I created an array and put the value into array as follow
int *ptr_int;
int list_int[10];
int i;
for (i=0; i<10; i++)
list_int[i] = i + 1;
and I assign a value into list_int array like this
list_int[17] = 18;
When I tried to get the count of array as follow
int size = sizeof(list_int ) / sizeof( int );
printf( "The size of int is %d.\n", size);
the result is only 10.
How could I get the array room count?
the result is only 10.
That's the real size. Assigning to list_int[17] is undefined behavior, and it does not magically extend the array.
You have already defined the max size of your array as 10 with the following definition
int list_int[10];
You can not assign value to list_int[17]. Becacuse list_int[17] is out of the memory of the array list_int[10] that you have defined.
You can only assign value to the element from 0 .. 9
list_int[17] = 18;
This is undefined behavior because the array size is only 10.
Note that with the exception of variable length arrays, sizeof operator is a compile time operator. The result of sizeof(list_int ) and sizeof(int) are determined in compile time.
To implement an array with dynamic size, you need to allocate the array dynamically, you may find realloc pretty helpful.
To Create Dynamic arrays ( allocated at run time )
int n;
scanf("%d",&n); // read n from the user at run time
int* x=(int*)malloc(sizeof(int)*n); // where n is the number of elements you need to allocate
////// after that you can access the array (x) using indexer
///// reading loop
for(int i=0;i<n;i++)
scanf("%d",&x[i]);
===============================================================================
Note: if you need more dynamic data structure which can allocate memory for each entry you can use linked lists
malloc()/calloc(),realloc()andfree()in particular. Do some tests leaning how to use them and if getting stuck using them come back here to Stackoverflow presenting your findings and issues posing another question and we'll try to help you.