How do I concatenate two arrays in C

Question

int   x   new int      1  2  3   int   y   new int      4  5     int   z      your answer here     Debug Assert z SequenceEqual new int     1  2  3  4  5          Right now I use   int   z   x Concat y  ToArray      Is there an easier or more efficient method

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You can take the ToArray   call off the end  Is there a reason you need it to be an array after the call to Concat   Calling Concat creates an iterator over both arrays  It does not create a new array so you have not used more memory for a new array  When you call ToArray you actually do create a new array and take up the memory for the new array   So if you just need to easily iterate over both then just call Concat

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The most efficient structure in terms of RAM  and CPU  to hold the combined array would be a special class that implements IEnumerable  or if you wish even derives from Array  and links internally to the original arrays to read the values  AFAIK Concat does just that   In your sample code you could omit the  ToArray   though  which would make it more efficient

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Be careful with the Concat method  The post Array Concatenation in C  explains that   var z   x Concat y  ToArray      Will be inefficient for large arrays  That means the Concat method is only for meduim-sized arrays  up to 10000 elements

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For smaller arrays  lt 10000 elements   using System Linq   int firstArray    5 4 2   int secondArray    3 2 1    int   result   firstArray ToList   Concat secondArray ToList    toArray

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You could write an extension method   public static T   Concat lt T gt  this T   x  T   y        if  x    null  throw new ArgumentNullException  x        if  y    null  throw new ArgumentNullException  y        int oldLen   x Length      Array Resize lt T gt  ref x  x Length   y Length       Array Copy y  0  x  oldLen  y Length       return x      Then   int   x    1 2 3   y    4 5   int   z   x Concat y       1 2 3 4 5

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I settled on a more general-purpose solution that allows concatenating an arbitrary set of one-dimensional arrays of the same type   I was concatenating 3  at a time    My function       public static T   ConcatArrays lt T gt  params T     list                var result   new T list Sum a   gt  a Length            int offset   0          for  int x   0  x  lt  list Length  x                          list x  CopyTo result  offset               offset    list x  Length                    return result          And usage           int   a   new int     1  2  3            int   b   new int     4  5  6            int   c   new int     7  8            var y   ConcatArrays a  b  c     Results in int    1 2 3 4 5 6 7 8

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What you need to remember is that when using LINQ you are utilizing delayed execution  The other methods described here all work perfectly  but they are executed immediately  Furthermore the Concat   function is probably optimized in ways you can t do yourself  calls to internal API s  OS calls etc    Anyway  unless you really need to try and optimize  you re currently on your path to  the root of all evil

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public static T   Concat lt T gt  this T   first  params T     arrays        int length   first Length      foreach  T   array in arrays                length    array Length            T   result   new T length       length   first Length      Array Copy first  0  result  0  first Length       foreach  T   array in arrays                Array Copy array  0  result  length  array Length           length    array Length            return result

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Try this   List lt int gt  list   new List lt int gt     list AddRange x   list AddRange y   int   z   list ToArray

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You can do it the way you have referred to  or if you want to get really manual about it  you can roll your own loop  string   one   new string      quot a quot    quot b quot     string   two   new string      quot c quot    quot d quot     string   three   three   new string one Length   two Length    int idx   0   for  int i   0  i  lt  one Length  i        three idx      one i   for  int j   0  j  lt  two Length  j        three idx      two j

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Sorry to revive an old thread  but how about this   static IEnumerable lt T gt  Merge lt T gt  params T     arrays        var merged   arrays SelectMany arr   gt  arr        foreach  var t in merged          yield return t      Then in your code   int   x  1  2  3   int   y  4  5  6    var z Merge x  y        z  is IEnumerable lt T gt   var za z ToArray        za  is int     Until you call  ToArray     ToList    or  ToDictionary       the memory is not allocated  you are free to  build your query  and either call one of those three to execute it or simply go through them all by using foreach  var i in z       clause which returns an item at a time from the yield return t  above     The above function can be made into an extension as follows   static IEnumerable lt T gt  Merge lt T gt  this T   array1  T   array2        var merged   array1 Concat array2        foreach  var t in merged          yield return t      So in the code  you can do something like   int   x1  1  2  3   int   x2  4  5  6   int   x3  7  8    var z x1 Merge x2  Merge x3         z  is IEnumerable lt T gt   var za z ToArray        za  is int     The rest is the same as before   One other improvement to this would be changing T   into IEnumerable lt T gt   so the params T     would become params IEnumerable lt T gt     to make these functions accept more than just arrays   Hope this helps

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int   x   new int      1  2  3   int   y   new int      4  5     int   z   x Union y  ToArray

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var z   new int x Length   y Length   x CopyTo z  0   y CopyTo z  x Length

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Try the following   T   r1   new T size1   T   r2   new T size2    List lt T gt  targetList   new List lt T gt  r1   targetList Concat r2   T   targetArray   targetList ToArray

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Here s my answer   int   z   new List lt string gt         Concat a       Concat b       Concat c       ToArray      This method can be used at initialization level  for example to define a static concatenation of static arrays   public static int   a   new int      1  2  3  4  5    public static int   b   new int      6  7  8    public static int   c   new int      9  10     public static int   z   new List lt string gt         Concat a       Concat b       Concat c       ToArray      However  it comes with two caveats that you need to consider    The Concat  method creates an iterator over both arrays  it does not create a new array  thus being efficient in terms of memory used  however  the subsequent  ToArray   will negate such advantage  since it will actually create a new array and take up the memory for the new array  As  Jodrell said  Concat would be rather inefficient for large arrays  it should only be used  for medium-sized arrays    If aiming for performance is a must  the following method can be used instead        lt summary gt      Concatenates two or more arrays into a single one       lt  summary gt  public static T   Concat lt T gt  params T     arrays           return  from array in arrays from arr in array select arr  ToArray         var result   new T arrays Sum a   gt  a Length        int offset   0      for  int x   0  x  lt  arrays Length  x                  arrays x  CopyTo result  offset           offset    arrays x  Length            return result      Or  for one-liners fans    int   z    from arrays in new     a  b  c   from arr in arrays select arr  ToArray      Although the latter method is much more elegant  the former one is definitely better for performance   For additional info  please refer to this post on my blog

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int   scores     100  90  90  80  75  60    int   alice     50  65  77  90  102    int   scoreBoard   new int scores Length   alice Length    int j   0  for  int i 0 i lt  scores Length alice Length  i        to combine two arrays       if i lt scores Length                scoreBoard i    scores i             else               scoreBoard i    alice j           j   j   1             for  int l   0  l  lt   scores Length   alice Length   l          Console WriteLine scoreBoard l

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I know the OP was only mildly curious about performance  That larger arrays may get a different result  see  kurdishTree   And that it usually does not matter   jordan peoples   None the less  I was curious and therefore lost my mind   as  TigerShark was explaining      I mean that I wrote a simple test based on the original question     and all the answers      using System  using System Collections Generic  using System Linq  using System Text  using System Threading Tasks   namespace concat       class Program               static void Main string   args                        int   x   new int      1  2  3               int   y   new int      4  5                  int itter   50000              Console WriteLine  test iterations   0    itter                DateTime startTest   DateTime Now              for int  i   0  i  lt  itter  i                                  int   z                  z   x Concat y  ToArray                              Console WriteLine   Concat Test Time in ticks   0     DateTime Now - startTest  Ticks                 startTest   DateTime Now              for int  i   0  i  lt  itter  i                                  var vz   new int x Length   y Length                   x CopyTo vz  0                   y CopyTo vz  x Length                             Console WriteLine   CopyTo Test Time in ticks   0     DateTime Now - startTest  Ticks                 startTest   DateTime Now              for int  i   0  i  lt  itter  i                                  List lt int gt  list   new List lt int gt                     list AddRange x                   list AddRange y                   int   z   list ToArray                              Console WriteLine  list AddRange Test Time in ticks   0     DateTime Now - startTest  Ticks                startTest   DateTime Now              for  int i   0  i  lt  itter  i                                  int   z   Methods Concat x  y                             Console WriteLine  Concat x  y  Test Time in ticks   0     DateTime Now - startTest  Ticks                startTest   DateTime Now              for  int i   0  i  lt  itter  i                                  int   z   Methods ConcatArrays x  y                             Console WriteLine  ConcatArrays Test Time in ticks   0     DateTime Now - startTest  Ticks                startTest   DateTime Now              for  int i   0  i  lt  itter  i                                  int   z   Methods SSConcat x  y                             Console WriteLine  SSConcat Test Time in ticks   0     DateTime Now - startTest  Ticks                startTest   DateTime Now              for  int k   0  k  lt  itter  k                                  int   three   new int x Length   y Length                    int idx   0                   for  int i   0  i  lt  x Length  i                        three idx      x i                   for  int j   0  j  lt  y Length  j                        three idx      y j                             Console WriteLine  Roll your own Test Time in ticks   0     DateTime Now - startTest  Ticks                 startTest   DateTime Now              for  int i   0  i  lt  itter  i                                  int   z   Methods ConcatArraysLinq x  y                             Console WriteLine  ConcatArraysLinq Test Time in ticks   0     DateTime Now - startTest  Ticks                startTest   DateTime Now              for  int i   0  i  lt  itter  i                                  int   z   Methods ConcatArraysLambda x  y                             Console WriteLine  ConcatArraysLambda Test Time in ticks   0     DateTime Now - startTest  Ticks                startTest   DateTime Now              for  int i   0  i  lt  itter  i                                  List lt int gt  targetList   new List lt int gt  x                   targetList Concat y                             Console WriteLine  targetList Concat y  Test Time in ticks   0     DateTime Now - startTest  Ticks                startTest   DateTime Now              for  int i   0  i  lt  itter  i                                  int   result   x ToList   Concat y ToList    ToArray                              Console WriteLine  x ToList   Concat y ToList    ToArray   Test Time in ticks   0     DateTime Now - startTest  Ticks                       static class Methods               public static T   Concat lt T gt  this T   x  T   y                        if  x    null  throw new ArgumentNullException  x                if  y    null  throw new ArgumentNullException  y                int oldLen   x Length              Array Resize lt T gt  ref x  x Length   y Length               Array Copy y  0  x  oldLen  y Length               return x                     public static T   ConcatArrays lt T gt  params T     list                        var result   new T list Sum a   gt  a Length                int offset   0              for  int x   0  x  lt  list Length  x                                  list x  CopyTo result  offset                   offset    list x  Length                            return result                      public static T   SSConcat lt T gt  this T   first  params T     arrays                        int length   first Length              foreach  T   array in arrays                                length    array Length                            T   result   new T length               length   first Length              Array Copy first  0  result  0  first Length               foreach  T   array in arrays                                Array Copy array  0  result  length  array Length                   length    array Length                            return result                     public static T   ConcatArraysLinq lt T gt  params T     arrays                        return  from array in arrays                     from arr in array                     select arr  ToArray                       public static T   ConcatArraysLambda lt T gt  params T     arrays                        return arrays SelectMany array   gt  array Select arr   gt  arr   ToArray                         The result was     Roll your own wins

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This is it   using System Linq   int   array1     1  3  5    int   array2     0  2  4        Concatenate array1 and array2  var result1   array1 Concat array2

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More efficient  faster  to use Buffer BlockCopy over Array CopyTo  int   x   new int      1  2  3   int   y   new int      4  5     int   z   new int x Length   y Length   var byteIndex   x Length   sizeof int   Buffer BlockCopy x  0  z  0  byteIndex   Buffer BlockCopy y  0  z  byteIndex  y Length   sizeof int      I wrote a simple test program that  quot warms up the Jitter quot   compiled in release mode and ran it without a debugger attached  on my machine  For 10 000 000 iterations of the example in the question  Concat took 3088ms CopyTo took 1079ms BlockCopy took 603ms  If I alter the test arrays to two sequences from 0 to 99 then I get results similar to this   Concat took 45945ms CopyTo took 2230ms BlockCopy took 1689ms  From these results I can assert that the CopyTo and BlockCopy methods are significantly more efficient than Concat and furthermore  if performance is a goal  BlockCopy has value over CopyTo  To caveat this answer  if performance doesn t matter  or there will be few iterations choose the method you find easiest  Buffer BlockCopy does offer some utility for type conversion beyond the scope of this question

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I ve found an elegant one line solution using LINQ or Lambda expression  both work the same  LINQ is converted to Lambda when program is compiled   The solution works for any array type and for any number of arrays   Using LINQ   public static T   ConcatArraysLinq lt T gt  params T     arrays        return  from array in arrays             from arr in array             select arr  ToArray        Using Lambda   public static T   ConcatArraysLambda lt T gt  params T     arrays        return arrays SelectMany array   gt  array Select arr   gt  arr   ToArray        I ve provided both for one s preference  Performance wise  Sergey Shteyn s or  deepee1 s solutions are a bit faster  Lambda expression being the slowest  Time taken is dependant on type s  of array elements  but unless there are millions of calls  there is no significant difference between the methods

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Late Answer  -    public static class ArrayExtention                public static T   Concatenate lt T gt  this T   array1  T   array2                        T   result   new T array1 Length   array2 Length               array1 CopyTo result  0               array2 CopyTo result  array1 Length               return result

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static class Extensions       public static T   Concat lt T gt  this T   array1  params T   array2    gt  ConcatArray array1  array2        public static T   ConcatArray lt T gt  params T     arrays                int l  i           for  l   i   0  i  lt  arrays Length  l    arrays i  Length  i              var a   new T l            for  l   i   0  i  lt  arrays Length  l    arrays i  Length  i                arrays i  CopyTo a  l            return a            I think the above solution is more general  amp  lighter than the others I saw here  It is more general because it doesn t limit concatenation for only two arrays and is lighter because it doesn t use LINQ nor List   Note the solution is concise and the added generality doesn t add significant runtime overhead

User · Answer

For int   what you ve done looks good to me  astander s answer would also work well for List lt int gt

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