Lession 27: C# Stack
C# includes a special type of collection which stores elements in LIFO style(Last In First Out). C# includes a generic and non-generic Stack. Here, you are going to learn about the non-generic stack.
Stack allows null value and also duplicate values. It provides a Push() method to add a value and Pop() or Peek() methods to retrieve values.
Important Properties and Methods of Stack:
| Property | Usage |
|---|---|
| Count | Returns the total count of elements in the Stack. |
| Method | Usage |
|---|---|
| Push | Inserts an item at the top of the stack. |
| Peek | Returns the top item from the stack. |
| Pop | Removes and returns items from the top of the stack. |
| Contains | Checks whether an item exists in the stack or not. |
| Clear | Removes all items from the stack. |
Add values into Stack:
The Push() method adds values into the Stack. It allows value of any datatype.
Push() method signature:
void Push(object obj);
Example: Push()
Stack myStack = new Stack();
myStack.Push("Hello!!");
myStack.Push(null);
myStack.Push(1);
myStack.Push(2);
myStack.Push(3);
myStack.Push(4);
myStack.Push(5);
Access Stack Elements:
You can retrieve stack elements by various ways. Use a foreach statement to iterate the Stack collection and get all the elements in LIFO style.
Example: Access Stack
Stack myStack = new Stack();
myStack.Push("Hello!!");
myStack.Push(null);
myStack.Push(1);
myStack.Push(2);
myStack.Push(3);
myStack.Push(4);
myStack.Push(5);
foreach (var itm in myStack)
Console.Write(itm);
Output:
5 4
3
2
1
Hello!!
Peek():
The Peek() method returns the last (top-most) value from the stack. Calling Peek() method on empty stack will throw InvalidOperationException. So always check for elements in the stack before retrieving elements using the Peek() method.
Peek() method signature:
object Peek();
Example: Stack.Peek()
Stack myStack = new Stack();
myStack.Push(1);
myStack.Push(2);
myStack.Push(3);
myStack.Push(4);
myStack.Push(5);
Console.WriteLine(myStack.Peek());
Console.WriteLine(myStack.Peek());
Console.WriteLine(myStack.Peek());
Output:
5 5
5
Pop():
You can also retrieve the value using the Pop() method. The Pop() method removes and returns the value that was added last to the Stack. The Pop() method call on an empty stack will raise an InvalidOperationException. So always check for number of elements in stack must be greater than 0 before calling Pop() method.
Pop() signature:
object Pop();
Example: Access Stack
Stack myStack = new Stack();
myStack.Push(1);
myStack.Push(2);
myStack.Push(3);
myStack.Push(4);
myStack.Push(5);
Console.Write("Number of elements in Stack: {0}", myStack.Count);
while (myStack.Count > 0)
Console.WriteLine(myStack.Pop());
Console.Write("Number of elements in Stack: {0}", myStack.Count);
Output:
Number of elements in Stack: 5 5
4
3
2
1
Number of elements in Stack: 0
Contains:
The Contains() method checks whether the specified item exists in a Stack collection or not. It returns true if it exists; otherwise it returns false.
Contains() method signature:
bool Contains(object obj);
Example: Stack.Contains()
Stack myStack = new Stack();
myStack.Push(1);
myStack.Push(2);
myStack.Push(3);
myStack.Push(4);
myStack.Push(5);
myStack.Contains(2); // returns true
myStack.Contains(10); // returns false
Clear:
The Clear() method removes all the values from the stack.
Clear() signature:
void Clear();
Example: Stack.Contains()
Stack myStack = new Stack();
myStack.Push(1);
myStack.Push(2);
myStack.Push(3);
myStack.Push(4);
myStack.Push(5);
myStack.Clear(); // removes all elements
Console.Write("Number of elements in Stack: {0}", myStack.Count);
Output:
Number of elements in Stack: 0Points to Remember :
- Stack stores the values in LIFO (Last in First out) style. The element which is added last will be the element to come out first.
- Use the Push() method to add elements into Stack.
- The Pop() method returns and removes elements from the top of the Stack. Calling the Pop() method on the empty Stack will throw an exception.
- The Peek() method always returns top most element in the Stack.
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