How to add element to C array

Question

I want to add an int into an array  but the problem is that I don t know what the index is now   int   arr   new int 15   arr 0    1  arr 1    2  arr 2    3  arr 3    4  arr 4    5    That code works because I know what index I am assigning to  but what if I don t know the index     In PHP  I can just do arr   22   which will automatically add 22 to the next empty index of the array  But in C   I can t do that  it gives me a compiler error  What do you guys suggest

User · Answer

You can use a variable to count places in the array  so when ever you add a new element  you put it in the right place  For example   int a   0  int arr 5         arr a    6  a

User · Answer

If you are writing in C   -- it is a way better to use data structures from standard library such as vector   C-style arrays are very error-prone  and should be avoided whenever possible

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Arrays in C   cannot change size at runtime  For that purpose  you should use vector lt int gt  instead   vector lt int gt  arr  arr push back 1   arr push back 2       arr size   will be the number of elements in the vector at the moment    As mentioned in the comments  vector is defined in vector header and std namespace  To use it  you should    include  lt vector gt   and also  either use std  vector in your code or add   using std  vector     or   using namespace std    after the  include  lt vector gt  line

User · Answer

Use a vector    include  lt vector gt   void foo         std  vector  lt int gt  v      v push back  1             equivalent to v 0    1

User · Answer

int arr     new int 15     The variable arr holds a memory address   At the memory address  there are 15 consecutive ints in a row   They can be referenced with index 0 to 14 inclusive       In php i can just do this arr   22    this will automatically add 22 to the   next empty index of array    There is no concept of  next  when dealing with arrays  One important thing that I think you are missing is that as soon as the array is created  all elements of the array already exist   They are uninitialized  but they all do exist already   So you aren t  filling  the elements of the array as you go  they are already filled  just with uninitialized values   There is no way to test for an uninitialized element in an array   It sounds like you want to use a data structure such as a queue or stack or vector

User · Answer

I may be missing the point of your question here  and if so I apologize  But  if you re not going to be deleting any items only adding them  why not simply assign a variable to the next empty slot  Every time you add a new value to the array  just increment the value to point to the next one   In C   a better solution is to use the standard library type std  list lt  type  gt   which also allows the array to grow dynamically  e g     include  lt list gt   std  list lt int gt  arr    for  int i   0  i  lt  10  i              add new value from 0 to 9 to next slot     arr push back i          add arbitrary value to the next free slot arr push back 22

User · Answer

Initialize all your array elements to null first  then look for the null to find the empty slot

User · Answer

I fully agree with the vector way when implementing a dynamic array  However  bear in mind that STL provides you with a host of containers that cater to different runtime requirements You should choose one with care  E g  For fast insertion at back you have the choice between a vector and a deque   And I almost forgot  with great power comes great responsibility  -  Since vectors are flexible in size  they often reallocate automagically to adjust for adding elements So beware about iterator invalidation  yes  it applies as well to pointers   However  as long as you are using operator   for accessing the individual elements you are safe

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You don t have to use vectors  If you want to stick with plain arrays  you can do something like this   int arr     new int 15   unsigned int arr length   0    Now  if you want to add an element to the end of the array  you can do this   if  arr length  lt  15      arr arr length       lt number gt     else        Handle a full array      It s not as short and graceful as the PHP equivalent  but it accomplishes what you were attempting to do  To allow you to easily change the size of the array in the future  you can use a  define    define ARRAY MAX 15  int arr     new int ARRAY MAX   unsigned int arr length   0   if  arr length  lt  ARRAY MAX      arr arr length       lt number gt     else        Handle a full array      This makes it much easier to manage the array in the future  By changing 15 to 100  the array size will be changed properly in the whole program  Note that you will have to set the array to the maximum expected size  as you can t change it once the program is compiled  For example  if you have an array of size 100  you could never insert 101 elements   If you will be using elements off the end of the array  you can do this   if  arr length  gt  0      int value   arr arr length--     else        Handle empty array      If you want to be able to delete elements off the beginning   ie a FIFO   the solution becomes more complicated  You need a beginning and end index as well    define ARRAY MAX 15  int arr     new int ARRAY MAX   unsigned int arr length   0  unsigned int arr start   0  unsigned int arr end   0      Insert number at end  if  arr length  lt  ARRAY MAX      arr arr end     lt number gt     arr end    arr end   1    ARRAY MAX    arr length       else        Handle a full array        Read number from beginning  if  arr length  gt  0      int value   arr arr start     arr start    arr start   1    ARRAY MAX    arr length --    else        Handle an empty array        Read number from end  if  arr length  gt  0      int value   arr arr end     arr end    arr end   ARRAY MAX - 1    ARRAY MAX    arr length --    else        Handle an empty array      Here  we are using the modulus operator     to cause the indexes to wrap  For example   99   1    100 is 0  a wrapping increment   And  99   99    100 is 98  a wrapping decrement   This allows you to avoid if statements and make the code more efficient   You can also quickly see how helpful the  define is as your code becomes more complex  Unfortunately  even with this solution  you could never insert over 100 items  or whatever maximum you set  in the array  You are also using 100 bytes of memory even if only 1 item is stored in the array   This is the primary reason why others have recommended vectors  A vector is managed behind the scenes and new memory is allocated as the structure expands  It is still not as efficient as an array in situations where the data size is already known  but for most purposes the performance differences will not be important  There are trade-offs to each approach and it s best to know both

User · Answer

Since I got so many negative feedback I realized my solution was wrong so i changed it   int arr 20     1 2 3 4 5   int index   5   void push int n       arr index    n      index       push 6    New array will contain value from 1 to 6 and you can make function for deletion like this as well

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There is no way to do what you say in C   with plain arrays  The C   solution for that is by using the STL library that gives you the std  vector  You can use a vector in this way   include  lt vector gt   std  vector lt  int  gt  arr   arr push back 1   arr push back 2   arr push back 3

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