Removing elements from an array in C

Question

I just have a simple question about arrays in C   What s the best way to remove elements from an array and in the process make the array smaller    i e the array is n size  then I take elements out of the array and then the array grows smaller by the amount that I removed it from    basically I m treating the array like a deck of cards and once I take a card off the top of the deck it shouldn t be there anymore    EDIT  I m going to drive myself crazy before the end of the day  thanks for all the help I m trying the value swapping thing but it s not working right     include  lt stdio h gt   include  lt string h gt   include  lt stdlib h gt   enum faces Ace   0  Jack   10  Queen  King   char   facecheck int d    int draw int deck  int i    int main           int deck 52   i  n      char suits 4  9                    Hearts            Diamonds            Clubs            Spades          n   0       for i   0  i lt 52  i                  deck i    n          n                   for i 0  i lt 52  i                         if i 13    0    i 13    10    i 13    11    i 13    12              printf   s    facecheck i 13             else printf   d    i 13 1           printf  of  s  n   suits i 13               draw deck  i         return 0        char   facecheck int d        static char   face                     Ace            Jack            Queen            King          if d    Ace          return face 0       else               if d    Jack               return face 1           else                       if d    Queen                  return face 2               else                                 if d    King                      return face 3                                     int draw int deck int i           int hand 5   j  temp j        for i 0  i lt 52  i                       j   i                       for i   0  i  lt  5  i                    deck i    hand               printf  A card has been drawn  n              deck i    temp j-1             temp j    deck i                           return deck

User · Answer

You don t really want to be reallocing memory every time you remove something  If you know the rough size of your deck then choose an appropriate size for your array and keep a pointer to the current end of the list  This is a stack   If you don t know the size of your deck  and think it could get really big as well as keeps changing size  then you will have to do something a little more complex and implement a linked-list   In C  you have two simple ways to declare an array    On the stack  as a static array  int myArray 16      Static array of 16 integers  On the heap  as a dynamically allocated array     Dynamically allocated array of 16 integers int  myArray   calloc 16  sizeof int       Standard C does not allow arrays of either of these types to be resized  You can either create a new array of a specific size  then copy the contents of the old array to the new one  or you can follow one of the suggestions above for a different abstract data type  ie  linked list  stack  queue  etc

User · Answer

Interestingly array is randomly accessible by the index  And removing randomly an element may impact the indexes of other elements as well       int remove element int from  int total  int index                if  total - index - 1   gt  0                          memmove from i  from i 1  sizeof int   total-index-1                              return total-1     return the new array size         Note that memcpy will not work in this case because of the overlapping memory    One of the efficient way  better than memory move  to remove one random element is swapping with the last element       int remove element int from  int total  int index                if index     total-1                       from index    from total-1               return total       DO NOT DECREASE   the total here         But the order is changed after the removal    Again if the removal is done in loop operation then the reordering may impact processing  Memory move is one expensive alternative to keep the order while removing an array element  Another of the way to keep the order while in a loop is to defer the removal  It can be done by validity array of the same size       int remove element int from  int total  int is valid  int index                is valid index    0              return total-1     return the number of elements         It will create a sparse array  Finally  the sparse array can be made compact that contains no two valid elements that contain invalid element between them  by doing some reordering       int sparse to compact int arr  int total  int is valid                int i   0              int last   total - 1                 trim the last invalid elements             for   last  gt   0  amp  amp   is valid last   last--      trim invalid elements from last                 now we keep swapping the invalid with last valid element             for i 0  i  lt  last  i                          if is valid i                               continue                      arr i    arr last      swap invalid with the last valid                     last--                      for   last  gt   0  amp  amp   is valid last   last--      trim invalid elements                           return last 1     return the compact length of the array

User · Answer

I  usually do this and works always      try this   for  i   res  i  lt   size-1  i            arrb i    arrb i   1       size    size - 1    in some ides size -- could give problems

User · Answer

What solution you need depends on whether you want your array to retain its order  or not  Generally  you never only have the array pointer  you also have a variable holding its current logical size  as well as a variable holding its allocated size  I m also assuming that the removeIndex is within the bounds of the array  With that given  the removal is simple  Order irrelevant array removeIndex    array --logicalSize    That s it  You simply copy the last array element over the element that is to be removed  decrementing the logicalSize of the array in the process  If removeIndex    logicalSize-1  i e  the last element is to be removed  this degrades into a self-assignment of that last element  but that is not a problem  Retaining order memmove array   removeIndex  array   removeIndex   1   --logicalSize - removeIndex  sizeof  array     A bit more complex  because now we need to call memmove   to perform the shifting of elements  but still a one-liner  Again  this also updates the logicalSize of the array in the process

User · Answer

There are really two separate issues   The first is keeping the elements of the array in proper order so that there are no  holes  after removing an element   The second is actually resizing the array itself   Arrays in C are allocated as a fixed number of contiguous elements   There is no way to actually remove the memory used by an individual element in the array but the elements can be shifted to fill the hole made by removing an element   For example   void remove element array type  array  int index  int array length       int i     for i   index  i  lt  array length - 1  i    array i    array i   1       Statically allocated arrays can not be resized   Dynamically allocated arrays can be resized with realloc     This will potentially move the entire array to another location in memory  so all pointers to the array or to its elements will have to be updated   For example   remove element array  index  array length       First shift the elements  then reallocate    array type  tmp   realloc array   array length - 1    sizeof array type     if  tmp    NULL  amp  amp  array length  gt  1          No memory available       exit EXIT FAILURE     array length   array length - 1  array   tmp    realloc will return a NULL pointer if the requested size is 0  or if there is an error   Otherwise it returns a pointer to the reallocated array   The temporary pointer is used to detect errors when calling realloc because instead of exiting it is also possible to just leave the original array as it was   When realloc fails to reallocate an array it does not alter the original array   Note that both of these operations will be fairly slow if the array is large or if a lot of elements are removed   There are other data structures like linked lists and hashes that can be used if efficient insertion and deletion is a priority

User · Answer

Try this simple code    include  lt stdio h gt   include  lt conio h gt  void main void         clrscr        int a 4   i  b      printf  enter nos         for  i   1  i  lt   5  i              scanf   d    amp a i              for i   1  i  lt   5  i              printf   n d   a i              printf   nenter element you want to delete         scanf   d    amp b       for  i   1  i  lt   5  i              if i    b                a i    i                      printf   n d   a i              getch

[c] Removing elements from an array in C

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