These examples are part of my guide The best way to become Ethereum Developer in 2018

and 30/30 challenge on polish ethereum programming 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!

• 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

• 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

• 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

• 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

• 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

• 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

• 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

• 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

• implement ERC20 token based on the EIP specification

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

• create your own ERC721 token using Open Zeppelin implementation
• inherit from "ERC721Token"
• add custom name and symbol

Check Truffle examples

• 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)

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 :)

If you like it please share it with others :)