How to create an array without knowing the size in c language

Here is an example showing one way to manage a dynamic array.

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


int main(void)
{
    /*  To manage an array of varying size, we keep a pointer to the first 
        element of the array and the number of elements in the array.

        The pointer is initially null, and there are no elements in the array.
    */
    int *Numbers = NULL;
    size_t NumberOfAllocatedElements = 0;

    /*  We get numbers one by one, using TemporaryNumber to hold them.  As long
        as scanf reports it was able to read and assign 1 item, we continue
        adding the number to the array.
    */
    int TemporaryNumber;
    while (1 == scanf("%d", &TemporaryNumber))
    {
        /*  To grow the array, increase the number of allocated elements and
            use realloc to request more space.
        */
        int *NewNumbers =
            realloc(Numbers, ++NumberOfAllocatedElements * sizeof *NewNumbers);

        /*  If realloc fails, we report an error and exit.  A more
            sophisticated program could do something else in this case.
        */
        if (!NewNumbers)
        {
            fprintf(stderr, "Error, unable to allocate memory.\n");
            exit(EXIT_FAILURE);
        }

        //  Update our pointer with the new address.
        Numbers = NewNumbers;

        //  Record the new element in the array.
        Numbers[NumberOfAllocatedElements - 1] = TemporaryNumber;
    }

    //  Show the contents of the array.
    for (size_t i = 0; i < NumberOfAllocatedElements; ++i)
        printf("Number %zu is %d.\n", i, Numbers[i]);

    //  Release the memory.
    free(Numbers);
}

This is largely a beginner example. An improvement would be to allocate large amounts of memory at a time, instead of just one more element each time. In this case, the program then needs to track two numbers about the array: The amount of space allocated and the number of elements currently used in it.

A variety of alternatives are also possible.

Solution 1: If you don't know the size of the array at either run-time or compile-time, you should go with linked lists

Solution 2: As Jean-Claude mentioned in his comment, Declare a fixed-size array using dynamic memory allocation functions like malloc() or calloc() and realloc() the size of array when needed.

Code for Solution 2:

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

int main(void)
{
    int *p = NULL, i = 0, j = 0;

    // Fixed-size array of size 1.
    if( (p = (int *) malloc (1 * sizeof(int)) ) == NULL)
    {
        printf("Memory Unavailable.\n");
        exit(1);
    }

    // Sentinel-loop and reallocation for the array.
    do
    {
        printf("Enter value\n-999 to end: ");
        scanf("%d", &p[i]);
        if( p[i] == -999)
            break;
        i++;
        if((p = (int *)realloc(p, sizeof(int)*(i+1))) == NULL)
        {
            printf("Memory can't be reallocated.\n");
            exit(2);
        }
    }
    while(1);

    printf("\nElements of the array:\n");
    for( ; i >= 0; i--)
        if(p[j] != -999)
            printf("%4d", p[j++]);
    free(p);
    return 0;
}

Output:

Enter value
-999 to end: 1
Enter value
-999 to end: 2
Enter value
-999 to end: 3
Enter value
-999 to end: 4
Enter value
-999 to end: 5
Enter value
-999 to end: -999

Elements of the array:
   1   2   3   4   5

I would love to know if you are using a user interface where you can take the array size as input and then proceed normally as if you knew the size of the array in advance.

Some code sample for you to follow : -

    int reqArraySize;
    printf("Enter the array size: ");
    scanf("%d", &reqArraySize);

  

After this you may proceed with this interger input array size :

    for(i=0;i<reqArraySize;i++)
      scanf("%d",&arr[i]);

Hope this helps you.

Cheers.