slower beyond that

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There is no way to set all elements in an array as a single operation  UNLESS  that value is the element types default value    Eg  if it is an array of integers you can set them all to zero with a single operation  like so  Array Clear

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I realize I m late to the party but here s an idea  Write a wrapper which has conversion operators to and from the wrapped value so that it can be used as a stand-in for the wrapped type  This was actually inspired by the silly-sounding answer from  l33t   First  coming from C    I realized that in C  a default ctor is not called when the elements of an array are constructed  Instead -- even in the presence of a user-defined default constructor  -- all array elements are zero-initialized  That did surprise me   So a wrapper class which simply provides a default ctor with the desired value would work for arrays in C   but not in C   A workaround is to let the wrapper type map 0 to the desired seed value upon conversion  That way zero initialized values appear to be initialized with the seed for all practical purposes   public struct MyBool       private bool  invertedValue       public MyBool bool b                     invertedValue    b             public static implicit operator MyBool bool b                return new MyBool b              public static implicit operator bool MyBool mb                return  mb  invertedValue            static void Main string   args            MyBool mb   false     should expose false          Console Out WriteLine  false init gives false                                      mb            MyBool   fakeBoolArray   new MyBool 100            Console Out WriteLine  Default array elems are true                                     fakeBoolArray All b   gt  b              fakeBoolArray 21    false          Console Out WriteLine  Assigning false worked                                      fakeBoolArray 21             fakeBoolArray 21    true             Should define ToString   on a MyBool             hence the    to force bool         Console Out WriteLine  Assigning true again worked                                       fakeBoolArray 21        This pattern is applicable to all value types  One could for example map 0 to 4 for ints if initialization with 4 was desired etc   I d love to make a template of it as would be possible in C    providing the seed value as template parameter  but I understand that s not possible in C   Or am I missing something   Of course in C   mapping is not necessary at all because one can provide a default ctor which will be called for array elements    FWIW  here s a C   equivalent  https   ideone com wG8yEh

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Boolean   data   new Boolean 25    new Action lt Boolean   gt   p    gt    BitArray seed   new BitArray p Length  true   seed CopyTo p  0      Invoke data

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What about a parallel implementation  public static void InitializeArray lt T gt  T   array  T value        var cores   Environment ProcessorCount       ArraySegment lt T gt    segments   new ArraySegment lt T gt  cores        var step   array Length   cores      for  int i   0  i  lt  cores  i                  segments i    new ArraySegment lt T gt  array  i   step  step             var remaining   array Length   cores      if  remaining    0                var lastIndex   segments Length - 1          segments lastIndex    new ArraySegment lt T gt  array  lastIndex   step  array Length -  lastIndex   step               var initializers   new Task cores       for  int i   0  i  lt  cores  i                  var index   i          var t   new Task      gt                        var s   segments index               for  int j   0  j  lt  s Count  j                                  array j   s Offset    value                                    initializers i    t          t Start               Task WaitAll initializers       When only initializing an array the power of this code can t be seen but I think you should definitely forget about the  pure  for

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this also works   but might be unnecessary   bool   abValues   new bool 1000    abValues   abValues Select  n   gt  n   true   ToArray lt bool gt

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You can use Array Fill in  NET Core 2 0  and  NET Standard 2 1

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Just a benchmark  BenchmarkDotNet v0 12 1  OS Windows 10 0 18363 997  1909 November2018Update 19H2  Intel Core i7-6700HQ CPU 2 60GHz  Skylake   1 CPU  8 logical and 4 physical cores  NET Core SDK 3 1 302    Host            NET Core 3 1 6  CoreCLR 4 700 20 26901  CoreFX 4 700 20 31603   X64 RyuJIT    NET Core 3 1    NET Core 3 1 6  CoreCLR 4 700 20 26901  CoreFX 4 700 20 31603   X64 RyuJIT  Job  NET Core 3 1  Runtime  NET Core 3 1              Method       Mean       Error      StdDev    -----------------  ---------  ----------  ----------     EnumerableRepeat   2 311 us   0 0228 us   0 0213 us      NewArrayForEach   2 007 us   0 0392 us   0 0348 us            ArrayFill   2 426 us   0 0103 us   0 0092 us         SimpleJob BenchmarkDotNet Jobs RuntimeMoniker NetCoreApp31       public class InitializeArrayBenchmark           const int ArrayLength   1600            Benchmark          public double   EnumerableRepeat                 return Enumerable Repeat double PositiveInfinity  ArrayLength  ToArray                        Benchmark          public double   NewArrayForEach                 var array   new double ArrayLength                for  int i   0  i  lt  array Length  i                      array i    double PositiveInfinity                             return array                      Benchmark          public double   ArrayFill                 var array   new double ArrayLength               Array Fill array  double PositiveInfinity                return array

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There are some more answers on this  duplicate   question  What is the equivalent of memset in C    Someone has benchmarked the alternatives  they included an unsafe version  but they didn t try memset   http   techmikael blogspot co uk 2009 12 filling-array-with-default-value html

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Here is another appraoch with System Collections BitArray which has such a constructor   bool   result   new BitArray 1000000  true  Cast lt bool gt    ToArray      or   bool   result   new bool 1000000   new BitArray 1000000  true  CopyTo result  0

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If your array is so large you should use BitArray  It uses 1 bit for every bool instead of a byte  like in an array of bools  also you can set the all the bits to true with bit operators  Or just initialize on true  If you only need to do it once  it will only cost more though   System Collections BitArray falses   new System Collections BitArray 100000  false   System Collections BitArray trues   new System Collections BitArray 100000  true       Now both contain only true values  falses And trues

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Don t know of a framework method but you could write a quick helper to do it for you   public static void Populate lt T gt  this T   arr  T value       for   int i   0  i  lt  arr Length i           arr i    value

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Make a private class inside where you make the array and the have a getter and setter for it  Unless you need each position in the array to be something unique  like random  then use int  as an array and then on get if the position is equal null fill that position and return the new random value   IsVisibleHandler      private bool   b   new bool 10000      public bool GetIsVisible int x        return  b x         public void SetIsVisibleTrueAt int x        b x    false    true         Or use   public void SetIsVisibleAt int x  bool isTrue    b x     isTrue      As setter

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If you are on  NET Core   NET Standard    2 1  or depend on the System Memory package  you can also use the Span lt T gt  Fill   method   var valueToFill   165  var data   new int 100    data AsSpan   Fill valueToFill       print array content for  int i   0  i  lt  data Length  i          Console WriteLine data i        https   dotnetfiddle net UsJ9bu

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For large arrays or arrays that will be variable sized you should probably use   Enumerable Repeat true  1000000  ToArray      For small array you can use the collection initialization syntax in C  3   bool   vals   new bool    false  false  false  false  false  false  false      The benefit of the collection initialization syntax  is that you don t have to use the same value in each slot and you can use expressions or functions to initialize a slot  Also  I think you avoid the cost of initializing the array slot to the default value  So  for example   bool   vals   new bool    false  true  false    a   b   amp  amp  c  SomeBoolMethod

[c#] How to populate/instantiate a C# array with a single value?

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