Why functions define contract behavior in Blockchain / Solidity - Visual Breakdown

Concept Flow - Why functions define contract behavior

User calls function

↓

Function receives input

↓

Function executes logic

↓

Function updates contract state or returns value

↓

Contract behavior changes or outputs result

↓

User sees outcome

Functions in a contract receive inputs, run code, update state or return values, defining how the contract behaves.

Execution Sample

Blockchain / Solidity

mapping(address => uint) balance;

function deposit(uint amount) public {
    balance[msg.sender] += amount;
}

function getBalance() public view returns (uint) {
    return balance[msg.sender];
}

A deposit function adds to the user's balance; getBalance returns the current balance.

Execution Table

Step	Action	Input	State Change	Output
1	User calls deposit	amount=10	balance[user] = 0 + 10 = 10	none
2	User calls getBalance	none	state unchanged	returns 10
3	User calls deposit	amount=5	balance[user] = 10 + 5 = 15	none
4	User calls getBalance	none	state unchanged	returns 15

💡 Execution stops after returning balance or updating state as per function logic.

Variable Tracker

Variable	Start	After Step 1	After Step 3	Final
balance[user]	0	10	15	15

Key Moments - 2 Insights

Why does calling deposit change the contract state but getBalance does not?

Because deposit updates the balance variable (see steps 1 and 3 in execution_table), while getBalance only reads and returns the balance without changing it (steps 2 and 4).

What happens if a function is called with no input?

Functions like getBalance that don't need input can be called without parameters and still return data, as shown in steps 2 and 4.

Visual Quiz - 3 Questions

Look at the execution_table, what is balance[user] after step 3?

A 10

B 5

C 15

D 0

💡 Hint

Check the 'State Change' column at step 3 in the execution_table.

At which step does the contract state NOT change?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look for 'state unchanged' in the 'State Change' column in execution_table.

If deposit is called with amount=20 at step 3 instead, what would balance[user] be after?

A 30

B 20

C 15

D 10

💡 Hint

Add the new amount to the previous balance from variable_tracker.

Concept Snapshot

Functions in blockchain contracts define behavior by:
- Receiving inputs (parameters)
- Executing code to update contract state or return data
- Changing contract state only when modifying variables
- Returning values without state change for view functions
- Acting as the interface users interact with

Full Transcript

In blockchain contracts, functions are the building blocks that define how the contract behaves. When a user calls a function, it receives inputs and runs code. Some functions update the contract's state variables, changing stored data. Others just read data and return it without changing anything. For example, a deposit function adds to a user's balance, changing the state, while a getBalance function returns the current balance without changing state. This step-by-step execution shows how functions control contract behavior by managing inputs, state changes, and outputs.