C dynamically growing array

I can use pointers, but I am a bit afraid of using them.

If you need a dynamic array, you can't escape pointers. Why are you afraid though? They won't bite (as long as you're careful, that is). There's no built-in dynamic array in C, you'll just have to write one yourself. In C++, you can use the built-in std::vector class. C# and just about every other high-level language also have some similar class that manages dynamic arrays for you.

If you do plan to write your own, here's something to get you started: most dynamic array implementations work by starting off with an array of some (small) default size, then whenever you run out of space when adding a new element, double the size of the array. As you can see in the example below, it's not very difficult at all: (I've omitted safety checks for brevity)

typedef struct {
  int *array;
  size_t used;
  size_t size;
} Array;

void initArray(Array *a, size_t initialSize) {
  a->array = malloc(initialSize * sizeof(int));
  a->used = 0;
  a->size = initialSize;
}

void insertArray(Array *a, int element) {
  // a->used is the number of used entries, because a->array[a->used++] updates a->used only *after* the array has been accessed.
  // Therefore a->used can go up to a->size 
  if (a->used == a->size) {
    a->size *= 2;
    a->array = realloc(a->array, a->size * sizeof(int));
  }
  a->array[a->used++] = element;
}

void freeArray(Array *a) {
  free(a->array);
  a->array = NULL;
  a->used = a->size = 0;
}

Using it is just as simple:

Array a;
int i;

initArray(&a, 5);  // initially 5 elements
for (i = 0; i < 100; i++)
  insertArray(&a, i);  // automatically resizes as necessary
printf("%d\n", a.array[9]);  // print 10th element
printf("%d\n", a.used);  // print number of elements
freeArray(&a);

---

One simple solution involves mmap. This is great if you can tolerate a POSIX solution. Just map a whole page and guard against overflows, since realloc would fail for such values anyway. Modern OSes won't commit to the whole lot until you use it, and you can truncate files if you want.

Alternatively, there's realloc. As with everything that seems scarier at first than it was later, the best way to get over the initial fear is to immerse yourself into the discomfort of the unknown! It is at times like that which we learn the most, after all.

Unfortunately, there are limitations. While you're still learning to use a function, you shouldn't assume the role of a teacher, for example. I often read answers from those who seemingly don't know how to use realloc (i.e. the currently accepted answer!) telling others how to use it incorrectly, occasionally under the guise that they've omitted error handling, even though this is a common pitfall which needs mention. Here's an answer explaining how to use realloc correctly. Take note that the answer is storing the return value into a different variable in order to perform error checking.

Every time you call a function, and every time you use an array, you are using a pointer. The conversions are occurring implicitly, which if anything should be even scarier, as it's the things we don't see which often cause the most problems. For example, memory leaks...

Array operators are pointer operators. array[x] is really a shortcut for *(array + x), which can be broken down into: * and (array + x). It's most likely that the * is what confuses you. We can further eliminate the addition from the problem by assuming x to be 0, thus, array[0] becomes *array because adding 0 won't change the value...

... and thus we can see that *array is equivalent to array[0]. You can use one where you want to use the other, and vice versa. Array operators are pointer operators.

malloc, realloc and friends don't invent the concept of a pointer which you've been using all along; they merely use this to implement some other feature, which is a different form of storage duration, most suitable when you desire drastic, dynamic changes in size.

It is a shame that the currently accepted answer also goes against the grain of some other very well-founded advice on StackOverflow, and at the same time, misses an opportunity to introduce a little-known feature which shines for exactly this usecase: flexible array members! That's actually a pretty broken answer... :(

When you define your struct, declare your array at the end of the structure, without any upper bound. For example:

struct int_list {
    size_t size;
    int value[];
};

This will allow you to unite your array of int into the same allocation as your count, and having them bound like this can be very handy!

sizeof (struct int_list) will act as though value has a size of 0, so it'll tell you the size of the structure with an empty list. You still need to add to the size passed to realloc to specify the size of your list.

Another handy tip is to remember that realloc(NULL, x) is equivalent to malloc(x), and we can use this to simplify our code. For example:

int push_back(struct int_list **fubar, int value) {
    size_t x = *fubar ? fubar[0]->size : 0
         , y = x + 1;

    if ((x & y) == 0) {
        void *temp = realloc(*fubar, sizeof **fubar
                                   + (x + y) * sizeof fubar[0]->value[0]);
        if (!temp) { return 1; }
        *fubar = temp; // or, if you like, `fubar[0] = temp;`
    }

    fubar[0]->value[x] = value;
    fubar[0]->size = y;
    return 0;
}

struct int_list *array = NULL;

The reason I chose to use struct int_list ** as the first argument may not seem immediately obvious, but if you think about the second argument, any changes made to value from within push_back would not be visible to the function we're calling from, right? The same goes for the first argument, and we need to be able to modify our array, not just here but possibly also in any other function/s we pass it to...

array starts off pointing at nothing; it is an empty list. Initialising it is the same as adding to it. For example:

struct int_list *array = NULL;
if (!push_back(&array, 42)) {
    // success!
}

P.S. Remember to free(array); when you're done with it!

---

There are a couple of options I can think of.

Linked List. You can use a linked list to make a dynamically growing array like thing. But you won't be able to do array[100] without having to walk through 1-99 first. And it might not be that handy for you to use either. Large array. Simply create an array with more than enough space for everything Resizing array. Recreate the array once you know the size and/or create a new array every time you run out of space with some margin and copy all the data to the new array. Linked List Array combination. Simply use an array with a fixed size and once you run out of space, create a new array and link to that (it would be wise to keep track of the array and the link to the next array in a struct).

It is hard to say what option would be best in your situation. Simply creating a large array is ofcourse one of the easiest solutions and shouldn't give you much problems unless it's really large.

---

Building on Matteo Furlans design, when he said "most dynamic array implementations work by starting off with an array of some (small) default size, then whenever you run out of space when adding a new element, double the size of the array". The difference in the "work in progress" below is that it doesn't double in size, it aims at using only what is required. I have also omitted safety checks for simplicity...Also building on brimboriums idea, I have tried to add a delete function to the code...

The storage.h file looks like this...

#ifndef STORAGE_H
#define STORAGE_H

#ifdef __cplusplus
extern "C" {
#endif

    typedef struct 
    {
        int *array;
        size_t size;
    } Array;

    void Array_Init(Array *array);
    void Array_Add(Array *array, int item);
    void Array_Delete(Array *array, int index);
    void Array_Free(Array *array);

#ifdef __cplusplus
}
#endif

#endif /* STORAGE_H */

The storage.c file looks like this...

#include <stdio.h>
#include <stdlib.h>
#include "storage.h"

/* Initialise an empty array */
void Array_Init(Array *array) 
{
    int *int_pointer;

    int_pointer = (int *)malloc(sizeof(int));

    if (int_pointer == NULL)
    {       
        printf("Unable to allocate memory, exiting.\n");
        free(int_pointer);
        exit(0);
    }
    else
    {
        array->array = int_pointer; 
        array->size = 0;
    }
}

/* Dynamically add to end of an array */
void Array_Add(Array *array, int item) 
{
    int *int_pointer;

    array->size += 1;

    int_pointer = (int *)realloc(array->array, array->size * sizeof(int));

    if (int_pointer == NULL)
    {       
        printf("Unable to reallocate memory, exiting.\n");
        free(int_pointer);
        exit(0);
    }
    else
    {
        array->array = int_pointer;
        array->array[array->size-1] = item;
    }
}

/* Delete from a dynamic array */
void Array_Delete(Array *array, int index) 
{
    int i;
    Array temp;
    int *int_pointer;

    Array_Init(&temp);

    for(i=index; i<array->size; i++)
    {
        array->array[i] = array->array[i + 1];
    }

    array->size -= 1;

    for (i = 0; i < array->size; i++)
    {
        Array_Add(&temp, array->array[i]);
    }

    int_pointer = (int *)realloc(temp.array, temp.size * sizeof(int));

    if (int_pointer == NULL)
    {       
        printf("Unable to reallocate memory, exiting.\n");
        free(int_pointer);
        exit(0);
    }
    else
    {
        array->array = int_pointer; 
    } 
}

/* Free an array */
void Array_Free(Array *array) 
{
  free(array->array);
  array->array = NULL;
  array->size = 0;  
}

The main.c looks like this...

#include <stdio.h>
#include <stdlib.h>
#include "storage.h"

int main(int argc, char** argv) 
{
    Array pointers;
    int i;

    Array_Init(&pointers);

    for (i = 0; i < 60; i++)
    {
        Array_Add(&pointers, i);        
    }

    Array_Delete(&pointers, 3);

    Array_Delete(&pointers, 6);

    Array_Delete(&pointers, 30);

    for (i = 0; i < pointers.size; i++)
    {        
        printf("Value: %d Size:%d \n", pointers.array[i], pointers.size);
    }

    Array_Free(&pointers);

    return (EXIT_SUCCESS);
}

Look forward to the constructive criticism to follow...

---

When you're saying

make an array holding an index number (int) of an indeterminate number of entities

you're basically saying you're using "pointers", but one which is a array-wide local pointer instead of memory-wide pointer. Since you're conceptually already using "pointers" (i.e. id numbers that refers to an element in an array), why don't you just use regular pointers (i.e. id numbers that refers to an element in the biggest array: the whole memory).

Instead of your objects storing a resource id numbers, you can make them store a pointer instead. Basically the same thing, but much more efficient since we avoid turning "array + index" into a "pointer".

Pointers are not scary if you think of them as array index for the whole memory (which is what they actually are)

---

To create an array of unlimited items of any sort of type:

typedef struct STRUCT_SS_VECTOR {
    size_t size;
    void** items;
} ss_vector;


ss_vector* ss_init_vector(size_t item_size) {
    ss_vector* vector;
    vector = malloc(sizeof(ss_vector));
    vector->size = 0;
    vector->items = calloc(0, item_size);

    return vector;
}

void ss_vector_append(ss_vector* vec, void* item) {
    vec->size++;
    vec->items = realloc(vec->items, vec->size * sizeof(item));
    vec->items[vec->size - 1] = item;
};

void ss_vector_free(ss_vector* vec) {
    for (int i = 0; i < vec->size; i++)
        free(vec->items[i]);

    free(vec->items);
    free(vec);
}

and how to use it:

// defining some sort of struct, can be anything really
typedef struct APPLE_STRUCT {
    int id;
} apple;

apple* init_apple(int id) {
    apple* a;
    a = malloc(sizeof(apple));
    a-> id = id;
    return a;
};


int main(int argc, char* argv[]) {
    ss_vector* vector = ss_init_vector(sizeof(apple));

    // inserting some items
    for (int i = 0; i < 10; i++)
        ss_vector_append(vector, init_apple(i));


    // dont forget to free it
    ss_vector_free(vector);

    return 0;
}

This vector/array can hold any type of item and it is completely dynamic in size.

---

Well, I guess if you need to remove an element you will make a copy of the array despising the element to be excluded.

// inserting some items
void* element_2_remove = getElement2BRemove();

for (int i = 0; i < vector->size; i++){
       if(vector[i]!=element_2_remove) copy2TempVector(vector[i]);
       }

free(vector->items);
free(vector);
fillFromTempVector(vector);
//

Assume that getElement2BRemove(), copy2TempVector( void* ...) and fillFromTempVector(...) are auxiliary methods to handle the temp vector.

---

These posts apparently are in the wrong order! This is #1 in a series of 3 posts. Sorry.

In attempting to use Lie Ryan's code, I had problems retrieving stored information. The vector's elements are not stored contiguously,as you can see by "cheating" a bit and storing the pointer to each element's address (which of course defeats the purpose of the dynamic array concept) and examining them.

With a bit of tinkering, via:

ss_vector* vector; // pull this out to be a global vector

// Then add the following to attempt to recover stored values.

int return_id_value(int i,apple* aa) // given ptr to component,return data item
{   printf("showing apple[%i].id = %i and  other_id=%i\n",i,aa->id,aa->other_id);
    return(aa->id);
}

int Test(void)  // Used to be "main" in the example
{   apple* aa[10]; // stored array element addresses
    vector = ss_init_vector(sizeof(apple));
    // inserting some items
    for (int i = 0; i < 10; i++)
    {   aa[i]=init_apple(i);
        printf("apple id=%i and  other_id=%i\n",aa[i]->id,aa[i]->other_id);
        ss_vector_append(vector, aa[i]);
     }   
 // report the number of components
 printf("nmbr of components in vector = %i\n",(int)vector->size);
 printf(".*.*array access.*.component[5] = %i\n",return_id_value(5,aa[5]));
 printf("components of size %i\n",(int)sizeof(apple));
 printf("\n....pointer initial access...component[0] = %i\n",return_id_value(0,(apple *)&vector[0]));
 //.............etc..., followed by
 for (int i = 0; i < 10; i++)
 {   printf("apple[%i].id = %i at address %i, delta=%i\n",i,    return_id_value(i,aa[i]) ,(int)aa[i],(int)(aa[i]-aa[i+1]));
 }   
// don't forget to free it
ss_vector_free(vector);
return 0;
}

It's possible to access each array element without problems, as long as you know its address, so I guess I'll try adding a "next" element and use this as a linked list. Surely there are better options, though. Please advise.

---

These posts apparently are in the wrong order! This is #3 in a series of 3 posts. Sorry.

I've "taken a few MORE liberties" with Lie Ryan's code. The linked list admittedly was time-consuming to access individual elements due to search overhead, i.e. walking down the list until you find the right element. I have now cured this by maintaining an address vector containing subscripts 0 through whatever paired with memory addresses. This works because the address vector is allocated all-at-once, thus contiguous in memory. Since the linked-list is no longer required, I've ripped out its associated code and structure.

This approach is not quite as efficient as a plain-and-simple static array would be, but at least you don't have to "walk the list" searching for the proper item. You can now access the elements by using a subscript. To enable this, I have had to add code to handle cases where elements are removed and the "actual" subscripts wouldn't be reflected in the pointer vector's subscripts. This may or may not be important to users. For me, it IS important, so I've made re-numbering of subscripts optional. If renumbering is not used, program flow goes to a dummy "missing" element which returns an error code, which users can choose to ignore or to act on as required.

From here, I'd advise users to code the "elements" portion to fit their needs and make sure that it runs correctly. If your added elements are arrays, carefully code subroutines to access them, seeing as how there's extra array structure that wasn't needed with static arrays. Enjoy!

#include <glib.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>


// Code from https://stackoverflow.com/questions/3536153/c-dynamically-growing-array
// For pointer-to-pointer info see:
// https://stackoverflow.com/questions/897366/how-do-pointer-to-pointers-work-in-c-and-when-might-you-use-them
typedef struct STRUCT_SS_VECTOR
{   size_t size; // # of vector elements
    void** items; // makes up one vector element's component contents
    int subscript; // this element's subscript nmbr, 0 thru whatever
 //   struct STRUCT_SS_VECTOR* this_element; // linked list via this ptr
 //   struct STRUCT_SS_VECTOR* next_element; // and next ptr
} ss_vector;

ss_vector* vector; // ptr to vector of components
ss_vector* missing_element(int subscript) // intercepts missing elements
{   printf("missing element at subscript %i\n",subscript);
    return NULL;
}

typedef struct TRACKER_VECTOR
{   int subscript;
    ss_vector* vector_ptr;
} tracker_vector;  // up to 20 or so, max suggested

tracker_vector* tracker;
int max_tracker=0; // max allowable # of elements in "tracker_vector"
int tracker_count=0; // current # of elements in "tracker_vector"
int tracker_increment=5; // # of elements to add at each expansion

void bump_tracker_vector(int new_tracker_count)
{   //init or lengthen tracker vector
    if(max_tracker==0) // not yet initialized
    { tracker=calloc(tracker_increment, sizeof(tracker_vector));
        max_tracker=tracker_increment;
printf("initialized %i-element tracker vector of size %lu at %lu\n",max_tracker,sizeof(tracker_vector),(size_t)tracker);
        tracker_count++;
        return;
    }
    else if (max_tracker<=tracker_count) // append to existing tracker vector by writing a new one, copying old one
    {   tracker_vector* temp_tracker=calloc(max_tracker+tracker_increment,sizeof(tracker_vector));  
        for(int i=0;(i<max_tracker);i++){   temp_tracker[i]=tracker[i];} // copy old tracker to new
        max_tracker=max_tracker+tracker_increment;
        free(tracker);
        tracker=temp_tracker;
printf("  re-initialized %i-element tracker vector of size %lu at %lu\n",max_tracker,sizeof(tracker_vector),(size_t)tracker);
        tracker_count++;
        return;
    } // else if
    // fall through for most "bumps"
    tracker_count++;
    return;
}  // bump_tracker_vector()

ss_vector* ss_init_vector(size_t item_size) // item_size is size of one array member
{   ss_vector* vector= malloc(sizeof(ss_vector)); 
    vector->size = 0; // initialize count of vector component elements
    vector->items = calloc(1, item_size); // allocate & zero out memory for one linked list element
    vector->subscript=0;
    bump_tracker_vector(0); // init/store the tracker vector
    tracker[0].subscript=0;
    tracker[0].vector_ptr=vector; 
    return vector; //->this_element;
} // ss_init_vector()

ss_vector* ss_vector_append( int i) // ptr to this element, element nmbr
{   ss_vector* local_vec_element=0;
    local_vec_element= calloc(1,sizeof(ss_vector)); // memory for one component
    local_vec_element->subscript=i; //vec_element->size; 
    local_vec_element->size=i; // increment # of vector components
    bump_tracker_vector(i);  // increment/store tracker vector
    tracker[i].subscript=i;
    tracker[i].vector_ptr=local_vec_element; //->this_element;
    return local_vec_element;
}  // ss_vector_append()

void bubble_sort(void)
{   //  bubble sort
    struct TRACKER_VECTOR local_tracker;
    int i=0;
    while(i<tracker_count-1)
    {   if(tracker[i].subscript>tracker[i+1].subscript)
        {   local_tracker.subscript=tracker[i].subscript; // swap tracker elements
            local_tracker.vector_ptr=tracker[i].vector_ptr;
            tracker[i].subscript=tracker[i+1].subscript;
            tracker[i].vector_ptr=tracker[i+1].vector_ptr;
            tracker[i+1].subscript=local_tracker.subscript;
            tracker[i+1].vector_ptr=local_tracker.vector_ptr;
            if(i>0) i--; // step back and go again
        }
        else 
        {   if(i<tracker_count-1) i++;
        }
    } // while()
} // void bubble_sort()

void move_toward_zero(int target_subscript) // toward zero
{   struct TRACKER_VECTOR local_tracker;
    // Target to be moved must range from 1 to max_tracker
    if((target_subscript<1)||(target_subscript>tracker_count)) return; // outside range
    // swap target_subscript ptr and target_subscript-1 ptr
    local_tracker.vector_ptr=tracker[target_subscript].vector_ptr;
    tracker[target_subscript].vector_ptr=tracker[target_subscript-1].vector_ptr;
    tracker[target_subscript-1].vector_ptr=local_tracker.vector_ptr;
}

void renumber_all_subscripts(gboolean arbitrary)
{   // assumes tracker_count has been fixed and tracker[tracker_count+1]has been zeroed out
    if(arbitrary)  // arbitrary renumber, ignoring "true" subscripts
    {   for(int i=0;i<tracker_count;i++) 
        {   tracker[i].subscript=i;}
    }
    else // use "true" subscripts, holes and all
    {   for(int i=0;i<tracker_count;i++) 
        {   if ((size_t)tracker[i].vector_ptr!=0) // renumbering "true" subscript tracker & vector_element
            {   tracker[i].subscript=tracker[i].vector_ptr->subscript;}
            else // renumbering "true" subscript tracker & NULL vector_element
            {   tracker[i].subscript=-1;}
        } // for()
        bubble_sort(); 
    } // if(arbitrary) ELSE
} // renumber_all_subscripts()

void collapse_tracker_higher_elements(int target_subscript)
{   // Fix tracker vector by collapsing higher subscripts toward 0.
    //  Assumes last tracker element entry is discarded.
    int j;
    for(j=target_subscript;(j<tracker_count-1);j++)
    {   tracker[j].subscript=tracker[j+1].subscript;
        tracker[j].vector_ptr=tracker[j+1].vector_ptr;
    }
    // Discard last tracker element and adjust count
    tracker_count--;
    tracker[tracker_count].subscript=0;
    tracker[tracker_count].vector_ptr=(size_t)0;
} // void collapse_tracker_higher_elements()

void ss_vector_free_one_element(int target_subscript, gboolean Keep_subscripts) 
{   // Free requested element contents.
    //      Adjust subscripts if desired; otherwise, mark NULL.
    // ----special case: vector[0]
    if(target_subscript==0) // knock out zeroth element no matter what
    {   free(tracker[0].vector_ptr);} 
    // ----if not zeroth, start looking at other elements
    else if(tracker_count<target_subscript-1)
    {   printf("vector element not found\n");return;}
    // Requested subscript okay. Freeit. 
    else
    {   free(tracker[target_subscript].vector_ptr);} // free element ptr
    // done with removal.
    if(Keep_subscripts) // adjust subscripts if required.
    {   tracker[target_subscript].vector_ptr=missing_element(target_subscript);} // point to "0" vector
    else // NOT keeping subscripts intact, i.e. collapsing/renumbering all subscripts toward zero
    {   collapse_tracker_higher_elements(target_subscript);
        renumber_all_subscripts(TRUE); // gboolean arbitrary means as-is, FALSE means by "true" subscripts
    } // if (target_subscript==0) else
// show the new list
// for(int i=0;i<tracker_count;i++){printf("   remaining element[%i] at %lu\n",tracker[i].subscript,(size_t)tracker[i].vector_ptr);}
} // void ss_vector_free_one_element()

void ss_vector_free_all_elements(void) 
{   // Start at "tracker[0]". Walk the entire list, free each element's contents, 
    //      then free that element, then move to the next one.
    //      Then free the "tracker" vector.
    for(int i=tracker_count;i>=0;i--) 
    {   // Modify your code to free vector element "items" here
        if(tracker[i].subscript>=0) free(tracker[i].vector_ptr);
    }
    free(tracker);
    tracker_count=0;
} // void ss_vector_free_all_elements()

// defining some sort of struct, can be anything really
typedef struct APPLE_STRUCT
{   int id; // one of the data in the component
    int other_id; // etc
    struct APPLE_STRUCT* next_element;
} apple; // description of component

apple* init_apple(int id) // make a single component
{   apple* a; // ptr to component
    a = malloc(sizeof(apple)); // memory for one component
    a->id = id; // populate with data
    a->other_id=id+10;
    a->next_element=NULL;
    // don't mess with aa->last_rec here
    return a; // return pointer to component
}

int return_id_value(int i,apple* aa) // given ptr to component, return single data item
{   printf("was inserted as apple[%i].id = %i     ",i,aa->id);
    return(aa->id);
}

ss_vector* return_address_given_subscript(int i) 
{   return tracker[i].vector_ptr;} 

int Test(void)  // was "main" in the example
{   int i;
    ss_vector* local_vector;
    local_vector=ss_init_vector(sizeof(apple)); // element "0"
    for (i = 1; i < 10; i++) // inserting items "1" thru whatever
    {local_vector=ss_vector_append(i);}   // finished ss_vector_append()
    // list all tracker vector entries
    for(i=0;(i<tracker_count);i++) {printf("tracker element [%i] has address %lu\n",tracker[i].subscript, (size_t)tracker[i].vector_ptr);}
    // ---test search function
    printf("\n NEXT, test search for address given subscript\n");
    local_vector=return_address_given_subscript(5);
printf("finished return_address_given_subscript(5) with vector at %lu\n",(size_t)local_vector);
    local_vector=return_address_given_subscript(0);
printf("finished return_address_given_subscript(0) with vector at %lu\n",(size_t)local_vector);
    local_vector=return_address_given_subscript(9);
printf("finished return_address_given_subscript(9) with vector at %lu\n",(size_t)local_vector);
    // ---test single-element removal
    printf("\nNEXT, test single element removal\n");
    ss_vector_free_one_element(5,TRUE); // keep subscripts; install dummy error element
printf("finished ss_vector_free_one_element(5)\n");
    ss_vector_free_one_element(3,FALSE);
printf("finished ss_vector_free_one_element(3)\n");
    ss_vector_free_one_element(0,FALSE);
    // ---test moving elements
printf("\n Test moving a few elements up\n");
    move_toward_zero(5);
    move_toward_zero(4);
    move_toward_zero(3);
    // show the new list
    printf("New list:\n");
    for(int i=0;i<tracker_count;i++){printf("   %i:element[%i] at %lu\n",i,tracker[i].subscript,(size_t)tracker[i].vector_ptr);}
    // ---plant some bogus subscripts for the next subscript test
    tracker[3].vector_ptr->subscript=7;
    tracker[3].subscript=5;
    tracker[7].vector_ptr->subscript=17;
    tracker[3].subscript=55;
printf("\n RENUMBER to use \"actual\" subscripts\n");   
    renumber_all_subscripts(FALSE);
    printf("Sorted list:\n");
    for(int i=0;i<tracker_count;i++)
    {   if ((size_t)tracker[i].vector_ptr!=0)
        {   printf("   %i:element[%i] or [%i]at %lu\n",i,tracker[i].subscript,tracker[i].vector_ptr->subscript,(size_t)tracker[i].vector_ptr);
        }
        else 
        {   printf("   %i:element[%i] at 0\n",i,tracker[i].subscript);
        }
    }
printf("\nBubble sort to get TRUE order back\n");
    bubble_sort();
    printf("Sorted list:\n");
    for(int i=0;i<tracker_count;i++)
    {   if ((size_t)tracker[i].vector_ptr!=0)
        {printf("   %i:element[%i] or [%i]at %lu\n",i,tracker[i].subscript,tracker[i].vector_ptr->subscript,(size_t)tracker[i].vector_ptr);}
        else {printf("   %i:element[%i] at 0\n",i,tracker[i].subscript);}
    }
    // END TEST SECTION
    // don't forget to free everything
    ss_vector_free_all_elements(); 
    return 0;
}

int main(int argc, char *argv[])
{   char cmd[5],main_buffer[50]; // Intentionally big for "other" I/O purposes
    cmd[0]=32; // blank = ASCII 32
    //  while(cmd!="R"&&cmd!="W"  &&cmd!="E"        &&cmd!=" ") 
    while(cmd[0]!=82&&cmd[0]!=87&&cmd[0]!=69)//&&cmd[0]!=32) 
    {   memset(cmd, '\0', sizeof(cmd));
        memset(main_buffer, '\0', sizeof(main_buffer));
        // default back to the cmd loop
        cmd[0]=32; // blank = ASCII 32
        printf("REad, TEst, WRITe, EDIt, or EXIt? ");
        fscanf(stdin, "%s", main_buffer);
        strncpy(cmd,main_buffer,4);
        for(int i=0;i<4;i++)cmd[i]=toupper(cmd[i]);
        cmd[4]='\0';
        printf("%s received\n ",cmd);
        // process top level commands
        if(cmd[0]==82) {printf("READ accepted\n");} //Read
        else if(cmd[0]==87) {printf("WRITe accepted\n");} // Write
        else if(cmd[0]==84) 
        {   printf("TESt accepted\n");// TESt
            Test();
        }
        else if(cmd[0]==69) // "E"
        {   if(cmd[1]==68) {printf("EDITing\n");} // eDit
            else if(cmd[1]==88) {printf("EXITing\n");exit(0);} // eXit
            else    printf("  unknown E command %c%c\n",cmd[0],cmd[1]);
        }
        else    printf("  unknown command\n");
        cmd[0]=32; // blank = ASCII 32
    } // while()
    // default back to the cmd loop
}   // main()

---

These posts may be in the wrong order! This is #2 in a series of 3 posts. Sorry.

I've "taken a few liberties" with Lie Ryan's code, implementing a linked list so individual elements of his vector can be accessed via a linked list. This allows access, but admittedly it is time-consuming to access individual elements due to search overhead, i.e. walking down the list until you find the right element. I'll cure this by maintaining an address vector containing subscripts 0 through whatever paired with memory addresses. This is still not as efficient as a plain-and-simple array would be, but at least you don't have to "walk the list" searching for the proper item.

    // Based on code from https://stackoverflow.com/questions/3536153/c-dynamically-growing-array
typedef struct STRUCT_SS_VECTOR
{   size_t size; // # of vector elements
    void** items; // makes up one vector element's component contents
    int subscript; // this element's subscript nmbr, 0 thru whatever
    struct STRUCT_SS_VECTOR* this_element; // linked list via this ptr
    struct STRUCT_SS_VECTOR* next_element; // and next ptr
} ss_vector;

ss_vector* vector; // ptr to vector of components

ss_vector* ss_init_vector(size_t item_size) // item_size is size of one array member
{   vector= malloc(sizeof(ss_vector)); 
    vector->this_element = vector; 
    vector->size = 0; // initialize count of vector component elements
    vector->items = calloc(1, item_size); // allocate & zero out memory for one linked list element
    vector->subscript=0;
    vector->next_element=NULL;
    //      If there's an array of element addresses/subscripts, install it now.
    return vector->this_element;
}

ss_vector* ss_vector_append(ss_vector* vec_element,                 int i) 
//                                                                          ^--ptr to this element  ^--element nmbr
{   ss_vector* local_vec_element=0;
    // If there is already a next element, recurse to end-of-linked-list
    if(vec_element->next_element!=(size_t)0) 
    {   local_vec_element= ss_vector_append(vec_element->next_element,i); // recurse to end of list
        return local_vec_element;
    }
    // vec_element is NULL, so make a new element and add at end of list
    local_vec_element= calloc(1,sizeof(ss_vector)); // memory for one component
    local_vec_element->this_element=local_vec_element; // save the address
    local_vec_element->next_element=0;
    vec_element->next_element=local_vec_element->this_element;
    local_vec_element->subscript=i; //vec_element->size; 
    local_vec_element->size=i; // increment # of vector components
    //      If there's an array of element addresses/subscripts, update it now.
    return local_vec_element;
}

void ss_vector_free_one_element(int i,gboolean Update_subscripts) 
{   // Walk the entire linked list to the specified element, patch up 
    //      the element ptrs before/next, then free its contents, then free it.
    //      Walk the rest of the list, updating subscripts, if requested.
    //      If there's an array of element addresses/subscripts, shift it along the way.
    ss_vector* vec_element;
    struct STRUCT_SS_VECTOR* this_one;
    struct STRUCT_SS_VECTOR* next_one;
    vec_element=vector;
    while((vec_element->this_element->subscript!=i)&&(vec_element->next_element!=(size_t) 0)) // skip
    {   this_one=vec_element->this_element; // trailing ptr
        next_one=vec_element->next_element; // will become current ptr
        vec_element=next_one;
    } 
    // now at either target element or end-of-list
    if(vec_element->this_element->subscript!=i)
    {   printf("vector element not found\n");return;}
    // free this one
    this_one->next_element=next_one->next_element;// previous element points to element after current one
    printf("freeing element[%i] at %lu",next_one->subscript,(size_t)next_one);
    printf(" between %lu and %lu\n",(size_t)this_one,(size_t)next_one->next_element);
    vec_element=next_one->next_element; 
    free(next_one); // free the current element
    // renumber if requested
    if(Update_subscripts)
    {   i=0;
        vec_element=vector;
        while(vec_element!=(size_t) 0)
        {   vec_element->subscript=i;
            i++;
            vec_element=vec_element->next_element; 
        }
    }
    //      If there's an array of element addresses/subscripts, update it now.
/*  // Check: temporarily show the new list
    vec_element=vector;
    while(vec_element!=(size_t) 0)
    {   printf("   remaining element[%i] at %lu\n",vec_element->subscript,(size_t)vec_element->this_element);
        vec_element=vec_element->next_element;
    } */
    return;
} // void ss_vector_free_one_element()

void ss_vector_insert_one_element(ss_vector* vec_element,int place) 
{   // Walk the entire linked list to specified element "place", patch up 
    //      the element ptrs before/next, then calloc an element and store its contents at "place".
    //      Increment all the following subscripts.
    //      If there's an array of element addresses/subscripts, make a bigger one, 
    //      copy the old one, then shift appropriate members.
    // ***Not yet implemented***
} // void ss_vector_insert_one_element()

void ss_vector_free_all_elements(void) 
{   // Start at "vector".Walk the entire linked list, free each element's contents, 
    //      free that element, then move to the next one.
    //      If there's an array of element addresses/subscripts, free it.
    ss_vector* vec_element;
    struct STRUCT_SS_VECTOR* next_one;
    vec_element=vector;
    while(vec_element->next_element!=(size_t) 0)
    {   next_one=vec_element->next_element;
        // free(vec_element->items) // don't forget to free these
        free(vec_element->this_element);
        vec_element=next_one;
        next_one=vec_element->this_element;
    }
    // get rid of the last one.
    // free(vec_element->items)
    free(vec_element);
    vector=NULL;
    //      If there's an array of element addresses/subscripts, free it now.
printf("\nall vector elements & contents freed\n");
} // void ss_vector_free_all_elements()

// defining some sort of struct, can be anything really
typedef struct APPLE_STRUCT
{   int id; // one of the data in the component
    int other_id; // etc
    struct APPLE_STRUCT* next_element;
} apple; // description of component

apple* init_apple(int id) // make a single component
{   apple* a; // ptr to component
    a = malloc(sizeof(apple)); // memory for one component
    a->id = id; // populate with data
    a->other_id=id+10;
    a->next_element=NULL;
    // don't mess with aa->last_rec here
    return a; // return pointer to component
};

int return_id_value(int i,apple* aa) // given ptr to component, return single data item
{   printf("was inserted as apple[%i].id = %i     ",i,aa->id);
    return(aa->id);
}

ss_vector* return_address_given_subscript(ss_vector* vec_element,int i) 
// always make the first call to this subroutine with global vbl "vector"
{   ss_vector* local_vec_element=0;
    // If there is a next element, recurse toward end-of-linked-list
    if(vec_element->next_element!=(size_t)0)
    {   if((vec_element->this_element->subscript==i))
        {   return vec_element->this_element;}
        local_vec_element= return_address_given_subscript(vec_element->next_element,i); // recurse to end of list
        return local_vec_element;
    }
    else
    {   if((vec_element->this_element->subscript==i)) // last element
        {   return vec_element->this_element;}
        // otherwise, none match
        printf("reached end of list without match\n");
        return (size_t) 0;
    }
} // return_address_given_subscript()

int Test(void)  // was "main" in the original example
{   ss_vector* local_vector;
    local_vector=ss_init_vector(sizeof(apple)); // element "0"
    for (int i = 1; i < 10; i++) // inserting items "1" thru whatever
    {   local_vector=ss_vector_append(vector,i);}   
    // test search function
    printf("\n NEXT, test search for address given subscript\n");
    local_vector=return_address_given_subscript(vector,5);
    printf("finished return_address_given_subscript(5) with vector at %lu\n",(size_t)local_vector);
    local_vector=return_address_given_subscript(vector,0);
    printf("finished return_address_given_subscript(0) with vector at %lu\n",(size_t)local_vector);
    local_vector=return_address_given_subscript(vector,9);
    printf("finished return_address_given_subscript(9) with vector at %lu\n",(size_t)local_vector);
    // test single-element removal
    printf("\nNEXT, test single element removal\n");
    ss_vector_free_one_element(5,FALSE); // without renumbering subscripts
    ss_vector_free_one_element(3,TRUE);// WITH renumbering subscripts
    // ---end of program---
    // don't forget to free everything
    ss_vector_free_all_elements(); 
    return 0;
}

---