Smart contract concept in Blockchain / Solidity - Step-by-Step Execution

Concept Flow - Smart contract concept

Start: Deploy Contract

↓

Contract Stored on Blockchain

↓

User Sends Transaction

↓

Contract Code Executes

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Check Conditions

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Update State

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Emit Events

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Transaction Complete

This flow shows how a smart contract is deployed, then how it runs when a user sends a transaction, checking conditions and updating state or rejecting the transaction.

Execution Sample

Blockchain / Solidity

contract SimpleStore {
  int storedData;
  function set(int x) public {
    storedData = x;
  }
  function get() public view returns (int) {
    return storedData;
  }
}

A simple smart contract that stores a number and lets users set or get it.

Execution Table

Step	Action	Input	State Before	State After	Output
1	Deploy contract	N/A	No contract	Contract deployed with storedData=0	N/A
2	Call set(42)	42	storedData=0	storedData=42	Transaction success
3	Call get()	N/A	storedData=42	storedData=42	Returns 42
4	Call set(100)	100	storedData=42	storedData=100	Transaction success
5	Call get()	N/A	storedData=100	storedData=100	Returns 100
6	Call set(-1)	-1 (invalid)	storedData=100	storedData=-1	Transaction success (no check in code)

💡 Execution stops after last transaction; smart contract runs code on each call updating or returning state.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6
storedData	0	42	42	100	100	-1

Key Moments - 3 Insights

Why does calling get() not change storedData?

Because get() is a view function that only reads storedData without modifying it, as shown in execution_table step 3 and 5 where state before and after remain the same.

What happens if we try to set a negative number?

In this simple contract, there is no check for negative numbers, so storedData changes to -1 anyway (step 6), but in real contracts you should add checks to prevent invalid data.

Why does the contract keep its state between calls?

Because the contract is stored on the blockchain and its variables like storedData persist between transactions, as seen in variable_tracker showing storedData values after each step.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of storedData after step 4?

A 42

B 100

C 0

D -1

💡 Hint

Check the 'State After' column in row for step 4 in execution_table.

At which step does the contract return the value 42?

A Step 2

B Step 4

C Step 3

D Step 5

💡 Hint

Look at the 'Output' column in execution_table for the step returning 42.

If we add a check to reject negative numbers in set(), what would happen at step 6?

A Transaction is rejected, storedData stays 100

B storedData changes to -1

C Contract crashes

D storedData resets to 0

💡 Hint

Refer to key_moments about input validation and execution_table step 6.

Concept Snapshot

Smart contracts are programs on the blockchain.
They run when users send transactions.
They store and update data persistently.
Functions can change state or just read it.
Transactions succeed or fail based on code logic.

Full Transcript

A smart contract is a program deployed on the blockchain that stores data and runs code when users send transactions. When deployed, it starts with initial state. Users call functions like set() to change stored data or get() to read it. Each call runs the contract code, checks conditions, updates state if allowed, and returns results. The contract keeps its state between calls because it lives on the blockchain. This example shows a simple contract storing a number. Calls to set() update the number, calls to get() return it without changing state. Without input checks, invalid data can be stored, so real contracts add validations. This visual trace helps understand how smart contracts execute step-by-step.