The Best way to become an Ethereum Developer in 2018 — part 3

5 min readAug 1, 2018

If you read previous parts of this guide (1, 2) you should know the basics of Solidity and you have your editor ready. Now it’s a time to write code yourself.

I have prepared a list of 30 challenges for you. I was publishing one challenge a day on Polish Ethereum FB group. Most of the challanges are single smart contracts, some of them are tests or even complete decentralized applications.

Here is the list of covered topics:

Solidity smart contracts (lottery, ICO, DAO…)
Tokens (ERC20, ERC721)
Testing (Solidity tests, JavaScript tests)
Security (Re-entry attack)
DAPPs & deployment (Truffle, Web3)
External services (Oraclize, Infura)
Storage (IPFS, Swarm)
and many more

How to learn

1. Read requirements.

It is good if you read through the requirements and extra materials I provided or google the subject yourself.

2. Write your code.

For most of the challenges (those without test coverage and frontend part) you can use online editor Remix. This might be easier to use and catch errors because of the auto debugger/compiler.

3. Check the solution.

Once you are done (or you completely got stuck) you can check my solution on the repository https://github.com/pbrudny/learning-solidity-2018 or click the challenge title.

4. Contribute (optional)

If your solution is different than mine. Feel free to make a Pull Request.

Solidity examples — 30 x challenges

01_say_hello

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

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

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

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

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_multiple_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

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

12_string_converter

create function to convert string to bytes32
add Solidity test for that function

13_route_manager

contract must store a list of bus stops
each bus stop has id, name and coordinates (i.e czn1, PKS Cieszyn, 10.123222, 33.212345)
owner should be able to add new stops
contract must have test coverage

14_testing_route_manager

add Solidity tests
add JavaScript tests

15_shared_wallet

contract has one owner
owner can add and remove managers
manager can withdraw all the funds
contract has a payable fallback function to receive transfers

16_multisig_wallet

create a multi signature wallet
transfers can only be done when 3 managers sign the transaction

17_crowdsale

create mintable coin (use Open Zeppelin)
create simple crowdsale (use Open Zeppelin)
set opening and closing time
set rate and wallet address

18_roles_management

create contract which inherites from Ownable (Open Zeppelin)
contract must have a list of managers
owner can add/update/remove manager
owner is a manager too
add modifier onlyManager

19_the_dao_attack

add very simplified The DAO contract (deposit, withdraw)
add attacker contract

20_lpg_price

get LPG price using Oraclize API

21_random_oraclize

get random number using Oraclize API

22_raffle_oraclize

users can join raffle once
owner can’t participate
winner is picked using RNG from Oraclize

23_ipfs_oraclize

add a simple json file to ipfs
use oraclize to read that file
store the result of one json attribute in a string variable

24_election

create one contract per election, providing a short name and address for each candidate.
creator of the contract gives the right to vote to each address individually
each voter can vote once
voter can not vote on himself