A. Stacks

This chapter introduces stacks and call stacks, which will help you understand block updates.

1 Understanding Stacks

A stack is a first-in-last-out (FILO) data structure, where elements can only be added or removed at one end.

1.1 First In Last Out

Imagine a Pringles can:

Chips can only be placed in from the top, and can only be taken out from the top.
Therefore, the chip placed at the bottom first will be the last one that can be taken out.

This is the "first in last out" principle.

1.2 Stack

A stack is like a Pringles can.
In data structure terminology:

"Putting chips into the can" corresponds to push;
"Taking chips from the top" corresponds to pop.

Additionally:

The top of the can is equivalent to the top of the stack
The bottom of the can is equivalent to the bottom of the stack

That's a stack.

2 Call Stacks

A call stack is a mechanism that programs use at runtime to manage function calls. It relies on the first-in-last-out rule to ensure functions return correctly.

3 How It Works

Imagine you're making phone calls to ask a question:

You call your friend A to ask a question.
A doesn't know the answer, so they call their friend B to ask.
B isn't sure either, so they call their friend C to ask.

This time, C finds the answer and tells B first.
B then tells A.
Finally, A tells you the answer.

This follows the call stack pattern:

Each phone call pushes a new "call task" onto the "call stack".
When C is searching for the answer, the task at the top of the stack is "C searching for the answer".
After C finds the answer, the most recent call completes first, then returns layer by layer to the callers (B → A → you).
Your initial phone call was the first task pushed onto the call stack, so it sits at the bottom of the stack.

In a program:

"Making a phone call" is equivalent to calling a function;
"Getting an answer and passing it back" is equivalent to a function returning a result.

This way, when functions are nested, the program can ensure each function completes and returns to its caller correctly.

A. Stacks

This chapter introduces stacks and call stacks, which will help you understand block updates.

1 Understanding Stacks

A stack is a first-in-last-out (FILO) data structure, where elements can only be added or removed at one end.

1.1 First In Last Out

Imagine a Pringles can:

Chips can only be placed in from the top, and can only be taken out from the top.
Therefore, the chip placed at the bottom first will be the last one that can be taken out.

This is the "first in last out" principle.

1.2 Stack

A stack is like a Pringles can.
In data structure terminology:

"Putting chips into the can" corresponds to push;
"Taking chips from the top" corresponds to pop.

Additionally:

The top of the can is equivalent to the top of the stack
The bottom of the can is equivalent to the bottom of the stack

That's a stack.

2 Call Stacks

A call stack is a mechanism that programs use at runtime to manage function calls. It relies on the first-in-last-out rule to ensure functions return correctly.

3 How It Works

Imagine you're making phone calls to ask a question:

You call your friend A to ask a question.
A doesn't know the answer, so they call their friend B to ask.
B isn't sure either, so they call their friend C to ask.

This time, C finds the answer and tells B first.
B then tells A.
Finally, A tells you the answer.

This follows the call stack pattern:

Each phone call pushes a new "call task" onto the "call stack".
When C is searching for the answer, the task at the top of the stack is "C searching for the answer".
After C finds the answer, the most recent call completes first, then returns layer by layer to the callers (B → A → you).
Your initial phone call was the first task pushed onto the call stack, so it sits at the bottom of the stack.

In a program:

"Making a phone call" is equivalent to calling a function;
"Getting an answer and passing it back" is equivalent to a function returning a result.

This way, when functions are nested, the program can ensure each function completes and returns to its caller correctly.

A. Stacks

This chapter introduces stacks and call stacks, which will help you understand block updates.

1 Understanding Stacks

A stack is a first-in-last-out (FILO) data structure, where elements can only be added or removed at one end.

1.1 First In Last Out

Imagine a Pringles can:

Chips can only be placed in from the top, and can only be taken out from the top.
Therefore, the chip placed at the bottom first will be the last one that can be taken out.

This is the "first in last out" principle.

1.2 Stack

A stack is like a Pringles can.
In data structure terminology:

"Putting chips into the can" corresponds to push;
"Taking chips from the top" corresponds to pop.

Additionally:

The top of the can is equivalent to the top of the stack
The bottom of the can is equivalent to the bottom of the stack

That's a stack.

2 Call Stacks

A call stack is a mechanism that programs use at runtime to manage function calls. It relies on the first-in-last-out rule to ensure functions return correctly.

3 How It Works

Imagine you're making phone calls to ask a question:

You call your friend A to ask a question.
A doesn't know the answer, so they call their friend B to ask.
B isn't sure either, so they call their friend C to ask.

This time, C finds the answer and tells B first.
B then tells A.
Finally, A tells you the answer.

This follows the call stack pattern:

Each phone call pushes a new "call task" onto the "call stack".
When C is searching for the answer, the task at the top of the stack is "C searching for the answer".
After C finds the answer, the most recent call completes first, then returns layer by layer to the callers (B → A → you).
Your initial phone call was the first task pushed onto the call stack, so it sits at the bottom of the stack.

In a program:

"Making a phone call" is equivalent to calling a function;
"Getting an answer and passing it back" is equivalent to a function returning a result.

This way, when functions are nested, the program can ensure each function completes and returns to its caller correctly.

A. Stacks

This chapter introduces stacks and call stacks, which will help you understand block updates.

1 Understanding Stacks

A stack is a first-in-last-out (FILO) data structure, where elements can only be added or removed at one end.

1.1 First In Last Out

Imagine a Pringles can:

Chips can only be placed in from the top, and can only be taken out from the top.
Therefore, the chip placed at the bottom first will be the last one that can be taken out.

This is the "first in last out" principle.

1.2 Stack

A stack is like a Pringles can.
In data structure terminology:

"Putting chips into the can" corresponds to push;
"Taking chips from the top" corresponds to pop.

Additionally:

The top of the can is equivalent to the top of the stack
The bottom of the can is equivalent to the bottom of the stack

That's a stack.

2 Call Stacks

A call stack is a mechanism that programs use at runtime to manage function calls. It relies on the first-in-last-out rule to ensure functions return correctly.

3 How It Works

Imagine you're making phone calls to ask a question:

You call your friend A to ask a question.
A doesn't know the answer, so they call their friend B to ask.
B isn't sure either, so they call their friend C to ask.

This time, C finds the answer and tells B first.
B then tells A.
Finally, A tells you the answer.

This follows the call stack pattern:

Each phone call pushes a new "call task" onto the "call stack".
When C is searching for the answer, the task at the top of the stack is "C searching for the answer".
After C finds the answer, the most recent call completes first, then returns layer by layer to the callers (B → A → you).
Your initial phone call was the first task pushed onto the call stack, so it sits at the bottom of the stack.

In a program:

"Making a phone call" is equivalent to calling a function;
"Getting an answer and passing it back" is equivalent to a function returning a result.

This way, when functions are nested, the program can ensure each function completes and returns to its caller correctly.