How can I create a dynamically sized array of structs

Question

I know how to create an array of structs but with a predefined size  However is there a way to create a dynamic array of structs such that the array could get bigger   For example       typedef struct               char  str        words       main                 words x 100      I do not want to use this  I want to dynamic increase the size of the array as data comes in          Is this possible     I ve researched this  words  array    words  malloc sizeof words    100    I want to get rid of the 100 and store the data as it comes in  Thus if 76 fields of data comes in  I want to store 76 and not 100  I m assuming that I don t know how much data is coming into my program  In the struct I defined above I could create the first  index  as       words  array    words  malloc sizeof words      However I want to dynamically add elements to the array after  I hope I described the problem area clearly enough  The major challenge is to dynamically add a second field  at least that is the challenge for the moment     I ve made a little progress however       typedef struct           char  str        words          Allocate first string      words x    words  malloc sizeof words        x 0  str    john           Allocate second string      x  words   realloc x  sizeof words        x 1  FirstName    bob           printf second string      printf   s   x 1  str   -- gt  This is working  it s printing out bob       free x      Free up memory       printf   s   x 1  str   -- gt  Not working since its still printing out BOB even though I freed up memory  What is wrong    I did some error checking and this is what I found  If after I free up memory for x I add the following       x NULL    then if I try to print x I get an error which is what I want  So is it that the free function is not working  at least on my compiler  I m using DevC      Thanks  I understand now due to      FirstName is a pointer to an array of char which is not being allocated by the malloc  only the pointer is being allocated and after you call free  it doesn t erase the memory  it just marks it as available on the heap to be over written later      MattSmith   Update  I m trying to modularize and put the creation of my array of structs in a function but nothing seems to work  I m trying something very simple and I don t know what else to do  It s along the same lines as before  just another function  loaddata that is loading the data and outside the method I need to do some printing  How can I make it work  My code is as follows         include  lt stdio h gt        include  lt stdlib h gt        include  lt string h gt        include  lt ctype h gt       typedef struct               char  str1          char  str2        words       void LoadData words    int          main                 words  x          int num           LoadData  amp x   amp num            printf   s  s   x 0  str1  x 0  str2           printf   s  s   x 1  str1  x 1  str2            getch                 void LoadData words  x  int   num                x    words   malloc sizeof words             x 0  str1    johnnie 0           x 0  str2    krapson 0            x    words   realloc x  sizeof words  2           x 1  str1    bob 0           x 1  str2    marley 0             num  num 1            This simple test code is crashing and I have no idea why  Where is the bug

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If you want to dynamically allocate arrays, you can use malloc from stdlib.h.

If you want to allocate an array of 100 elements using your words struct, try the following:

words* array = (words*)malloc(sizeof(words) * 100);

The size of the memory that you want to allocate is passed into malloc and then it will return a pointer of type void (void*). In most cases you'll probably want to cast it to the pointer type you desire, which in this case is words*.

The sizeof keyword is used here to find out the size of the words struct, then that size is multiplied by the number of elements you want to allocate.

Once you are done, be sure to use free() to free up the heap memory you used in order to prevent memory leaks:

free(array);

If you want to change the size of the allocated array, you can try to use realloc as others have mentioned, but keep in mind that if you do many reallocs you may end up fragmenting the memory. If you want to dynamically resize the array in order to keep a low memory footprint for your program, it may be better to not do too many reallocs.

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Your code in the last update should not compile  much less run   You re passing  amp x to LoadData   amp x has the type of   words  but LoadData expects words    Of course it crashes when you call realloc on a pointer that s pointing into stack   The way to fix it is to change LoadData to accept words     Thi sway  you can actually modify the pointer in main    For example  realloc call would look like   x    words   realloc  x  sizeof words  2     It s the same principlae as in  num  being int  rather than int   Besides this  you need to really figure out how the strings in words ere stored  Assigning a const string to char    as in str2    marley 0   is permitted  but it s rarely the right solution  even in C   Another point  non need to have  marley 0  unless you really need two 0s at the end of string  Compiler adds 0 tho the end of every string literal

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If you want to dynamically allocate arrays, you can use malloc from stdlib.h.

If you want to allocate an array of 100 elements using your words struct, try the following:

words* array = (words*)malloc(sizeof(words) * 100);

The size of the memory that you want to allocate is passed into malloc and then it will return a pointer of type void (void*). In most cases you'll probably want to cast it to the pointer type you desire, which in this case is words*.

The sizeof keyword is used here to find out the size of the words struct, then that size is multiplied by the number of elements you want to allocate.

Once you are done, be sure to use free() to free up the heap memory you used in order to prevent memory leaks:

free(array);

If you want to change the size of the allocated array, you can try to use realloc as others have mentioned, but keep in mind that if you do many reallocs you may end up fragmenting the memory. If you want to dynamically resize the array in order to keep a low memory footprint for your program, it may be better to not do too many reallocs.

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Your code in the last update should not compile  much less run   You re passing  amp x to LoadData   amp x has the type of   words  but LoadData expects words    Of course it crashes when you call realloc on a pointer that s pointing into stack   The way to fix it is to change LoadData to accept words     Thi sway  you can actually modify the pointer in main    For example  realloc call would look like   x    words   realloc  x  sizeof words  2     It s the same principlae as in  num  being int  rather than int   Besides this  you need to really figure out how the strings in words ere stored  Assigning a const string to char    as in str2    marley 0   is permitted  but it s rarely the right solution  even in C   Another point  non need to have  marley 0  unless you really need two 0s at the end of string  Compiler adds 0 tho the end of every string literal

User · Answer

If you want to dynamically allocate arrays, you can use malloc from stdlib.h.

If you want to allocate an array of 100 elements using your words struct, try the following:

words* array = (words*)malloc(sizeof(words) * 100);

The size of the memory that you want to allocate is passed into malloc and then it will return a pointer of type void (void*). In most cases you'll probably want to cast it to the pointer type you desire, which in this case is words*.

The sizeof keyword is used here to find out the size of the words struct, then that size is multiplied by the number of elements you want to allocate.

Once you are done, be sure to use free() to free up the heap memory you used in order to prevent memory leaks:

free(array);

If you want to change the size of the allocated array, you can try to use realloc as others have mentioned, but keep in mind that if you do many reallocs you may end up fragmenting the memory. If you want to dynamically resize the array in order to keep a low memory footprint for your program, it may be better to not do too many reallocs.

User · Answer

In C    use a vector  It s like an array but you can easily add and remove elements and it will take care of allocating and deallocating memory for you   I know the title of the question says C  but you tagged your question with C and C

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This looks like an academic exercise which unfortunately makes it harder since you can t use C    Basically you have to manage some of the overhead for the allocation and keep track how much memory has been allocated if you need to resize it later  This is where the C   standard library shines   For your example  the following code allocates the memory and later resizes it      initial size int count   100  words  testWords    words   malloc count   sizeof words       resize the array count   76  testWords    words   realloc testWords  count  sizeof words      Keep in mind  in your example you are just allocating a pointer to a char and you still need to allocate the string itself and more importantly to free it at the end  So this code allocates 100 pointers to char and then resizes it to 76  but does not allocate the strings themselves   I have a suspicion that you actually want to allocate the number of characters in a string which is very similar to the above  but change word to char   EDIT  Also keep in mind it makes a lot of sense to create functions to perform common tasks and enforce consistency so you don t copy code everywhere  For example  you might have a  allocate the struct  b  assign values to the struct  and c  free the struct  So you might have      Allocate a words struct words  CreateWords int size      Assign a value void AssignWord word  dest  char  str      Clear a words structs  and possibly internal storage  void FreeWords words  w     EDIT  As far as resizing the structs  it is identical to resizing the char array  However the difference is if you make the struct array bigger  you should probably initialize the new array items to NULL  Likewise  if you make the struct array smaller  you need to cleanup before removing the items -- that is free items that have been allocated  and only the allocated items  before you resize the struct array  This is the primary reason I suggested creating helper functions to help manage this      Resize words  must know original and new size if shrinking    if you need to free internal storage first  void ResizeWords words  w  size t oldsize  size t newsize

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You ve tagged this as C   as well as C   If you re using C   things are a lot easier  The standard template library has a template called vector which allows you to dynamically build up a list of objects    include  lt stdio h gt   include  lt vector gt   typedef std  vector lt char  gt  words   int main int argc  char   argv             words myWords           myWords push back  Hello            myWords push back  World             words  iterator iter          for  iter   myWords begin    iter    myWords end      iter                    printf   s     iter                      return 0      If you re using C things are a lot harder  yes malloc  realloc and free are the tools to help you  You might want to consider using a linked list data structure instead  These are generally easier to grow but don t facilitate random access as easily    include  lt stdio h gt   include  lt stdlib h gt   typedef struct s words           char  str          struct s words  next    words   words  create words char  word            words  newWords   malloc sizeof words            if  NULL    newWords                   newWords- gt str   word                  newWords- gt next   NULL                    return newWords     void delete words words  oldWords            if  NULL    oldWords- gt next                    delete words oldWords- gt next                     free oldWords      words  add word words  wordList  char  word            words  newWords   create words word           if  NULL    newWords                    newWords- gt next   wordList                    return newWords     int main int argc  char   argv             words  myWords   create words  Hello            myWords   add word myWords   World             words  iter          for  iter   myWords  NULL    iter  iter   iter- gt next                    printf   s    iter- gt str                     delete words myWords           return 0      Yikes  sorry for the worlds longest answer  So WRT to the  don t want to use a linked list comment     include  lt stdio h gt     include  lt stdlib h gt   typedef struct       char   words      size t nWords      size t size      size t block size    word list   word list  create word list size t block size        word list  pWordList   malloc sizeof word list        if  NULL    pWordList            pWordList- gt nWords   0          pWordList- gt size   block size          pWordList- gt block size   block size          pWordList- gt words   malloc sizeof char   block size           if  NULL    pWordList- gt words                free pWordList               return NULL                          return pWordList     void delete word list word list  pWordList        free pWordList- gt words       free pWordList      int add word to word list word list  pWordList  char  word        size t nWords   pWordList- gt nWords      if  nWords  gt   pWordList- gt size            size t newSize   pWordList- gt size   pWordList- gt block size          void  newWords   realloc pWordList- gt words  sizeof char   newSize            if  NULL    newWords                return 0            else                   pWordList- gt size   newSize              pWordList- gt words    char   newWords                        pWordList- gt words nWords    word        pWordList- gt nWords        return 1     char   word list start word list  pWordList            return pWordList- gt words     char   word list end word list  pWordList            return  amp pWordList- gt words pWordList- gt nWords      int main int argc  char   argv             word list  myWords   create word list 2           add word to word list myWords   Hello            add word to word list myWords   World            add word to word list myWords   Goodbye             char   iter          for  iter   word list start myWords   iter    word list end myWords     iter                    printf   s     iter                      delete word list myWords            return 0

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You ve tagged this as C   as well as C   If you re using C   things are a lot easier  The standard template library has a template called vector which allows you to dynamically build up a list of objects    include  lt stdio h gt   include  lt vector gt   typedef std  vector lt char  gt  words   int main int argc  char   argv             words myWords           myWords push back  Hello            myWords push back  World             words  iterator iter          for  iter   myWords begin    iter    myWords end      iter                    printf   s     iter                      return 0      If you re using C things are a lot harder  yes malloc  realloc and free are the tools to help you  You might want to consider using a linked list data structure instead  These are generally easier to grow but don t facilitate random access as easily    include  lt stdio h gt   include  lt stdlib h gt   typedef struct s words           char  str          struct s words  next    words   words  create words char  word            words  newWords   malloc sizeof words            if  NULL    newWords                   newWords- gt str   word                  newWords- gt next   NULL                    return newWords     void delete words words  oldWords            if  NULL    oldWords- gt next                    delete words oldWords- gt next                     free oldWords      words  add word words  wordList  char  word            words  newWords   create words word           if  NULL    newWords                    newWords- gt next   wordList                    return newWords     int main int argc  char   argv             words  myWords   create words  Hello            myWords   add word myWords   World             words  iter          for  iter   myWords  NULL    iter  iter   iter- gt next                    printf   s    iter- gt str                     delete words myWords           return 0      Yikes  sorry for the worlds longest answer  So WRT to the  don t want to use a linked list comment     include  lt stdio h gt     include  lt stdlib h gt   typedef struct       char   words      size t nWords      size t size      size t block size    word list   word list  create word list size t block size        word list  pWordList   malloc sizeof word list        if  NULL    pWordList            pWordList- gt nWords   0          pWordList- gt size   block size          pWordList- gt block size   block size          pWordList- gt words   malloc sizeof char   block size           if  NULL    pWordList- gt words                free pWordList               return NULL                          return pWordList     void delete word list word list  pWordList        free pWordList- gt words       free pWordList      int add word to word list word list  pWordList  char  word        size t nWords   pWordList- gt nWords      if  nWords  gt   pWordList- gt size            size t newSize   pWordList- gt size   pWordList- gt block size          void  newWords   realloc pWordList- gt words  sizeof char   newSize            if  NULL    newWords                return 0            else                   pWordList- gt size   newSize              pWordList- gt words    char   newWords                        pWordList- gt words nWords    word        pWordList- gt nWords        return 1     char   word list start word list  pWordList            return pWordList- gt words     char   word list end word list  pWordList            return  amp pWordList- gt words pWordList- gt nWords      int main int argc  char   argv             word list  myWords   create word list 2           add word to word list myWords   Hello            add word to word list myWords   World            add word to word list myWords   Goodbye             char   iter          for  iter   word list start myWords   iter    word list end myWords     iter                    printf   s     iter                      delete word list myWords            return 0

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Your code in the last update should not compile  much less run   You re passing  amp x to LoadData   amp x has the type of   words  but LoadData expects words    Of course it crashes when you call realloc on a pointer that s pointing into stack   The way to fix it is to change LoadData to accept words     Thi sway  you can actually modify the pointer in main    For example  realloc call would look like   x    words   realloc  x  sizeof words  2     It s the same principlae as in  num  being int  rather than int   Besides this  you need to really figure out how the strings in words ere stored  Assigning a const string to char    as in str2    marley 0   is permitted  but it s rarely the right solution  even in C   Another point  non need to have  marley 0  unless you really need two 0s at the end of string  Compiler adds 0 tho the end of every string literal

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In C    use a vector  It s like an array but you can easily add and remove elements and it will take care of allocating and deallocating memory for you   I know the title of the question says C  but you tagged your question with C and C

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Another option for you is a linked list   You ll need to analyze how your program will use the data structure  if you don t need random access it could be faster than reallocating

User · Answer

If you want to dynamically allocate arrays, you can use malloc from stdlib.h.

If you want to allocate an array of 100 elements using your words struct, try the following:

words* array = (words*)malloc(sizeof(words) * 100);

The size of the memory that you want to allocate is passed into malloc and then it will return a pointer of type void (void*). In most cases you'll probably want to cast it to the pointer type you desire, which in this case is words*.

The sizeof keyword is used here to find out the size of the words struct, then that size is multiplied by the number of elements you want to allocate.

Once you are done, be sure to use free() to free up the heap memory you used in order to prevent memory leaks:

free(array);

If you want to change the size of the allocated array, you can try to use realloc as others have mentioned, but keep in mind that if you do many reallocs you may end up fragmenting the memory. If you want to dynamically resize the array in order to keep a low memory footprint for your program, it may be better to not do too many reallocs.

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Another option for you is a linked list   You ll need to analyze how your program will use the data structure  if you don t need random access it could be faster than reallocating

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Every coder need to simplify their code to make it easily understood    even for beginners   So array of structures using dynamically is easy  if you understand the concepts      Dynamically sized array of structures   include  lt stdio h gt   include  lt stdlib h gt   struct book        char name 20       int p                    Declaring book structure  int main           int n  i             struct book  b         Initializing pointer to a structure     scanf    d n    amp n        b    struct book    calloc  n  sizeof  struct book        Creating memory for array of structures dynamically      for  i   0  i  lt  n  i                  scanf    s  d n    b   i - gt name   amp  b   i - gt p      Getting values for array of structures  no error check                       for  i   0  i  lt  n  i                  printf    s  d t    b   i - gt name   b   i - gt p      Printing values in array of structures            scanf    d n    amp n         Get array size to re-allocate         b    struct book    realloc  b  n   sizeof  struct book       change the size of an array using realloc function     printf    n         for  i   0  i  lt  n  i                  printf    s  d t    b   i - gt name   b   i - gt p      Printing values in array of structures            return 0

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This looks like an academic exercise which unfortunately makes it harder since you can t use C    Basically you have to manage some of the overhead for the allocation and keep track how much memory has been allocated if you need to resize it later  This is where the C   standard library shines   For your example  the following code allocates the memory and later resizes it      initial size int count   100  words  testWords    words   malloc count   sizeof words       resize the array count   76  testWords    words   realloc testWords  count  sizeof words      Keep in mind  in your example you are just allocating a pointer to a char and you still need to allocate the string itself and more importantly to free it at the end  So this code allocates 100 pointers to char and then resizes it to 76  but does not allocate the strings themselves   I have a suspicion that you actually want to allocate the number of characters in a string which is very similar to the above  but change word to char   EDIT  Also keep in mind it makes a lot of sense to create functions to perform common tasks and enforce consistency so you don t copy code everywhere  For example  you might have a  allocate the struct  b  assign values to the struct  and c  free the struct  So you might have      Allocate a words struct words  CreateWords int size      Assign a value void AssignWord word  dest  char  str      Clear a words structs  and possibly internal storage  void FreeWords words  w     EDIT  As far as resizing the structs  it is identical to resizing the char array  However the difference is if you make the struct array bigger  you should probably initialize the new array items to NULL  Likewise  if you make the struct array smaller  you need to cleanup before removing the items -- that is free items that have been allocated  and only the allocated items  before you resize the struct array  This is the primary reason I suggested creating helper functions to help manage this      Resize words  must know original and new size if shrinking    if you need to free internal storage first  void ResizeWords words  w  size t oldsize  size t newsize

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Your code in the last update should not compile  much less run   You re passing  amp x to LoadData   amp x has the type of   words  but LoadData expects words    Of course it crashes when you call realloc on a pointer that s pointing into stack   The way to fix it is to change LoadData to accept words     Thi sway  you can actually modify the pointer in main    For example  realloc call would look like   x    words   realloc  x  sizeof words  2     It s the same principlae as in  num  being int  rather than int   Besides this  you need to really figure out how the strings in words ere stored  Assigning a const string to char    as in str2    marley 0   is permitted  but it s rarely the right solution  even in C   Another point  non need to have  marley 0  unless you really need two 0s at the end of string  Compiler adds 0 tho the end of every string literal

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For the test code  if you want to modify a pointer in a function  you should pass a  pointer to pointer  to the function  Corrected code is as follows    include  lt stdio h gt   include  lt stdlib h gt   include  lt ctype h gt   typedef struct       char  str1      char  str2    words   void LoadData words    int     main         words   x      int num       LoadData x   amp num        printf   s  s n     x 0   str1    x 0   str2       printf   s  s n     x 1   str1    x 1   str2      void LoadData words   x  int  num         x    words   malloc sizeof words           x 0   str1    johnnie 0         x 0   str2    krapson 0         x    words   realloc  x  sizeof words    2         x 1   str1    bob 0         x 1   str2    marley 0         num    num   1

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If you want to grow the array dynamically  you should use malloc   to dynamically allocate some fixed amount of memory  and then use realloc   whenever you run out  A common technique is to use an exponential growth function such that you allocate some small fixed amount and then make the array grow by duplicating the allocated amount    Some example code would be   size   64  i   0  x   malloc sizeof words  size      enough space for 64 words    while  read words          if    i  gt  size            size    2          x   realloc sizeof words    size              done with x    free x

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Here is how I would do it in C    size t size   500  char  dynamicAllocatedString   new char  size      Use same principal for any struct or c   class

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If you want to grow the array dynamically  you should use malloc   to dynamically allocate some fixed amount of memory  and then use realloc   whenever you run out  A common technique is to use an exponential growth function such that you allocate some small fixed amount and then make the array grow by duplicating the allocated amount    Some example code would be   size   64  i   0  x   malloc sizeof words  size      enough space for 64 words    while  read words          if    i  gt  size            size    2          x   realloc sizeof words    size              done with x    free x

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Here is how I would do it in C    size t size   500  char  dynamicAllocatedString   new char  size      Use same principal for any struct or c   class

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You ve tagged this as C   as well as C   If you re using C   things are a lot easier  The standard template library has a template called vector which allows you to dynamically build up a list of objects    include  lt stdio h gt   include  lt vector gt   typedef std  vector lt char  gt  words   int main int argc  char   argv             words myWords           myWords push back  Hello            myWords push back  World             words  iterator iter          for  iter   myWords begin    iter    myWords end      iter                    printf   s     iter                      return 0      If you re using C things are a lot harder  yes malloc  realloc and free are the tools to help you  You might want to consider using a linked list data structure instead  These are generally easier to grow but don t facilitate random access as easily    include  lt stdio h gt   include  lt stdlib h gt   typedef struct s words           char  str          struct s words  next    words   words  create words char  word            words  newWords   malloc sizeof words            if  NULL    newWords                   newWords- gt str   word                  newWords- gt next   NULL                    return newWords     void delete words words  oldWords            if  NULL    oldWords- gt next                    delete words oldWords- gt next                     free oldWords      words  add word words  wordList  char  word            words  newWords   create words word           if  NULL    newWords                    newWords- gt next   wordList                    return newWords     int main int argc  char   argv             words  myWords   create words  Hello            myWords   add word myWords   World             words  iter          for  iter   myWords  NULL    iter  iter   iter- gt next                    printf   s    iter- gt str                     delete words myWords           return 0      Yikes  sorry for the worlds longest answer  So WRT to the  don t want to use a linked list comment     include  lt stdio h gt     include  lt stdlib h gt   typedef struct       char   words      size t nWords      size t size      size t block size    word list   word list  create word list size t block size        word list  pWordList   malloc sizeof word list        if  NULL    pWordList            pWordList- gt nWords   0          pWordList- gt size   block size          pWordList- gt block size   block size          pWordList- gt words   malloc sizeof char   block size           if  NULL    pWordList- gt words                free pWordList               return NULL                          return pWordList     void delete word list word list  pWordList        free pWordList- gt words       free pWordList      int add word to word list word list  pWordList  char  word        size t nWords   pWordList- gt nWords      if  nWords  gt   pWordList- gt size            size t newSize   pWordList- gt size   pWordList- gt block size          void  newWords   realloc pWordList- gt words  sizeof char   newSize            if  NULL    newWords                return 0            else                   pWordList- gt size   newSize              pWordList- gt words    char   newWords                        pWordList- gt words nWords    word        pWordList- gt nWords        return 1     char   word list start word list  pWordList            return pWordList- gt words     char   word list end word list  pWordList            return  amp pWordList- gt words pWordList- gt nWords      int main int argc  char   argv             word list  myWords   create word list 2           add word to word list myWords   Hello            add word to word list myWords   World            add word to word list myWords   Goodbye             char   iter          for  iter   word list start myWords   iter    word list end myWords     iter                    printf   s     iter                      delete word list myWords            return 0

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In C    use a vector  It s like an array but you can easily add and remove elements and it will take care of allocating and deallocating memory for you   I know the title of the question says C  but you tagged your question with C and C

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You ve tagged this as C   as well as C   If you re using C   things are a lot easier  The standard template library has a template called vector which allows you to dynamically build up a list of objects    include  lt stdio h gt   include  lt vector gt   typedef std  vector lt char  gt  words   int main int argc  char   argv             words myWords           myWords push back  Hello            myWords push back  World             words  iterator iter          for  iter   myWords begin    iter    myWords end      iter                    printf   s     iter                      return 0      If you re using C things are a lot harder  yes malloc  realloc and free are the tools to help you  You might want to consider using a linked list data structure instead  These are generally easier to grow but don t facilitate random access as easily    include  lt stdio h gt   include  lt stdlib h gt   typedef struct s words           char  str          struct s words  next    words   words  create words char  word            words  newWords   malloc sizeof words            if  NULL    newWords                   newWords- gt str   word                  newWords- gt next   NULL                    return newWords     void delete words words  oldWords            if  NULL    oldWords- gt next                    delete words oldWords- gt next                     free oldWords      words  add word words  wordList  char  word            words  newWords   create words word           if  NULL    newWords                    newWords- gt next   wordList                    return newWords     int main int argc  char   argv             words  myWords   create words  Hello            myWords   add word myWords   World             words  iter          for  iter   myWords  NULL    iter  iter   iter- gt next                    printf   s    iter- gt str                     delete words myWords           return 0      Yikes  sorry for the worlds longest answer  So WRT to the  don t want to use a linked list comment     include  lt stdio h gt     include  lt stdlib h gt   typedef struct       char   words      size t nWords      size t size      size t block size    word list   word list  create word list size t block size        word list  pWordList   malloc sizeof word list        if  NULL    pWordList            pWordList- gt nWords   0          pWordList- gt size   block size          pWordList- gt block size   block size          pWordList- gt words   malloc sizeof char   block size           if  NULL    pWordList- gt words                free pWordList               return NULL                          return pWordList     void delete word list word list  pWordList        free pWordList- gt words       free pWordList      int add word to word list word list  pWordList  char  word        size t nWords   pWordList- gt nWords      if  nWords  gt   pWordList- gt size            size t newSize   pWordList- gt size   pWordList- gt block size          void  newWords   realloc pWordList- gt words  sizeof char   newSize            if  NULL    newWords                return 0            else                   pWordList- gt size   newSize              pWordList- gt words    char   newWords                        pWordList- gt words nWords    word        pWordList- gt nWords        return 1     char   word list start word list  pWordList            return pWordList- gt words     char   word list end word list  pWordList            return  amp pWordList- gt words pWordList- gt nWords      int main int argc  char   argv             word list  myWords   create word list 2           add word to word list myWords   Hello            add word to word list myWords   World            add word to word list myWords   Goodbye             char   iter          for  iter   word list start myWords   iter    word list end myWords     iter                    printf   s     iter                      delete word list myWords            return 0

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This looks like an academic exercise which unfortunately makes it harder since you can t use C    Basically you have to manage some of the overhead for the allocation and keep track how much memory has been allocated if you need to resize it later  This is where the C   standard library shines   For your example  the following code allocates the memory and later resizes it      initial size int count   100  words  testWords    words   malloc count   sizeof words       resize the array count   76  testWords    words   realloc testWords  count  sizeof words      Keep in mind  in your example you are just allocating a pointer to a char and you still need to allocate the string itself and more importantly to free it at the end  So this code allocates 100 pointers to char and then resizes it to 76  but does not allocate the strings themselves   I have a suspicion that you actually want to allocate the number of characters in a string which is very similar to the above  but change word to char   EDIT  Also keep in mind it makes a lot of sense to create functions to perform common tasks and enforce consistency so you don t copy code everywhere  For example  you might have a  allocate the struct  b  assign values to the struct  and c  free the struct  So you might have      Allocate a words struct words  CreateWords int size      Assign a value void AssignWord word  dest  char  str      Clear a words structs  and possibly internal storage  void FreeWords words  w     EDIT  As far as resizing the structs  it is identical to resizing the char array  However the difference is if you make the struct array bigger  you should probably initialize the new array items to NULL  Likewise  if you make the struct array smaller  you need to cleanup before removing the items -- that is free items that have been allocated  and only the allocated items  before you resize the struct array  This is the primary reason I suggested creating helper functions to help manage this      Resize words  must know original and new size if shrinking    if you need to free internal storage first  void ResizeWords words  w  size t oldsize  size t newsize

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If you want to grow the array dynamically  you should use malloc   to dynamically allocate some fixed amount of memory  and then use realloc   whenever you run out  A common technique is to use an exponential growth function such that you allocate some small fixed amount and then make the array grow by duplicating the allocated amount    Some example code would be   size   64  i   0  x   malloc sizeof words  size      enough space for 64 words    while  read words          if    i  gt  size            size    2          x   realloc sizeof words    size              done with x    free x

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If you want to grow the array dynamically  you should use malloc   to dynamically allocate some fixed amount of memory  and then use realloc   whenever you run out  A common technique is to use an exponential growth function such that you allocate some small fixed amount and then make the array grow by duplicating the allocated amount    Some example code would be   size   64  i   0  x   malloc sizeof words  size      enough space for 64 words    while  read words          if    i  gt  size            size    2          x   realloc sizeof words    size              done with x    free x

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This looks like an academic exercise which unfortunately makes it harder since you can t use C    Basically you have to manage some of the overhead for the allocation and keep track how much memory has been allocated if you need to resize it later  This is where the C   standard library shines   For your example  the following code allocates the memory and later resizes it      initial size int count   100  words  testWords    words   malloc count   sizeof words       resize the array count   76  testWords    words   realloc testWords  count  sizeof words      Keep in mind  in your example you are just allocating a pointer to a char and you still need to allocate the string itself and more importantly to free it at the end  So this code allocates 100 pointers to char and then resizes it to 76  but does not allocate the strings themselves   I have a suspicion that you actually want to allocate the number of characters in a string which is very similar to the above  but change word to char   EDIT  Also keep in mind it makes a lot of sense to create functions to perform common tasks and enforce consistency so you don t copy code everywhere  For example  you might have a  allocate the struct  b  assign values to the struct  and c  free the struct  So you might have      Allocate a words struct words  CreateWords int size      Assign a value void AssignWord word  dest  char  str      Clear a words structs  and possibly internal storage  void FreeWords words  w     EDIT  As far as resizing the structs  it is identical to resizing the char array  However the difference is if you make the struct array bigger  you should probably initialize the new array items to NULL  Likewise  if you make the struct array smaller  you need to cleanup before removing the items -- that is free items that have been allocated  and only the allocated items  before you resize the struct array  This is the primary reason I suggested creating helper functions to help manage this      Resize words  must know original and new size if shrinking    if you need to free internal storage first  void ResizeWords words  w  size t oldsize  size t newsize

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Every coder need to simplify their code to make it easily understood    even for beginners   So array of structures using dynamically is easy  if you understand the concepts      Dynamically sized array of structures   include  lt stdio h gt   include  lt stdlib h gt   struct book        char name 20       int p                    Declaring book structure  int main           int n  i             struct book  b         Initializing pointer to a structure     scanf    d n    amp n        b    struct book    calloc  n  sizeof  struct book        Creating memory for array of structures dynamically      for  i   0  i  lt  n  i                  scanf    s  d n    b   i - gt name   amp  b   i - gt p      Getting values for array of structures  no error check                       for  i   0  i  lt  n  i                  printf    s  d t    b   i - gt name   b   i - gt p      Printing values in array of structures            scanf    d n    amp n         Get array size to re-allocate         b    struct book    realloc  b  n   sizeof  struct book       change the size of an array using realloc function     printf    n         for  i   0  i  lt  n  i                  printf    s  d t    b   i - gt name   b   i - gt p      Printing values in array of structures            return 0

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Another option for you is a linked list   You ll need to analyze how your program will use the data structure  if you don t need random access it could be faster than reallocating

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In C    use a vector  It s like an array but you can easily add and remove elements and it will take care of allocating and deallocating memory for you   I know the title of the question says C  but you tagged your question with C and C

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Another option for you is a linked list   You ll need to analyze how your program will use the data structure  if you don t need random access it could be faster than reallocating

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For the test code  if you want to modify a pointer in a function  you should pass a  pointer to pointer  to the function  Corrected code is as follows    include  lt stdio h gt   include  lt stdlib h gt   include  lt ctype h gt   typedef struct       char  str1      char  str2    words   void LoadData words    int     main         words   x      int num       LoadData x   amp num        printf   s  s n     x 0   str1    x 0   str2       printf   s  s n     x 1   str1    x 1   str2      void LoadData words   x  int  num         x    words   malloc sizeof words           x 0   str1    johnnie 0         x 0   str2    krapson 0         x    words   realloc  x  sizeof words    2         x 1   str1    bob 0         x 1   str2    marley 0         num    num   1

[c] How can I create a dynamically sized array of structs?

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