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learning-solidity-2018

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Basic examples of Solidity smart contracts

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# learning solidity ~~2018~~ [We are updating this project in 2024 😎]

These examples are part of my guide [The best way to become Ethereum Developer in 2018](https://medium.com/@pbrudny/the-best-way-to-become-ethereum-solidity-developer-in-2018-5606e54646e6)

and `30/30 challenge` on [polish ethereum programming](https://www.facebook.com/groups/531936723867447/?ref=bookmarks) FB group (members write one contract a day to improve Solidity skills).

In Ethereum world everything is changing dynamically.
I decided to make a set of Smart Contracts for learning purposes.

I know it's funny to name a repo this way but I am quite convinced that in 2019 these examples may not work with the latest Solidity.

Try to write a contract yourself before checking the code in the repo!

## Challenges

### 01_say_hello.sol
* set greeting on creation and allow to change it by the owner (creator of the contract)
* return greeting to everyone who calls `sayHello` method
* return `Hello Daddy` to the creator

### 02_balance_address_checker.sol
* return address of the contract
* return address of the contract's owner
* return address of the sender
* return balance of the contract
* return balance of the contract's owner (ONLY if you are the owner)
* return balance of the sender

### 03_basic_random.sol
* return random number 0-99
* the "financial importance" of this calculation is less than miner would get for mining single block
* https://ethereum.stackexchange.com/questions/419/when-can-blockhash-be-safely-used-for-a-random-number-when-would-it-be-unsafe?noredirect=1&lq=1

### 04_lottery_10_users.sol
* 10 users limit
* user has to pay 0.1 ether to join the lottery
* same user can join once
* owner of the contract can join the lottery
* when 10 users join then the winner is picked
* winner receives all the money
* new lottery starts when the winner is pickecd

### 05_lottery_no_limit.sol
* user has to pay 0.1 ether to join the lottery
* no limit for users number   
* same user can join multiple times
* owner of the contract can join too
* owner decides when to pick the winner
* winner receives all the money
* new lottery starts when the winner is pickecd

### 06_lottery_muliple_winners
* no limit for users
* user joins paying 0.1 eth and picking number 1-100
* owner decides when to generate random number (1-100) 
* users who picked generated numbers win
* total prize is distributed to all winners

### 07_fundraising
* fundraising has the goal to reach (amount is set on creation)
* fundraising has the time limit (time is set on creation)
* anyone can add any amount until time is up or the goal is reached
* when the time is up but the goal is not reached users can withdraw their funds
* when the goal is reached owner can withdraw all the money

### 08_basic_token
* intial supply of tokens is set on creation
* contract creator gets initial tokens
* tokens can be transferred to any account
* there is a protection from overflow
* everyone can check balances

### 09_ERC_20_token
* implement ERC20 token based on [the EIP specification]( https://github.com/ethereum/EIPs/blob/master/EIPS/eip-20.md)

### 10_ERC20_usage
* create your own ERC20 token using Open Zeppelin implementation
* inherit from "StandardToken"
* add custom name, symbol, decimals and initial supply

### 11_ERC_721_usage
* create your own ERC721 token using Open Zeppelin implementation
* inherit from "ERC721Token"
* add custom name and symbol

## Other challenges
* [Truffle challenges: 12-29](https://github.com/pbrudny/truffle-challanges)
* [Web3 deploy challenge: 30](https://github.com/pbrudny/web3-deploy-example)

## Contribution
* feel to free to add new challenges in the README.
* feel free to add your solution to any challenge (of course if it's different than the existing one)

### Example: 
If you want to add another Basic Token contract:
1. Fork the repo
2. Add `basic_token_b.sol` to the `07_basic_token` directory
3. Make a PR
4. Let me review it and merge it
5. Enjoy having a nice contribution to the great project :)

## Like it?
If you like it please share it with others :)

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