An array is a data structure that stores elements of the same type in a contiguous block of memory with indexes to access each element. Arrays have fixed sizes that must be defined upon creation. Retrieving an element from an array has constant complexity but inserting or deleting has linear complexity as it may require shifting elements. A linked list stores elements in nodes with pointers to the next node. It has variable size and faster insertion/deletion in the middle but slower retrieval by index than arrays.

1. Array
• A data structure that contains a list of elements
accessed by an index
– Array: a set of pairs (index and value)
– For each index, there is a value associated with that index.
• Arrays are generally a fixed size, which means you must
specify the maximum size when the array is created
• Increasing the size of an array normally involves
creating a new array of the desired size and copying
the values from the old array to the new array

2. Array
• An array of size 5
String arr = new String[5];
Int nextIndex = 0;
• Set some of the values
arr[nextIndex ] = “Hi”
++nextIndex;
arr[nextIndex ] = “there”
++nextIndex;
arr[nextIndex ] = “Bob”
++nextIndex;
arr[nextIndex ] = “.”
++nextIndex;
Hi there Bob .

3. Array
• Insert into the middle of a array
– Insert “Billy” between “there” and “Bob”
• Push “Bob” and “.” toward the end of the array to make space for “Bob” in
element 2
insertIndex = 2;
for ( int i = nextIndex; i > = insertIndex; --i ) {
arr[i] = arr[i-1];
}
++nextIndex;
• Set the new value for element 2
arr[insertIndex] = “Billy”;
Hi there Bob .
Hi there Bob Bob .
Hi there Billy Bob .

4. Questions
• What is the complexity of “retrieve” in an
array?
• What is the complexity of “store” in an array?
• What about insertion and deletion for ordered
elements in an arrary?

5. Linked List
• A linked list is a data structure that contains a set of nodes
linked with pointers.
• Singly-linked list has links in one direction. To the next node in
the list.
• A doubly-linked list had links in both directions. To the next
and previous nodes in the list.
Data
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Variable
Variable Data
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6. Linked List
• A Circularly-linked list has a complete cycles of
links.
Variable Data
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Data
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7. Linked List
• Insert at the end of a linked list
• Create new node
• Change the links to
include the new node
Billy
Next
Hi
Next
Sentence there
Next
Bob
Next
.
Next

8. Linked List
• Insert into the middle of a linked list
• Create new node
• Change the links to
include the new node
Hi
Next
Sentence there
Next
Bob
Next
.
Next
Billy
Next

9. Array versus Linked List
• Some operations are faster with one versus the other
– Find element at position X – Array faster
– Find element Y at unknown position – Tie
– Insert element in the middle – Linked list faster
– Insert element at the end – Tie, if doubly-linked and circular, Array
otherwise
– Insert element at the beginning – Linked list
• Arrays are (typically) a fixed size, linked lists are variable
length
• Arrays are defined as holding a specific number of elements
whether or not that many elements are currently in the array.
Doing that wastes space