Shadowing Practice: Stack Data Structure In STL | C++ Tutorial - Learn English Speaking with Video

In this video, we're going to learn about the stack data structure class template that's built into the C++ standard template library.

So a data structure stores a collection of data and stacks are a type of data structure.

Stacks are very commonly used in practice, so they're included directly in the C++ standard template library.

Stacks are a last in first out data structure.

What

that means is the last element that's put into the data

structure is going to be the first element out of the data structure.

So a stack data structure works like a stack of plates or books.

For example, we could have a stack data structure to store integer values.

And when we insert elements onto the stack, we say that we push them onto the stack.

When we remove elements from the stack, we say that we pop them from the stack.

So for example, if we pushed the element 5 onto the stack, 5 would be here.

We could call this the bottom of the stack.

We could then push the element 7 onto the stack, and 7 would go here.

We would say that 7 is at the top of the stack.

Then we could push another element onto the stack.

So we could push 9 onto the stack.

9 would then be the new top of the stack.

Now if we popped an element off this stack, it's always going to be removed from the top of the stack.

So if we popped an element off this stack here it would be this element here that's popped

and then seven would become the new top of the stack

and we could keep popping elements from the stack

and eventually the stack would be empty where an empty stack

is a stack with no elements we would say this stack here has a size of two

because the stack has two elements and there's also typically an operation which allows us to view the top of the stack.

It's usually called peak.

In C++, it's called top.

So let's try out the stack data structure that's built into the C++ standard template library.

The first thing we'll do is include the stack library.

This will allow us to declare and use stack objects.

Then down here, we'll declare a stack object to store in values called numbers.

So this will declare a stack object to store in values called numbers.

Now initially this stack is going to be empty and it's going to have a size of zero.

Stack objects have size and empty methods

that we can use to check the size of a stack and to check whether it's empty or not.

So let's call those methods.

Here we'll output size colon

and then we'll call numbers .size where size is going to return the size of the stack

and we'll output that then we'll also call the empty method to check

if the stack is empty so

if numbers .empty is true then we'll output here stack is empty followed by an end line

so the empty method is going to return true if the stack is empty and false otherwise.

So right now, if we save compile and run the program, we'll get that the stack has a size of zero and that the stack is empty, both of which are correct.

So now let's push an element onto the stack.

We'll have here numbers .push and we'll push the value eight onto the stack.

Now, if we check the size of the stack, the size should now be one.

Let's try that we'll copy this and down here we'll paste it

and if we save compile and run the program now

we see the size is now one we could also check

if the stack is not empty so now we'll have if

not numbers dot empty is true so in other words if the empty function now returns false

then we'll output stack is not empty followed by an end line

and if we save compile

and run the program we'll now get stack is not empty

we can also retrieve the value at the top of the stack using the top method

so we could call here numbers dot top

and this is going to return the value that's at the top of the stack.

We'll output that.

We'll have here top colon and we'll output the value at the top of the stack followed by an end line.

And if we save compile and run the program, we'll now get the value at the top of the stack is eight.

And that makes sense because right now eight is the only thing on the stack.

We could try to push a few more values onto the stack

so right now we have eight at the bottom of the stack which is also the top of the stack.

We could push on, let's say, the value 9.

We'll have numbers .push and then 9.

So then at this point, 9 would become the new top of the stack and 8 would still be at the bottom of the stack.

We could also push on the value 5.

We could have numbers .push 5.

And then 5 would be at the top of the stack and the stack would now have a size of 3.

So let's output the size and top after performing these operations.

Down here, we'll output the top again with cout top and numbers .top.

We'll also output an endline for spacing and we'll also output the new size.

So we'll have cout and size colon and we'll have numbers .size to output the size followed by an end line.

And now if we save compile and run the program, we'll get that the new top of the stack is five and the new size of the stack is three,

both of which are correct.

So then we could also pop a value from the stack.

So for example, with this stack here, if we pop a value from the stack, it's going to be the element five that's removed from the stack.

so let's try that we'll use the pop method of the stack object to do that we'll have here numbers .pop

now this method is only going to remove the element at

the top of the stack it's not going to return it

so we'll output here the new top of the stack

and the new size of the stack we'll copy these and paste them after the pop.

So now if we save compile and run the program, we'll get that the new top of the stack is nine and the new size of the stack is two,

both of which are correct because here we have nine at the top of the stack

and the size of the stack is now two.

Now again, the pop method is not going to return the value that's removed from the stack.

So we're going to lose it if we don't save it.

So what we may want to do is save the value in a variable.

For example, we could have here int popped value is equal to,

and we'll call numbers .top to return the value that's currently at the top of the stack

and save it into this variable here.

Then we can pop the value, but we still have the value saved in this variable here.

So we could output that we could have cout

and let's say popped value colon

and we'll output the pop value followed by an inline and

if we save compile

and run the program we'll see the pop value is five

the previous top of the stack before we pop the value

off the stack now we can also use the swap method to swap the contents of two stacks.

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So for example, let's declare another stack to store int values.

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We'll have here stack int, and we'll call this stack other stack.

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Then we'll push the value four onto the stack.

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So we'll have other stack dot push four.

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So this stack only has one value on the stack.

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We could output the size of the stack here.

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We could have cout

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and then other stack size colon we'll output the size of the other stack

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and we'll also output an end line as well.

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So now if we save compile and run the program, we'll get here that this other stack has a size of one.

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We can now swap this stack with our original stack numbers.

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So we could have other stack and then dot swap to call the swap method and we'll pass it numbers.

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And this will swap the contents of the two stacks.

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So other stack should now have the two values nine and eight and numbers should now have the one value four.

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Let's output the size of the stacks to confirm this.

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So we'll output here an end line and then we'll output the size of other stack.

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So we'll copy this and paste it here

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and we'll also put the size of the number stack with numbers size colon

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and we'll output numbers dot size

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and then we'll output end lines as well one after outputting the other stack size

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and another after outputting the number stack size so now if we save compile and run the program,

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we'll see the other stack size is two and the number stack size is one.

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So we can tell these stacks have been swapped.

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Now the type int is a primitive type in C++.

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We can also push objects onto a stack using the push method.

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If we use the push method to push objects onto the stack, a copy of the object is going to be created.

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There's also a method called in place.

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The in place method will actually forward its arguments to the constructor of the object

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and create an object for the purposes of the stack.

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Let me show you the difference.

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The first thing we'll do is define our own class called employee.

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So up here we'll define a class called employee and this is going to be a pretty simple class.

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We'll have one public member variable called days.

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The constructor for employee objects

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is going to accept an int as an argument called days

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and we'll set the days member variable to the days value that's provided.

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We'll also output in this constructor constructor called

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and the number of days just

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so we can verify the constructor has been called followed by

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an inline we'll also create what's called a copy constructor the

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copy constructor is called to create a copy of the object

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its parameter is going to be a reference parameter of the same type of this object here employee

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so we'll have const employee and then end employee and we'll output here that the copy constructor has been called.

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So we'll have copy constructor called and we'll output the days in this object here followed by an end line.

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And we'll also set this copy objects days to match the days of the object that it's copying.

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So we're making a copy of this object here.

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So we're going to take its days value and set the days member variable of this employee object to that value.

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So then down here, we'll declare a stack to store employee objects.

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We'll have stack employee, and we'll call this stack to store employee objects, employee underscore stack.

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We'll create an employee object called Joe.

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We'll have employee Joe, and we'll have 10, and maybe 10 is the number of days that Joe worked, then we'll push Joe onto the stack.

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So we'll have employee stack dot push Joe.

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So now if we save compile and run the program,

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we'll get this constructor called 10 and then again, copy constructor called 10.

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So what's happened here is two objects have been made.

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Originally, we made the Joe object here.

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then when we push Joe onto the stack, a copy of the Joe object is made.

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So be aware of the fact that's how it works.

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Now we could also use a method called emplace,

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and the emplace method will actually forward its arguments to the constructor of this type of object that the stack is storing, in this case here, employee.

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And it's going to create an object that's going to exist on the stack.

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So if we have here employee stack and we call the emplace method

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and we pass it let's say 20 what's going to happen

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is 20 is going to be passed to an employee object constructor

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and the employee object that's created is going to be pushed onto the stack.

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So if we save compile and run the program notice this this time we only get constructor called 20

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No copy constructor is called.

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So in the case of Joe, the Joe object was created, and when we push Joe onto the stack, a copy of that object was created.

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Here, that's not occurring.

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Here, what's happening is the object is being created and placed directly on the stack.

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Now, notably, if we call the top method to retrieve the element at the top of the stack, and we assign the result to an employee variable,

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at that point the copy constructor is going to be called.

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So if we had let's say employee and we'll have maybe top employee is equal to employee stack dot top.

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At this point the copy constructor will be called to make a copy of that employee object.

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So if we save compile

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and run the program we'll find now a copy has been made of that object at the top of the stack.

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Now when we pop an object from the stack, the destructor for that object is going to be called.

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So we could have a destructor in our employee class.

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We'll have here tilde employee.

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And again, we'll make a very simple destructor.

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What we'll do is just output that the destructor has been called.

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So we'll have cout and then destructor.

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and called and we'll put the days followed by an inline.

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So now down here, if we pop the top element from the stack, we'll see that destructor is called.

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So what we'll have is employee and then stack and we'll have dot pop to pop the top element from the stack.

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I'm actually going to comment out this line here and this line here and this line here.

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So we can just focus on this one element at the top of the stack.

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So right now, if we save compile and run the program, we'll see here that we get destructor called 20.

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So when we pop that object from the stack, the destructor was called.

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So in C++, the implementations of different data structures like stacks and queues that come with the standard template library are called containers.

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In particular, the stack container is called a container adapter.

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It's called a container adapter because underlying a stack object is going to be a container object of a different type.

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So there might be something like a vector

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or a DQ container object that's really behind the scenes where

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the stack object is kind of like a limited interface

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that allows us to access that other object in a limited way that conforms to the rules of a stack, which is important.

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The last in first out behavior of stack data structure elements is what makes stacks useful in certain scenarios like parsing.

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So this is how we can use stacks in the C++ standard template library.

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Check out portfoliocourses .com where we'll help you build a portfolio that will impress employers.

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Thank you.

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