I'm building an array that increments its size when it's totally full.

The situation is, the user calls append with value of 5, but the array (with 4 slots) is full. My program calls my resize function, which turns the array into 8 slots maintaining the 4 old values.

The resize implementation is working, but the part where it appends the new value, the slot specified continues empty. Like this:

Initial array:

[1, 2, 3, 4]

Array after resize and append:

[1, 2, 3, 4, -1, -1, -1, -1]

Expected result:

[1, 2, 3, 4, 5, -1, -1, -1]

#define EMPTY -1
size_t capacity = 4;
int size = 0;

int main(void)
{
    int *arr = malloc(capacity * sizeof(int));
    arr[0] = 1;
    arr[1] = 2;
    arr[2] = 3;
    arr[3] = 4;
    size = 4;

    appendInTheEnd(arr, 5, &arr);
    for (int i = 0; i < capacity; i  )
    {
        printf("%d ", arr[i]);
    }
}


void appendInTheEnd(int *arr, int value, int **array)
{
    if (size == capacity)
    {
        resizeArray(array, capacity);
        capacity *= 2;
        arr[size] = value;
        size  ;
    }
}

void resizeArray(int **arr, size_t capacity)
{
    int *newArr = malloc(2 * capacity * sizeof(int));
    for (int i = capacity; i < 2 * capacity; i  )
    {
        newArr[i] = EMPTY;
    }
    memcpy(newArr, *arr, capacity * sizeof(int));
    memset(newArr   capacity, EMPTY, capacity);
    free(*arr);
    *arr = newArr;
}

CodePudding user response：

I would do it this way:

typedef struct
{
    size_t size; 
    int data[];
} int_arr_type;


int_arr_type *append(int_arr_type *arr, int val)
{
    size_t new_size = arr ? arr -> size   1 : 1;
    int_arr_type *new_arr = realloc(arr, sizeof(*new_arr)   new_size * sizeof(new_arr -> data[0]));

    if(new_arr)
    {
        new_arr -> data[new_size - 1] = val;
        new_arr -> size = new_size;
    }
    return new_arr;
}


int main(void)
{
    int_arr_type *a = NULL;

    a = append(a, 1);
    a = append(a, 2);
    a = append(a, 3);
    a = append(a, 4);
    a = append(a, 5);
    a = append(a, 6);

    for(size_t index = 0; index < a -> size; index  )
    {
        printf("a[%zu]=%d\n", index, a -> data[index]);
    }
}

https://godbolt.org/z/YT6hdnMd3

CodePudding user response：

appendInTheEnd() doesn't do anything unless size == capacity. Don't pass in both arr and array. You just need **arr. Here is a minimal change to fix the issue (see @0___________'s answer for a bigger step):

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

#define EMPTY -1
size_t capacity = 4;
int size = 0;

void resizeArray(int **arr, size_t capacity) {
    int *newArr = malloc(2 * capacity * sizeof(int));
    for (int i = capacity; i < 2 * capacity; i  )
    {
        newArr[i] = EMPTY;
    }
    memcpy(newArr, *arr, capacity * sizeof(int));
    memset(newArr   capacity, EMPTY, capacity * sizeof(int));
    free(*arr);
    *arr = newArr;
}

void appendInTheEnd(int **arr, int value) {
    if(size == capacity) {
        resizeArray(arr, capacity);
        capacity *= 2;
    }
    (*arr)[size  ] = value;
}


int main(void) {
    int *arr = malloc(capacity * sizeof(int));
    arr[0] = 1;
    arr[1] = 2;
    arr[2] = 3;
    arr[3] = 4;
    size = 4;

    appendInTheEnd(&arr, 5);
    for (int i = 0; i < capacity; i  ) {
        printf("%d ", arr[i]);
    }
}

and it will print:

1 2 3 4 5 -1 -1 -1

I would change the print loop to iterate till size instead of capacity. It's perfectly ok to leave the values >= size uninitialized (no need for memset in resizeArray(); note that in memset it should be capacity * sizeof(int) not just capacity; I had a mistake in previous answer since fixed),

The next step with your program, btw, is to make a struct to hold your state so it's not global values:

struct int_array {
   int *data;
   size_t size;
   size_t capacity;
};

Then you write a int_array_init() function that initialize your struct, and you change the signature of resizeArray() and appendInTheEnd to take a pointer to struct *int_array; along with whatever else information is needed. It's a good idea to then rename your functions a prefix of your struct so:

appendInTheEnd() renamed to int_array_append()
resizeArray() renamed to int_array_resize()