adding 2st animation

lectures/06_memory/README.md

@@ -78,7 +78,57 @@ similar to the examples covered in the pointers lecture except that there is no
 other than the pointer variable.
 - Dynamic allocation of primitives like ints and doubles is not very interesting or significant. What’s more important is dynamic allocation of arrays and class objects.
 
-- play this [animation](https://jidongxiao.github.io/CSCI1200-DataStructures/animations/dynamic_memory/example1/index.html).
+- play this [animation](https://jidongxiao.github.io/CSCI1200-DataStructures/animations/dynamic_memory/example1/index.html) to see what exactly the above code snippet does.
+
+## 6.3 Dynamic Allocation of Arrays
+- How do we allocate an array on the stack? What is the code? What memory diagram is produced by the code?
+- Declaring the size of an array at compile time doesn’t offer much flexibility. Instead we can dynamically allocate an array based on data. This gets us part-way toward the behavior of the standard library vector class. Here’s an example:
+
+<table>
+ <tr>
+  <td>
+
+ <pre>
+int main() {
+    std::cout << "Enter the size of the array: ";
+    int n,i;
+    std::cin >> n;
+    double *a = new double[n];
+    for (i=0; i<n; ++i) { a[i] = sqrt(i); }
+    for (i=0; i<n; ++i) {
+        if ( double(int(a[i])) == a[i] )
+            std::cout << i << " is a perfect square " << std::endl;
+    }
+    delete [] a;
+    return 0;
+}
+ </pre>
+
+</td>
+<td><img src="array.png" alt="array"</td>
+</tr>
+</table>
+
+- The expression new double[n] asks the system to dynamically allocate enough consecutive memory to hold n
+double’s (usually 8n bytes).
+  - What’s crucially important is that n is a variable. Therefore, its value and, as a result, the size of the
+array are not known until the program is executed and the the memory must be allocated dynamically.
+  - The address of the start of the allocated memory is assigned to the pointer variable a.
+- After this, a is treated as though it is an array. For example: a[i] = sqrt( i );
+In fact, the expression a[i] is exactly equivalent to the pointer arithmetic and dereferencing expression *(a+i)
+which we have seen several times before.
+- After we are done using the array, the line: delete [] a; releases the memory allocated for the entire
+array and calls the destructor (we’ll learn about these soon!) for each slot of the array. Deleting a dynamically
+allocated array without the [] is an error (but it may not cause a crash or other noticeable problem, depending
+on the type stored in the array and the specific compiler implementation).
+- Since the program is ending, releasing the memory is not a major concern. However, to demonstrate
+that you understand memory allocation & deallocation, you should always delete dynamically allocated
+memory in this course, even if the program is terminating.
+- In more substantial programs it is ABSOLUTELY CRUCIAL. If we forget to release memory repeatedly
+the program can be said to have a memory leak. Long-running programs with memory leaks will eventually
+run out of memory and crash.
+
+- play this [animation](https://jidongxiao.github.io/CSCI1200-DataStructures/animations/dynamic_memory/example2/index.html) to see what exactly the above code snippet does.
 
 ## 6.3 Exercises