How do I determine the size of my array in C

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How do I determine the size of my array in C    That is  the number of elements the array can hold

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The function sizeof returns the number of bytes which is used by your array in the memory  If you want to calculate the number of elements in your array  you should divide that number with the sizeof variable type of the array  Let s say int array 10    if variable type integer in your computer is 32 bit  or 4 bytes   in order to get the size of your array  you should do the following   int array 10   int sizeOfArray   sizeof array  sizeof int

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define SIZE OF ARRAY  array   sizeof  array    sizeof  array 0

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It is worth noting that sizeof doesn t help when dealing with an array value that has decayed to a pointer  even though it points to the start of an array  to the compiler it is the same as a pointer to a single element of that array  A pointer does not  remember  anything else about the array that was used to initialize it   int a 10   int  p   a   assert sizeof a    sizeof a 0      10   assert sizeof p     sizeof int     assert sizeof  p     sizeof int

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You can use sizeof operator but it will not work for functions because it will take the reference of pointer you can do the following to find the length of an array   len   sizeof arr  sizeof arr 0     Code originally found here   C program to find the number of elements in an array

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You can use the  amp  operator  Here is the source code    include lt stdio h gt   include lt stdlib h gt  int main         int a 10        int  p        printf   p n    void   a        printf   p n    void     amp a 1        printf  ---- diff---- n        printf   zu n   sizeof a 0         printf  The size of array a is  zu n     char     amp a 1 - char   a   sizeof a 0            return 0       Here is the sample output  1549216672 1549216712 ---- diff---- 4 The size of array a is 10

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int size     amp arr  1  - arr    Check out this link for explanation

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If you know the data type of the array  you can use something like   int arr      23  12  423  43  21  43  65  76  22    int noofele   sizeof arr  sizeof int     Or if you don t know the data type of array  you can use something like   noofele   sizeof arr  sizeof arr 0      Note  This thing only works if the array is not defined at run time  like malloc  and the array is not passed in a function  In both cases  arr  array name  is a pointer

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I would advise to never use sizeof  even if it can be used  to get any of the two different sizes of an array  either in number of elements or in bytes  which are the last two cases I show here   For each of the two sizes  the macros shown below can be used to make it safer   The reason is to make obvious the intention of the code to maintainers  and difference sizeof ptr  from sizeof arr  at first glance  which written this way isn t obvious   so that bugs are then obvious for everyone reading the code   TL DR   define ARRAY SIZE arr       sizeof arr    sizeof  arr  0     must be array arr     define ARRAY SSIZE arr       ptrdiff t ARRAY SIZE arr     define ARRAY BYTES arr      sizeof arr    must be array arr     define ARRAY SBYTES arr      ssize t ARRAY BYTES arr    must be array arr   defined below  IS needed as -Wsizeof-pointer-div is buggy  as of april 2020    define is same type a  b     builtin types compatible p typeof a   typeof b    define is array arr          is same type  arr    amp  arr  0     define must be e                                                     0    int sizeof                                                                   struct                                                                             Static assert  e     VA OPT         VA ARGS                              char ISO C forbids a struct with no members                                                                                                                                                              define must be array arr   must be is array arr    quot Not a        quot     There have been important bugs regarding this topic  https   lkml org lkml 2015 9 3 428 I disagree with the solution that Linus provides  which is to never use array notation for parameters of functions  I like array notation as documentation that a pointer is being used as an array   But that means that a fool-proof solution needs to be applied so that it is impossible to write buggy code  From an array we have three sizes which we might want to know   The size of the elements of the array The number of elements in the array The size in bytes that the array uses in memory   The size of the elements of the array The first one is very simple  and it doesn t matter if we are dealing with an array or a pointer  because it s done the same way  Example of usage  void foo ptrdiff t nmemb  int arr static nmemb             qsort arr  nmemb  sizeof arr 0    cmp      qsort   needs this value as its third argument   For the other two sizes  which are the topic of the question  we want to make sure that we re dealing with an array  and break the compilation if not  because if we re dealing with a pointer  we will get wrong values   When the compilation is broken  we will be able to easily see that we weren t dealing with an array  but with a pointer instead  and we will just have to write the code with a variable or a macro that stores the size of the array behind the pointer   The number of elements in the array This one is the most common  and many answers have provided you with the typical macro ARRAY SIZE   define ARRAY SIZE arr       sizeof arr    sizeof  arr  0     Given that the result of ARRAY SIZE is commonly used with signed variables of type ptrdiff t  it is good to define a signed variant of this macro   define ARRAY SSIZE arr       ptrdiff t ARRAY SIZE arr    Arrays with more than PTRDIFF MAX members are going to give invalid values for this signed version of the macro  but from reading C17  6 5 6 9  arrays like that are already playing with fire  Only ARRAY SIZE and size t should be used in those cases  Recent versions of compilers  such as GCC 8  will warn you when you apply this macro to a pointer  so it is safe  there are other methods to make it safe with older compilers   It works by dividing the size in bytes of the whole array by the size of each element  Examples of usage  void foo ptrdiff t nmemb            char buf nmemb            fgets buf  ARRAY SIZE buf   stdin      void bar ptrdiff t nmemb            int arr nmemb            for  ptrdiff t i   0  i  lt  ARRAY SSIZE arr   i                    arr i    i     If these functions didn t use arrays  but got them as parameters instead  the former code would not compile  so it would be impossible to have a bug  given that a recent compiler version is used  or that some other trick is used   and we need to replace the macro call by the value  void foo ptrdiff t nmemb  char buf nmemb              fgets buf  nmemb  stdin      void bar ptrdiff t nmemb  int arr nmemb              for  ptrdiff t i   0  i  lt  nmemb  i                    arr i    i      The size in bytes that the array uses in memory ARRAY SIZE is commonly used as a solution to the previous case  but this case is rarely written safely  maybe because it s less common  The common way to get this value is to use sizeof arr    The problem  the same as with the previous one  if you have a pointer instead of an array  your program will go nuts  The solution to the problem involves using the same macro as before  which we know to be safe  it breaks compilation if it is applied to a pointer    define ARRAY BYTES arr          sizeof  arr  0     ARRAY SIZE arr    Given that the result of ARRAY BYTES is sometimes compared to the output of functions that return ssize t  it is good to define a signed variant of this macro   define ARRAY SBYTES arr      ssize t ARRAY BYTES arr    How it works is very simple   it undoes the division that ARRAY SIZE does  so after mathematical cancellations you end up with just one sizeof arr   but with the added safety of the ARRAY SIZE construction  Example of usage  void foo ptrdiff t nmemb            int arr nmemb            memset arr  0  ARRAY BYTES arr       memset   needs this value as its third argument  As before  if the array is received as a parameter  a pointer   it won t compile  and we will have to replace the macro call by the value  void foo ptrdiff t nmemb  int arr nmemb              memset arr  0  sizeof arr 0     nmemb       Update  23 apr 2020   -Wsizeof-pointer-div is buggy  Today I found out that the new warning in GCC only works if the macro is defined in a header that is not a system header   If you define the macro in a header that is installed in your system  usually  usr local include  or  usr include     include  lt foo h gt    the compiler will NOT emit a warning  I tried GCC 9 3 0   So we have  define ARRAY SIZE arr   sizeof arr    sizeof  arr  0    and want to make it safe   We will need C11  Static assert   and some GCC extensions  Statements and Declarations in Expressions    builtin types compatible p   define is same type a  b         builtin types compatible p typeof a   typeof b    define is array arr              is same type  arr    amp  arr  0     define Static assert array arr   Static assert is array arr    quot Not a        quot     define ARRAY SIZE arr                                                                                                                                      Static assert array arr                                                   sizeof arr    sizeof  arr  0                                                                                                                  Now ARRAY SIZE   is completely safe  and therefore all its derivatives will be safe   Update  libbsd provides   arraycount    Libbsd provides the macro   arraycount   in  lt sys cdefs h gt   which is unsafe because it lacks a pair of parentheses  but we can add those parentheses ourselves  and therefore we don t even need to write the division in our header  why would we duplicate code that already exists     That macro is defined in a system header  so if we use it we are forced to use the macros above   include  lt stddef h gt   include  lt sys cdefs h gt   include  lt sys types h gt     define is same type a  b         builtin types compatible p typeof a   typeof b    define is array arr              is same type  arr    amp  arr  0     define Static assert array arr   Static assert is array arr    quot Not a        quot     define ARRAY SIZE arr                                                                                                                                      Static assert array arr                                                     arraycount  arr                                                                                                                               define ARRAY SSIZE arr           ptrdiff t ARRAY SIZE arr    define ARRAY BYTES arr          sizeof  arr  0     ARRAY SIZE arr    define ARRAY SBYTES arr          ssize t ARRAY BYTES arr    Some systems provide nitems   in  lt sys param h gt  instead  and some systems provide both   You should check your system  and use the one you have  and maybe use some preprocessor conditionals for portability and support both   Update  Allow the macro to be used at file scope  Unfortunately  the      gcc extension cannot be used at file scope  To be able to use the macro at file scope  the static assertion must be inside sizeof struct       Then  multiply it by 0 to not affect the result   A cast to  int  might be good to simulate a function that returns  int 0  in this case it is not necessary  but then it is reusable for other things   Additionally  the definition of ARRAY BYTES   can be simplified a bit   include  lt stddef h gt   include  lt sys cdefs h gt   include  lt sys types h gt     define is same type a  b        builtin types compatible p typeof a   typeof b    define is array arr             is same type  arr    amp  arr  0     define must be e                                                                 0    int sizeof                                                                   struct                                                                             Static assert  e     VA OPT         VA ARGS                              char ISO C forbids a struct with no members                                                                                                                                                              define must be array arr       must be is array arr    quot Not a        quot     define ARRAY SIZE arr             arraycount  arr     must be array arr    define ARRAY SSIZE arr           ptrdiff t ARRAY SIZE arr    define ARRAY BYTES arr          sizeof arr    must be array arr    define ARRAY SBYTES arr          ssize t ARRAY BYTES arr     Notes  This code makes use of the following extensions  which are completely necessary  and their presence is absolutely necessary to achieve safety   If your compiler doesn t have them  or some similar ones  then you can t achieve this level of safety     builtin types compatible p   typeof    I also make use of the following C11 feature   However  its absence by using an older standard can be overcome using some dirty tricks  see for example  What is     -      in C code      Static assert    I also use the following extension  but there s a standard C way of doing the same thing     VA OPT

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Beside the answers already provided  I want to point out a special case by the use of   sizeof a    sizeof  a 0       If a is either an array of char  unsigned char or signed char you do not need to use sizeof twice since a sizeof expression with one operand of these types do always result to 1   Quote from C18 6 5 3 4 4         When sizeof is applied to an operand that has type char  unsigned char  or signed char   or a qualified version thereof  the result is 1     Thus  sizeof a    sizeof  a 0   would be equivalent to NUMBER OF ARRAY ELEMENTS   1 if a is an array of type char  unsigned char or signed char  The division through 1 is redundant   In this case  you can simply abbreviate and do   sizeof a    For example   char a 10   size t length   sizeof a     If you want a proof  here is a link to GodBolt     Nonetheless  the division maintains safety  if the type significantly changes  although these cases are rare

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sizeof array    sizeof array 0

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Note  This one can give you undefined behaviour as pointed by M M in the comment   int a 10   int size       amp a 1 -a      For more details see here and also here

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A more elegant solution will be  size t size   sizeof a    sizeof  a

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Size of an array in C   int a 10   size t size of array   sizeof a           Size of array a int n   sizeof  a    sizeof  a 0          Number of elements in array a size t size of element   sizeof a 0       Size of each element in array a                                                                                     Size of each element   size of type

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The sizeof  trick  is the best way I know  with one small but  to me  this being a major pet peeve  important change in the use of parenthesis   As the Wikipedia entry makes clear  C s sizeof is not a function  it s an operator  Thus  it does not require parenthesis around its argument  unless the argument is a type name  This is easy to remember  since it makes the argument look like a cast expression  which also uses parenthesis   So  If you have the following   int myArray 10     You can find the number of elements with code like this   size t n   sizeof myArray   sizeof  myArray    That  to me  reads a lot easier than the alternative with parenthesis  I also favor use of the asterisk in the right-hand part of the division  since it s more concise than indexing   Of course  this is all compile-time too  so there s no need to worry about the division affecting the performance of the program  So use this form wherever you can   It is always best to use sizeof on an actual object when you have one  rather than on a type  since then you don t need to worry about making an error and stating the wrong type   For instance  say you have a function that outputs some data as a stream of bytes  for instance across a network  Let s call the function send    and make it take as arguments a pointer to the object to send  and the number of bytes in the object  So  the prototype becomes   void send const void  object  size t size     And then you need to send an integer  so you code it up like this   int foo   4711  send  amp foo  sizeof  int      Now  you ve introduced a subtle way of shooting yourself in the foot  by specifying the type of foo in two places  If one changes but the other doesn t  the code breaks  Thus  always do it like this   send  amp foo  sizeof foo     Now you re protected  Sure  you duplicate the name of the variable  but that has a high probability of breaking in a way the compiler can detect  if you change it

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If you really want to do this to pass around your array I suggest implementing a structure to store a pointer to the type you want an array of and an integer representing the size of the array  Then you can pass that around to your functions  Just assign the array variable value  pointer to first element  to that pointer  Then you can go Array arr i  to get the i-th element and use Array size to get the number of elements in the array   I included some code for you  It s not very useful but you could extend it with more features  To be honest though  if these are the things you want you should stop using C and use another language with these features built in      Absolutely no one should use this       By the time you re done implementing it you ll wish you just passed around    an array and size to your functions       This is a static implementation  You can get a dynamic implementation and     cut out the array in main by using the stdlib memory allocation methods     but it will work much slower since it will store your array on the heap      include  lt stdio h gt   include  lt string h gt      include  MyTypeArray h        MyTypeArray h   ifndef MYTYPE ARRAY  define MYTYPE ARRAY    typedef struct MyType      int age     char name 20     MyType  typedef struct MyTypeArray      int size     MyType  arr    MyTypeArray   MyType new MyType int age  char  name   MyTypeArray newMyTypeArray int size  MyType  first       endif End MyTypeArray h        MyTypeArray c    MyType new MyType int age  char  name       MyType d     d age   age     strcpy d name  name      return d     MyTypeArray new MyTypeArray int size  MyType  first       MyTypeArray d     d size   size     d arr   first     return d       End MyTypeArray c      void print MyType names MyTypeArray d       int i     for  i   0  i  lt  d size  i               printf  Name   s  Age   d n   d arr i  name  d arr i  age           int main           First create an array on the stack to store our elements in        Note we could create an empty array with a size instead and       set the elements later        MyType arr      new MyType 10   Sam    new MyType 3   Baxter           Now create a  MyTypeArray  which will use the array we just       created internally  Really it will just store the value of the pointer        arr   Here we are manually setting the size  You can use the sizeof       trick here instead if you re sure it will work with your compiler        MyTypeArray array   new MyTypeArray 2  arr         MyTypeArray array   new MyTypeArray sizeof arr  sizeof arr 0    arr         print MyType names array      return 0

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Executive summary   int a 17   size t n   sizeof a  sizeof a 0        Full answer   To determine the size of your array in bytes  you can use the sizeof operator   int a 17   size t n   sizeof a     On my computer  ints are 4 bytes long  so n is 68   To determine the number of elements in the array  we can divide the total size of the array by the size of the array element  You could do this with the type  like this   int a 17   size t n   sizeof a    sizeof int     and get the proper answer  68   4   17   but if the type of a changed you would have a nasty bug if you forgot to change the sizeof int  as well   So the preferred divisor is sizeof a 0   or the equivalent sizeof  a   the size of the first element of the array   int a 17   size t n   sizeof a    sizeof a 0      Another advantage is that you can now easily parameterize the array name in a macro and get    define NELEMS x    sizeof x    sizeof  x  0     int a 17   size t n   NELEMS a

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The macro ARRAYELEMENTCOUNT x  that everyone is making use of evaluates incorrectly  This  realistically  is just a sensitive matter  because you can t have expressions that result in an  array  type      Compile as  CL  P  macro c       define ARRAYELEMENTCOUNT x   sizeof  x    sizeof  x 0     ARRAYELEMENTCOUNT p   1     Actually evaluates as    sizeof  p   1    sizeof  p   1 0       Whereas     Compile as  CL  P  macro c       define ARRAYELEMENTCOUNT x   sizeof  x    sizeof  x  0    ARRAYELEMENTCOUNT p   1     It correctly evaluates to    sizeof  p   1    sizeof  p   1  0      This really doesn t have a lot to do with the size of arrays explicitly  I ve just noticed a lot of errors from not truly observing how the C preprocessor works  You always wrap the macro parameter  not an expression in might be involved in     This is correct  my example was a bad one  But that s actually exactly what should happen  As I previously mentioned p   1 will end up as a pointer type and invalidate the entire macro  just like if you attempted to use the macro in a function with a pointer parameter    At the end of the day  in this particular instance  the fault doesn t really matter  so I m just wasting everyone s time  huzzah    because you don t have expressions with a type of  array   But really the point about preprocessor evaluation subtles I think is an important one

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For multidimensional arrays it is a tad more complicated  Oftenly people define explicit macro constants  i e     define g rgDialogRows   2  define g rgDialogCols   7  static char const  g rgDialog g rgDialogRows  g rgDialogCols                                    494     210     Generic Sample Dialog                  1     330     174     88              OK                      But these constants can be evaluated at compile-time too with sizeof    define rows of array name               sizeof name       sizeof name 0  0     columns of array name    define columns of array name            sizeof name 0     sizeof name 0  0     static char  g rgDialog   7                    assert    rows of array g rgDialog     2   assert columns of array g rgDialog     7     Note that this code works in C and C    For arrays with more than two dimensions use  sizeof name 0  0  0   sizeof name 0  0  0  0     etc   ad infinitum

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The best way is you save this information  for example  in a structure   typedef struct        int  array       int elements    list s    Implement all necessary functions such as create  destroy  check equality  and everything else you need  It is easier to pass as a parameter

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The simplest Answer    include  lt stdio h gt   int main void         int a      2 3 4 5 4 5 6 78 9 91 435 4 5 76 7 34    for Example only     int size       size   sizeof a  sizeof a 0     Method      printf   size    d  size       return 0

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The sizeof way is the right way iff you are dealing with arrays not received as parameters  An array sent as a parameter to a function is treated as a pointer  so sizeof will return the pointer s size  instead of the array s   Thus  inside functions this method does not work  Instead  always pass an additional parameter size t size indicating the number of elements in the array   Test    include  lt stdio h gt   include  lt stdlib h gt   void printSizeOf int intArray     void printLength int intArray      int main int argc  char  argv          int array       0  1  2  3  4  5  6         printf  sizeof of array   d n    int  sizeof array        printSizeOf array        printf  Length of array   d n    int   sizeof array    sizeof array 0           printLength array      void printSizeOf int intArray          printf  sizeof of parameter   d n    int  sizeof intArray       void printLength int intArray          printf  Length of parameter   d n    int   sizeof intArray    sizeof intArray 0           Output  in a 64-bit Linux OS    sizeof of array  28 sizeof of parameter  8 Length of array  7 Length of parameter  2   Output  in a 32-bit windows OS    sizeof of array  28 sizeof of parameter  4 Length of array  7 Length of parameter  1

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you ve introduced a subtle way of shooting yourself in the foot   C  native  arrays do not store their size  It is therefore recommended to save the length of the array in a separate variable const  and pass it whenever you pass the array  that is    define MY ARRAY LENGTH   15 int myArray MY ARRAY LENGTH     You SHOULD always avoid native arrays  unless you can t  in which case  mind your foot   If you are writing C    use the STL s  vector  container   Compared to arrays  they provide almost the same performance   and they are far more useful      vector is a template  the  lt int gt  means it is a vector of ints vector lt int gt  numbers        push back   puts a new value at the end  or back  of the vector for  int i   0  i  lt  10  i        numbers push back i       Determine the size of the array cout  lt  lt  numbers size      See  http   www cplusplus com reference stl vector

[c] How do I determine the size of my array in C?

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