How can I find the number of elements in an array

Question

I have an int array and I need to find the number of elements in it  I know it has something to do with sizeof but I m not sure how to use it exactly

User · Answer

Actually  there is no proper way to count the elements in a dynamic integer array  However  the sizeof command works properly in Linux  but it does not work properly in Windows  From a programmer s point of view  it is not recommended to use sizeof to take the number of elements in a dynamic array  We should keep track of the number of elements when making the array

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In real we can t count how many elements are store in array But u can find the array length or size using sizeof operator  But why we can t find how many elements are present in my array  Because when we initialise a array compiler give memory on our program like a 10   10 blocks of 4 size   and every block has garbage value if we put some value in some index like a 0  1 a 1  2 a 3  8  and other block has garbage value no one can tell which value Is garbage and which value is not garbage that s a reason we cannot calculate how many elements in a array  I hope this will help u to understand   Little concept

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I personally think that sizeof a    sizeof  a  looks cleaner   I also prefer to define it as a macro    define NUM a   sizeof a    sizeof  a     Then you can use it in for-loops  thusly   for  i   0  i  lt  NUM a   i

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I used following code as suggested above to evaluate number of elements in my 2-dimensional array    include  lt stdio h gt   include  lt string h gt   void main void        char strs 3  20                    January              February                             int arraysize   sizeof strs  sizeof strs 0         for  int i   0  i  lt  arraysize  i                  printf  Month  d is   s n   i  strs i               It works nicely  As far as I know you can t mix up different data types in C arrays and also you should have the same size of all array elements  if I am right   therefore you can take advantage of that with this little trick    count number of bytes with sizeof   function from whole 2d array  in this case 3 20   60 bytes  count number of bytes with sizeof   function from first array element strs 0   in this case 20 bytes  divide whole size with size of one element what will give you number of elements   This snipped should be portable for 2d arrays in C however in other programming languages it could not work because you can use different data types within array with different sizes  like in JAVA

User · Answer

Super easy  Just divide the number of allocated bytes by the number of bytes of the array s data type using sizeof    For example  given an integer array called myArray int numArrElements   sizeof myArray    sizeof int    Now  if the data type of your array isn t constant and could possibly change  make the divisor in the equation use the size of the first value as the size of the data type For example  int numArrElements   sizeof myArray    sizeof myArray 0

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If we don t know the number of elements in the array and when the input is given by the user at the run time  Then we can write the code as  C CODE   while scanf   d   amp array count    1       count        C   CODE   while cin gt  gt a count       count        Now the count will be having the count of number of array elements which are entered

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i mostly found a easy way to execute the length of array inside a loop just like that   int array      10  20  30  40    int i   for  i   0  i  lt  array i   i          printf   d n   array i

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void numel int array1 100  100         int count 0      for int i 0 i lt 100 i                  for int j 0 j lt 100 j                          if array1 i  j     0                                  count                      printf   n d- d  array1 i  j  count                             else                  break                      printf  Number of elements  d  count     int main            int r arr 100  100   0  c      printf  Enter the no  of rows          scanf   d   amp r       printf   nEnter the no  of columns          scanf   d   amp c       printf   nEnter the elements          for int i 0 i lt r i                  for int j 0 j lt c j                          scanf   d   amp arr i  j                        numel arr       This shows the exact number of elements in matrix irrespective of the array size you mentioned while initilasing IF that s what you meant

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sizeof returns the size in bytes of it s argument  This is not what you want  but it can help   Let s say you have an array   int array 4     If you apply sizeof to the array  sizeof array    it will return its size in bytes  which in this case is 4   the size of an int  so a total of maybe 16 bytes  depending on your implementation    If you apply sizeof to an element of the array  sizeof array 0     it will return its size in bytes  which in this case is the size of an int  so a total of maybe 4 bytes  depending on your implementation    If you divide the first one by the second one  it will be   4   the size of an int     the size of an int    4  That s exactly what you wanted   So this should do   sizeof array    sizeof array 0     Now you would probably like to have a macro to encapsulate this logic and never have to think again how it should be done    define ARRAY SIZE arr   sizeof arr    sizeof  arr  0      You need the parentheses enclosing all the macro as in any other complex macro  and also enclosing every variable  just to avoid unexpected bugs related to operators precedence   Now you can use it on any array like this   int array 6   ptrdiff t nmemb   nmemb   ARRAY SIZE array      nmemb    6      Remember that arguments of functions declared as arrays are not really arrays  but pointers to the first element of the array  so this will NOT work on them   void foo int false array 6             ptrdiff t nmemb           nmemb   ARRAY SIZE false array              nmemb    sizeof int      sizeof int                 maybe    2         But it can be used in functions if you pass a pointer to an array instead of just the array   void bar int   arrptr  7             ptrdiff t nmemb           nmemb   ARRAY SIZE  arrptr              nmemb    7

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int a 20   int length  length   sizeof a    sizeof int     and you can use another way to make your code not be hard-coded to int  Say if you have an array array  you just need to   int len   sizeof array    sizeof array 0

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It is not possible to find the number of elements in an array unless it is a character array  Consider the below example   int main         int arr 100   1 2 3 4 5       int size   sizeof arr  sizeof arr 0        printf   d   size       return 1      The above value gives us value 100 even if the number of elements is five  If it is a character array  you can search linearly for the null string at the end of the array and increase the counter as you go through

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include lt stdio h gt  int main         int arr    10 20 30 40 50 60       int  p      int count 0       for p arr p lt  amp arr 1 p            count         printf  The no of elements in array  d  count        return 0         OUTPUT 6   EXPLANATION  p is a pointer to a 1-D array  and in the loop for p arr p lt  amp arr 1 p    I made p point to the base address  Suppose its base address is 1000  if we increment p then it points to 1002 and so on  Now coming to the concept of  amp arr - It basically represents the whole array  and if we add 1 to the whole array i e   amp arr 1  it gives the address 1012 i e  the address of next 1-D array  in our case the size of int is 2   so the condition becomes 1000 lt 1012   So  basically the condition becomes  for p 1000 p lt 1012 p      And now let s check the condition and count the value   1st time p 1000 and p lt 1012 condition is true  enter in the loop  increment the value of count to 1  2nd time p 1002 and p lt 1012 condition is true  enter in the loop  increment the value of count to 2      6th time p 1010 and p lt 1012 condition is true  enter in the loop  increment the value of count to 6  Last time p 1012 and p lt 1012 condition is false  print the value of count 6 in printf statement

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The question is simple  given a C   array  e g  x as in int x 10    how would you get the number of elements in it   An obvious solution is the following macro  definition 1     define countof  array     sizeof  array   sizeof  array 0        I cannot say this isn   t correct  because it does give the right answer when you give it an array   However  the same expression gives you something bogus when you supply something that is not an array   For example  if you have  int   p    then countof  p   always give you 1 on a machine where an int pointer and an int have the same size  e g  on a Win32 platform    This macro also wrongfully accepts any object of a class that has a member function operator     For example  suppose you write  class IntArray   private      int   p      size t size  public      int  amp  operator      size t i      x    then sizeof  x   will be the size of the x object  not the size of the buffer pointed to by x p   Therefore you won   t get a correct answer by countof  x     So we conclude that definition 1 is not good because the compiler does not prevent you from misusing it   It fails to enforce that only an array can be passed in   What is a better option   Well  if we want the compiler to ensure that the parameter to countof is always an array  we have to find a context where only an array is allowed   The same context should reject any non-array expression   Some beginners may try this  definition 2    template  lt typename T  size t N gt  size t countof  T array N         return N      They figure  this template function will accept an array of N elements and return N   Unfortunately  this doesn   t compile because C   treats an array parameter the same as a pointer parameter  i e  the above definition is equivalent to   template  lt typename T  size t N gt  size t countof  T   array         return N      It now becomes obvious that the function body has no way of knowing what N is   However  if a function expects an array reference  then the compiler does make sure that the size of the actual parameter matches the declaration   This means we can make definition 2 work with a minor modification  definition 3    template  lt typename T  size t N gt  size t countof  T   amp array  N          return N      This countof works very well and you cannot fool it by giving it a pointer   However  it is a function  not a macro   This means you cannot use it where a compile time constant is expected   In particular  you cannot write something like   int x 10    int y  2 countof x        twice as big as x   Can we do anything about it   Someone  I don   t know who it is     I just saw it in a piece of code from an unknown author  came up with a clever idea  moving N from the body of the function to the return type  e g  make the function return an array of N elements   then we can get the value of N without actually calling the function   To be precise  we have to make the function return an array reference  as C   does not allow you to return an array directly   The implementation of this is   template  lt typename T  size t N gt  char    amp  ArraySizeHelper  T   amp array  N     N     define countof  array    sizeof   ArraySizeHelper  array        Admittedly  the syntax looks awful   Indeed  some explanation is necessary   First  the top-level stuff  char    amp  ArraySizeHelper         N     says  ArraySizeHelper is a function that returns a reference  note the  amp   to a char array of N elements   Next  the function parameter is  T   amp array  N    which is a reference to a T array of N elements   Finally  countof is defined as the size of the result of the function  ArraySizeHelper   Note we don   t even need to define  ArraySizeHelper    -- a declaration is enough   With this new definition   int x 10    int y  2 countof x        twice as big as x   becomes valid  just as we desire   Am I happy now   Well  I think this definition is definitely better than the others we have visited  but it is still not quite what I want   For one thing  it doesn   t work with types defined inside a function   That   s because the template function  ArraySizeHelper expects a type that is accessible in the global scope   I don   t have a better solution   If you know one  please let me know

User · Answer

If you have your array in scope you can use sizeof to determine its size in bytes and use the division to calculate the number of elements    define NUM OF ELEMS 10 int arr NUM OF ELEMS   size t NumberOfElements   sizeof arr  sizeof arr 0      If you receive an array as a function argument or allocate an array in heap you can not determine its size using the sizeof  You ll have to store pass the size information somehow to be able to use it   void DoSomethingWithArray int  arr  int NumOfElems        for int i   0  i  lt  NumOfElems    i            arr i

[c] How can I find the number of elements in an array?

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