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C# Stack Collection: Push, Pop

Use the Stack. Loop over the elements in a Stack, and manipulate the Stack with Push and Pop.

Stack.

Each element is added to the top. Each element we remove is removed from the top. This is a LIFO collection—the stack is last-in-first-out.

The Stack

can help you develop parsers quickly and also replace complex recursive algorithms. Stack is a generic type—we must specify its stored element type.Generic Class, Method

Push.

Usually the first action you need to do on Stack is Push elements into it. The word Push is a computer science term that means "add to the top." Here we create a Stack.

Start: A local variable is assigned to a new Stack containing 3 integers. The ints were added with the 10000 last.

Then: We write each value of the stack to the Console in a foreach-loop with Console.WriteLine.

Console.WriteLine

Tip: You can see that 10000 is displayed first. This is explained by the LIFO concept—last in first out.

LIFO: The last element added (with Push) to Stack is the first one removed (with Pop).

C# program that creates new Stack of integers using System; using System.Collections.Generic; class Program { static Stack<int> GetStack() { Stack<int> stack = new Stack<int>(); stack.Push(100); stack.Push(1000); stack.Push(10000); return stack; } static void Main() { var stack = GetStack(); Console.WriteLine("--- Stack contents ---"); foreach (int i in stack) { Console.WriteLine(i); } } } Output --- Stack contents --- 10000 1000 100

Pop.

Here we Pop and Peek. When you call Pop, the elements from the top of the Stack is returned, and the element is removed from the collection.

Next: This example uses the same Stack collection as above, which means Pop returns 10000.

Also: It uses Peek, which does the same thing as Pop but does not remove the element.

C# program that uses Pop method using System; using System.Collections.Generic; class Program { static void Main() { // Get the stack. // ... See above definition of this method. Stack<int> stack = GetStack(); // Pop the top element. int pop = stack.Pop(); // Write to the console. Console.WriteLine("--- Element popped from top of Stack ---"); Console.WriteLine(pop); // Now look at the top element. int peek = stack.Peek(); Console.WriteLine("--- Element now at the top ---"); Console.WriteLine(peek); } } Output --- Element popped from top of Stack --- 10000 --- Element now at the top --- 1000

Notes, Pop and Peek.

Pop and Peek both act on the top of Stack, meaning the element most recently added. They also both return that top value.

Peek: This does not remove the element from the Stack collection. It only gets the value—it "peeks" at the value.

Pop: This removes the reference. If you call Pop, then afterwards Peek and Pop will return the next value when they are called.

Clear, Count.

Let us test more parts of Stack. The Count property is used without parentheses, while Clear() is a parameterless method. It erases the Stack's contents.

Note: The example receives the Stack used in the above examples, then counts it, clears it, and finally counts it again.

C# program that uses Clear and Count methods using System; using System.Collections.Generic; class Program { static void Main() { // Get the stack [See method definition above] Stack<int> stack = GetStack(); // Count the number of elements in the Stack int count = stack.Count; Console.WriteLine("--- Element count ---"); Console.WriteLine(count); // Clear the Stack stack.Clear(); Console.WriteLine("--- Stack was cleared ---"); Console.WriteLine(stack.Count); } } Output --- Element count --- 3 --- Stack was cleared --- 0

Null.

The value null is allowed in Stacks with reference types such as string. You can also assign your Stack to null instead of calling Clear.Null

Performance: When assigning to null, the contents are not changed. Instead the reference is unrooted in the garbage collector.

Exceptions.

When you call Pop or Peek on your Stack, the runtime will throw an exception if the Stack has zero elements. This can be solved.

Tip: To work around this problem, you must check the Count property. Here we catch the exception raised by this situation.

InvalidOperationException
C# program that uses Stack incorrectly, correctly using System; using System.Collections.Generic; class Program { static void Main() { // Create an empty Stack. var stack = new Stack<int>(); try { // This throws an exception. int pop = stack.Pop(); } catch (Exception ex) { Console.WriteLine("--- Exception raised by Pop ---"); Console.WriteLine(ex.ToString()); } // Here we safely Pop the stack. if (stack.Count > 0) { int safe = stack.Pop(); } else { Console.WriteLine("--- Avoided exception by using Count method ---"); } } } Output --- Exception raised by Pop --- System.InvalidOperationException: Stack empty. ... ... --- Avoided exception by using Count method

Copy, search.

You can use constructors of Stack to streamline your code. One constructor accepts an IEnumerable parameter, which is an interface that most collections implement.ConstructorIEnumerable

Also: We search the Stack with the Contains method. The Contains method on Stack returns true if the element is found.

True, False

Contains: With Contains we search for a string. The object reference is not compared. Instead the string contents are.

C# program that uses the Stack constructor using System; using System.Collections.Generic; class Program { static void Main() { // An example string array. string[] values = { "Dot", "Net", "Perls" }; // Copy an array into a Stack. var stack = new Stack<string>(values); // Display the Stack. Console.WriteLine("--- Stack contents ---"); foreach (string value in stack) { Console.WriteLine(value); } // See if the stack contains "Perls" Console.WriteLine("--- Stack Contains method result ---"); bool contains = stack.Contains("Perls"); Console.WriteLine(contains); } } Output --- Stack contents --- Perls Net Dot --- Stack Contains method result --- True

Discussion.

There are several other methods on Stack in System.Collections.Generic. You can copy your Stack to a new array with ToArray(). Also, you can use TrimExcess to reduce memory.

Internally: TrimExcess will check the Stack's fill rate, and then resize the internal array.

Info: When looking inside Stack with IL Disassembler, we see it is implemented with an array of type T[]. This is type you specify.

IL Disassembler

Tip: Stack is basically a wrapper around an array. No boxing or unboxing will occur.

Parsers.

Stack can be used for parsers, such as HTML parsers. Many parsers push new elements they encounter, and then when they exit the element, they pop elements.

Research.

Stacks are well-researched in computer science literature. Method calls are stored within stacks. A pushdown stack is an abstract type that we can use in programs.

Quote: A pushdown stack is an abstract data type that comprises two basic operations: insert (push) a new item, and remove (pop) the item that was most recently inserted (Algorithms in C++ Third Edition).

A summary.

The Stack collection, found in the System.Collections.Generic namespace, provides a simple wrapper on an array. Stack is a useful abstraction of the classic stack data structure.Queue
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