Fastest way to set all values of an array

Question

I have a char     and I want to set the value of every index to the same char value  There is the obvious way to do it  iteration      char f          char    c   new char  50     for int i   0  i  lt  c length  i           c i    f        But I was wondering if there s a way that I can utilize System arraycopy or something equivalent that would bypass the need to iterate  Is there a way to do that   EDIT   From Arrays java  public static void fill char   a  int fromIndex  int toIndex  char val            rangeCheck a length  fromIndex  toIndex           for  int i   fromIndex  i  lt  toIndex  i                a i    val          This is exactly the same process  which shows that there might not be a better way to do this   1 to everyone who suggested fill anyway - you re all correct and thank you

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If you have another array of char  char   b and you want to replace c with b  you can use c b clone

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Arrays fill is the best option for general purpose use  If you need to fill large arrays though as of latest idk 1 8 u102  there is a faster way that leverages System arraycopy  You can take a look at this alternate Arrays fill implementation   According to the JMH benchmarks you can get almost 2x performance boost for large arrays  1000     In any case  these implementations should be used only where needed  JDKs Arrays fill should be the preferred choice

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As of Java-8  there are four variants of the setAll method which sets all elements of the specified array  using a provided generator function to compute each element   Of those four overloads only three of them accept an array of primitives declared as such        setAll double   array  IntToDoubleFunction generator         setAll int   array  IntUnaryOperator generator        setAll long   array  IntToLongFunction generator      Examples of how to use the aforementioned methods      given an index  set the element at the specified index with the provided value double    doubles   new double 50   Arrays setAll doubles  index - gt  30D       given an index  set the element at the specified index with the provided value int    ints   new int 50   Arrays setAll ints  index - gt  60        given an index  set the element at the specified index with the provided value long    longs   new long 50   Arrays setAll longs  index - gt  90L     The function provided to the setAll method receives the element index and returns a value for that index    you may be wondering how about characters array   This is where the fourth overload of the setAll method comes into play  As there is no overload that consumes an array of character primitives  the only option we have is to change the declaration of our character array to a type Character     If changing the type of the array to Character is not appropriate then you can fall back to the Arrays fill method   Example of using the setAll method with Character        given an index  set the element at the specified index with the provided value Character   character   new Character 50   Arrays setAll characters  index - gt           Although  it s simpler to use the Arrays fill method rather than the setAll method to set a specific value    The setAll method has the advantage that you can either set all the elements of the array to have the same value or generate an array of even numbers  odd numbers or any other formula   e g   int   evenNumbers   new int 10    Arrays setAll evenNumbers  i - gt  i   2     There s also several overloads of the parallelSetAll method which is executed in parallel  although it s important to note that the function passed to the parallelSetAll method must be side-effect free   Conclusion  If your goal is simply to set a specific value for each element of the array then using the Arrays fill overloads would be the most appropriate option  However  if you want to be more flexible or generate elements on demand then using the Arrays setAll or Arrays parallelSetAll  when appropriate  would be the option to go for

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Use Arrays fill    char f          char    c   new char  50     Arrays fill c  f

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I have a minor improvement on Ross Drew s answer   For a small array  a simple loop is faster than the System arraycopy approach  because of the overhead associated with setting up System arraycopy   Therefore  it s better to fill the first few bytes of the array using a simple loop  and only move to System arraycopy when the filled array has a certain size   The optimal size of the initial loop will be JVM specific and system specific of course    private static final int SMALL   16   public static void arrayFill byte   array  byte value      int len   array length    int lenB   len  lt  SMALL   len   SMALL     for  int i   0  i  lt  lenB  i          array i    value         for  int i   SMALL  i  lt  len  i    i        System arraycopy array  0  array  i  len  lt  i   i   len - i   i

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System arraycopy is my answer  Please let me know is there any better ways  Thx  private static long   r1   new long 64   private static long     r2   new long 64  64       Proved       link Arrays fill long    long     makes r2 has 64 references to r1 - not the answer       link Arrays fill long    long   sometimes slower than deep 2 looping  lt br  gt      private static void testFillPerformance         SimpleDateFormat sdf   new SimpleDateFormat  HH mm ss        System out println sdf format new Date          Arrays fill r1  0l        long stamp0   System nanoTime                Arrays fill r2  0l   -- exception     long stamp1   System nanoTime                System out println String format  Arrays fill takes  s nano-seconds    stamp1 - stamp0         stamp0   System nanoTime        for  int i   0  i  lt  64  i              for  int j   0  j  lt  64  j                r2 i  j    0l            stamp1   System nanoTime        System out println String format  Arrays  2-looping takes  s nano-seconds    stamp1 - stamp0         stamp0   System nanoTime        for  int i   0  i  lt  64  i              System arraycopy r1  0  r2 i   0  64             stamp1   System nanoTime        System out println String format  System arraycopy looping takes  s nano-seconds    stamp1 - stamp0         stamp0   System nanoTime        Arrays fill r2  r1       stamp1   System nanoTime        System out println String format  One round Arrays fill takes  s nano-seconds    stamp1 - stamp0         stamp0   System nanoTime        for  int i   0  i  lt  64  i            Arrays fill r2 i   0l       stamp1   System nanoTime        System out println String format  Two rounds Arrays fill takes  s nano-seconds    stamp1 - stamp0        12 33 18 Arrays  2-looping takes 133536 nano-seconds  System arraycopy looping takes 22070 nano-seconds  One round Arrays fill takes 9777 nano-seconds  Two rounds Arrays fill takes 93028 nano-seconds   12 33 38 Arrays  2-looping takes 133816 nano-seconds  System arraycopy looping takes 22070 nano-seconds  One round Arrays fill takes 17042 nano-seconds  Two rounds Arrays fill takes 95263 nano-seconds   12 33 51 Arrays  2-looping takes 199187 nano-seconds  System arraycopy looping takes 44140 nano-seconds  One round Arrays fill takes 19555 nano-seconds  Two rounds Arrays fill takes 449219 nano-seconds   12 34 16 Arrays  2-looping takes 199467 nano-seconds  System arraycopy looping takes 42464 nano-seconds  One round Arrays fill takes 17600 nano-seconds  Two rounds Arrays fill takes 170971 nano-seconds   12 34 26 Arrays  2-looping takes 198907 nano-seconds  System arraycopy looping takes 24584 nano-seconds  One round Arrays fill takes 10616 nano-seconds  Two rounds Arrays fill takes 94426 nano-seconds

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Arrays fill might suit your needs

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You could use arraycopy but it depends on whether you can predefine the source array  - do you need a different character fill each time  or are you filling arrays repeatedly with the same char   Clearly the length of the fill matters - either you need a source that is bigger than all possible destinations  or you need a loop to repeatedly arraycopy a chunk of data until the destination is full       char f            char   c   new char 50       for  int i   0  i  lt  c length  i                  c i    f             char   d   new char 50       System arraycopy c  0  d  0  d length

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Try Arrays fill c  f    Arrays javadoc

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See Arrays fill method   char f        char    c   new char  50   Arrays fill c  f

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Arrays fill myArray   c     Arrays fill  Although it is quite possible that this is doing the loop in the background and is therefore not any more efficient than what you have  other than the lines of code savings   If you really care about efficiency  try the following in comparison to the above   int size   50  char   array   new char size   for  int i 0  i lt size  i       array i     c       Notice that the above doesn t call array size   for each iteration

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Assigns the specified char value to each element of the specified array        of chars                 param a the array to be filled         param val the value to be stored in all elements of the array             public static void fill char   a  char val            for  int i   0  len   a length  i  lt  len  i                a i    val          That s the way Arrays fill does it    I suppose you could drop into JNI and use memset

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As another option and for posterity I was looking into this recently and found a solution that allows a much shorter loop by handing some of the work off to the System class  which  if the JVM you re using is smart enough  can be turned into a memset operation -       initialize a smaller piece of the array and use the System arraycopy     call to fill in the rest of the array in an expanding binary fashion     public static void bytefill byte   array  byte value      int len   array length     if  len  gt  0       array 0    value           Value of i will be  1  2  4  8  16  32       len    for  int i   1  i  lt  len  i    i        System arraycopy array  0  array  i    len - i   lt  i     len - i    i          This solution was taken from the IBM research paper  quot Java server performance  A case study of building efficient  scalable Jvms quot  by R  Dimpsey  R  Arora  K  Kuiper  Simplified explanation As the comment suggests  this sets index 0 of the destination array to your value then uses the System class to copy one object i e  the object at index 0 to index 1 then those two objects  index 0 and 1  into 2 and 3  then those four objects  0 1 2 and 3  into 4 5 6 and 7 and so on    Efficiency  at the point of writing  In a quick run through  grabbing the System nanoTime   before and after and calculating a duration I came up with -  This method   332 617 - 390 262   highest - lowest  from 10 tests  Float   n   new Float array length     Fill with null   666 650 Setting via loop   3 743 488 - 9 767 744    highest - lowest  from 10 tests  Arrays fill   12 539 336  The JVM and JIT compilation It should be noted that as the JVM and JIT evolves  this approach may well become obsolete as library and runtime optimisations could reach or even exceed these numbers simply using fill    At the time of writing  this was the fastest option I had found   It has been mentioned this might not be the case now but I have not checked   This is the beauty and the curse of Java

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Java Programmer s FAQ Part B Sect 6 suggests   public static void bytefill byte   array  byte value        int len   array length      if  len  gt  0      array 0    value      for  int i   1  i  lt  len  i    i          System arraycopy  array  0  array  i                len - i   lt  i     len - i    i       This essentially makes log2 array length  calls to System arraycopy which hopefully utilizes an optimized memcpy implementation    However  is this technique still required on modern Java JITs such as the Oracle Android JIT

[java] Fastest way to set all values of an array?

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