Fastest way to zero out a 2d array in C

Question

I want to repeatedly zero a large 2d array in C  This is what I do at the moment      Array of size n   m  where n may not equal m for j   0  j  lt  n  j          for i   0  i  lt  m  i                    array i  j    0            I ve tried using memset   memset array  0  sizeof array     But this only works for 1D arrays  When I printf the contents of the 2D array  the first row is zeroes  but then I got a load of random large numbers and it crashes

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int array N  M     0       at least in GCC 4 8

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memset array  0  sizeof int  n  n

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If you are really  really obsessed with speed  and not so much with portability  I think the absolute fastest way to do this would be to use SIMD vector intrinsics  e g  on Intel CPUs  you could use these SSE2 instructions     m128i  mm setzero si128                          Create a quadword with a value of 0  void  mm storeu si128    m128i  p    m128i a       Write a quadword to the specified address    Each store instruction will set four 32-bit ints to zero in one hit   p must be 16-byte aligned  but this restriction is also good for speed because it will help the cache  The other restriction is that p must point to an allocation size that is a multiple of 16-bytes  but this is cool too because it allows us to unroll the loop easily   Have this in a loop  and unroll the loop a few times  and you will have a crazy fast initialiser      Assumes int is 32-bits  const int mr   roundUpToNearestMultiple m  4           This isn t the optimal modification of m and n  but done this way here for clarity      const int nr   roundUpToNearestMultiple n  4        int i   0  int array mr  nr    attribute     aligned  16          GCC directive    m128i  px      m128i  array  const int incr   s  gt  gt  2                                Unroll it 4 times  const   m128i zero128    mm setzero si128     for i   0  i  lt  s  i    incr         mm storeu si128 px    zero128        mm storeu si128 px    zero128        mm storeu si128 px    zero128        mm storeu si128 px    zero128       There is also a variant of  mm storeu that bypasses the cache  i e  zeroing the array won t pollute the cache  which could give you some secondary performance benefits in some circumstances    See here for SSE2 reference  http   msdn microsoft com en-us library kcwz153a v vs 80  aspx

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If array is truly an array  then you can  zero it out  with   memset array  0  sizeof array     But there are two points you should know    this works only if array is really a  two-d array   i e   was declared T array M  N   for some type T  it works only in the scope where array was declared   If you pass it to a function  then the name array decays to a pointer  and sizeof will not give you the size of the array    Let s do an experiment    include  lt stdio h gt   void f int   arr  5         printf  f     sizeof arr         zu n   sizeof arr       printf  f     sizeof arr 0       zu n   sizeof arr 0        printf  f     sizeof arr 0  0    zu n   sizeof arr 0  0       int main void        int arr 10  5       printf  main  sizeof arr         zu n   sizeof arr       printf  main  sizeof arr 0       zu n   sizeof arr 0        printf  main  sizeof arr 0  0    zu n n   sizeof arr 0  0        f arr       return 0      On my machine  the above prints   main  sizeof arr        200 main  sizeof arr 0      20 main  sizeof arr 0  0   4  f     sizeof arr        8 f     sizeof arr 0      20 f     sizeof arr 0  0   4   Even though arr is an array  it decays to a pointer to its first element when passed to f    and therefore the sizes printed in f   are  wrong    Also  in f   the size of arr 0  is the size of the array arr 0   which is an  array  5  of int    It is not the size of an int    because the  decaying  only happens at the first level  and that is why we need to declare f   as taking a pointer to an array of the correct size   So  as I said  what you were doing originally will work only if the two conditions above are satisfied   If not  you will need to do what others have said   memset array  0  m n sizeof array 0  0      Finally  memset   and the for loop you posted are not equivalent in the strict sense   There could be  and have been  compilers where  all bits zero  does not equal zero for certain types  such as pointers and floating-point values   I doubt that you need to worry about that though

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You can try this     int array 20 30      0

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memset array  0  sizeof array 0  0     m   n     Where m and n are the width and height of the two-dimensional array  in your example  you have a square two-dimensional array  so m    n

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This happens because sizeof array  gives you the allocation size of the object pointed to by array   array is just a pointer to the first row of your multidimensional array   However  you allocated j arrays of size i  Consequently  you need to multiply the size of one row  which is returned by sizeof array  with the number of rows you allocated  e g    bzero array  sizeof array    j     Also note that sizeof array  will only work for statically allocated arrays  For a dynamically allocated array you would write  size t arrayByteSize   sizeof int    i   j   int  array   malloc array2dByteSite   bzero array  arrayByteSize

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Well  the fastest way to do it is to not do it at all   Sounds odd I know  here s some pseudocode   int array       bool array is empty    void ClearArray         array is empty   true     int ReadValue  int x  int y       return array is empty   0   array  x  y      void SetValue  int x  int y  int value       if  array is empty             memset  array  0  number of byte the array uses         array is empty   false          array  x  y    value      Actually  it s still clearing the array  but only when something is being written to the array  This isn t a big advantage here  However  if the 2D array was implemented using  say  a quad tree  not a dynamic one mind   or a collection of rows of data  then you can localise the effect of the boolean flag  but you d need more flags  In the quad tree just set the empty flag for the root node  in the array of rows just set the flag for each row   Which leads to the question  why do you want to repeatedly zero a large 2d array   What is the array used for  Is there a way to change the code so that the array doesn t need zeroing   For example  if you had   clear array for each set of data   for each element in data set     array    element    that is  use it for an accumulation buffer  then changing it like this would improve the performance no end    for set 0 and set 1    for each element in each set      array   element1   element2   for remaining data sets    for each element in data set      array    element    This doesn t require the array to be cleared but still works  And that will be far faster than clearing the array  Like I said  the fastest way is to not do it in the first place

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Use calloc instead of malloc   calloc will initiate all fields to 0   int  a    int   calloc n size of int        all cells of a have been initialized to 0

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I think that the fastest way to do it by hand is following code  You can compare it s speed to memset function  but it shouldn t be slower    change type of ptr and ptr1 pointers if your array type is different then int     define SIZE X 100  define SIZE Y 100  int  ptr   ptr1  ptr    array 0  0   ptr1   ptr   SIZE X SIZE Y sizeof array 0  0       while ptr  lt  ptr1         ptr     0

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If you initialize the array with malloc  use calloc instead  it will zero your array for free   Same perf obviously as memset  just less code for you

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How was your 2D array declared   If it something like   int arr 20  30     You can zero it by doing   memset arr  sizeof int  20 30

[c] Fastest way to zero out a 2d array in C?

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