Build Your First Smart Contract

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Bitcoin is the 1 crypto-currency and usually dominates the headlines but there's a 2 contender called Ethereum that does everything 1 does and more. In this post we will explain what Ethereum is using an analogy from computing history. Then we will use a business scenario and sample code to illustrate how Ethereum can help the business be more effective. We will also mention the fees for Ethereum's services.

Let's go back in time forty to fifty years. Before personal computers arrived, most businesses that could afford one had a mainframe computer. This was shared by all programmers in the organisation. A programmer of yore would submit a job at a teletype and retrieve the output from the printer. Along with the output came a bill for the job's use of CPU, memory and disk resources.

The problem with sharing one mainframe was that all business stopped whenever the mainframe stopped. So manufacturers created combinations of two or more computers designed to provide continuous service in case one had a fault. This service appeared to the business as the work of one virtual computer. Forward through time to now. Ethereum is a modern global massively fault-tolerant virtual mainframe. It is made up of many thousands of computers operating peer-to-peer and managing a blockchain examples of ethereum contracts the internet.

This is the same kind of blockchain used by Bitcoin and gives Ethereum the ability to synchronize peers and do crypto-currency transfers. So Examples of ethereum contracts is a distributed and decentralized virtual machine with the built-in ability to receive and send money.

A program running on this machine can operate like a virtual vending machine. Accepting tokens in cyberspace can be thought of as placing a coin in a slot. The service is virtual, but the real computers behind Ethereum get some wear-and-tear and incur real costs for use of space, electricity and network.

Ethereum collects a examples of ethereum contracts from the user to run a program. This enables Ethereum to pay the computer operators called miners for providing service.

Let's consider a fictitious charitable organisation directed by a council of five trustees. The traditional approach is to designate one trustee as Treasurer and trust them with examples of ethereum contracts control of the organisation's bank accounts.

Ethereum provides a platform to build an automated system that does all this in a consistent and transparent manner. Our system removes the need for the Treasurer role and frees the organization from needing a bank account. Let's introduce the examples of ethereum contracts contract in pieces. The complete version is included at the end.

We should mention that the code presented is immature and not ready for real operation. We'll begin with donations. This part is quite simple as any smart contract can be built to receive Ether. We introduce two functions. The one with the same name as the contract is examples of ethereum contracts constructor. It will be called by Ethereum when our contract is created.

The second function has no name. It is referred to as the fallback function because it is called whenever the contract receives a message and no other function matches the request. Such is the case whenever the contract receives an Ether transfer. Any function that is marked payable is allowed to receive Ether. Someone donating to OurCharity first buys Ether and sends some from their wallet to our contract.

They need to know our contract's address so the organization publishes that on the web and by other means. As an added benefit, all money transactions will be examples of ethereum contracts viewable on the blockchain, so it should no longer be necessary to issue receipts and audits will be easier - software does most of it.

Ether can come in to the smart contract but there is no way to spend yet. Let's look into the disbursement side now. Our approach requires that someone initate a "spend proposal". This includes the amount to spend and the payee's Ethereum address to receive the funds. We will also store the approvals within the same structure. Ethereum doesn't do floating point yet so we store the amount in units of weithe smallest possible units of Ether. Our organisation has a council of examples of ethereum contracts and requires at least three councillors to approve a spend.

Our contract knows each councillor by an Ethereum address and includes a function to add a councillor:. A councillor may approve at most once. The approve function will send the payment at most once, and only if it the contract has enough funds. The logic zeroes the examples of ethereum contracts amount after the send so the payment will be sent only once. Anyone can use their wallet software or the Ethereum node software to send a donation.

Here is a command-line example of donating one Ether:. The receipt for the donation is publicly listed on the blockchain: We can also call contract functions from the command-line though any complexity can make that difficult. A better approach is to run our node in the background and use simple javascript programs. Here is some code we might use to submit a spending proposal:.

Our contract is deployed from an Ethereum node which automatically includes one Ethereum account. Ethereum deducts some Ether from this account when we run the script examples of ethereum contracts create the contract. The amount of Ether required to run the machine is called gas. To convert from gas to Ether we multiply by the network's gasprice. When we compiled our contract, the compiler estimated our contract would require gas to construct so we provided just to be certain.

Any unused gas is automatically examples of ethereum contracts. To determine the dollar cost we have to do some math. Assume we burned the full gas. So our calculation goes as follows:. We spent one dollar to deploy our contract. The cost of calling the addProposal method was merely a few cents.

It cost us nothing to receive Ether because the sender pays the mining fees. Despite the fees it seems that the cost of doing business through our contract is much less than the cost of employing a Treasurer and paying bank fees. The other benefit is no waiting; Ethereum transactions complete within a few seconds whereas traditional business transactions could take days or even weeks.

Before we declare a complete victory we have to admit our design has a big drawback. Users pay Ethereum for storage as well as execution. The cost for storage increases significantly as it passes certain examples of ethereum contracts. This is an Ethereum feature that encourages programmers to minimize use of disk space. The initial marginal cost of storing a SpendProposal is very examples of ethereum contracts but will grow exponentially as more are added.

A better design, perhaps examples of ethereum contracts a future post, might store the proposals off of the blockchain but remain available to our contract.

Here is the smart contract. As stated previously, the code is illustrative and not suitable for production use. The Analogy Let's go back in time examples of ethereum contracts to fifty years. The Scenario Let's consider a fictitious charitable organisation directed by a council of five trustees. The Solution Our solution includes one Ethereum program known as a smart contract.

Donations We'll begin with donations. Disbursements Ether can come in to the smart contract but there is no way to spend yet.

Our contract knows each councillor by an Ethereum address and includes a examples of ethereum contracts to add a councillor: Here is a command-line example of donating examples of ethereum contracts Ether: Here is some code we might use to submit a spending proposal: Let's review the costs. So our calculation goes as follows: Room for Improvement Before we declare a complete victory we have to admit our design has a big drawback.

The Full Code Here is the smart contract. Authors get paid when people like you upvote their post. Simple for beginner to learn how to code smart contract in ethereum Blockchain.

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Blockchain open source code

This page will help you build a Hello, World contract on the ethereum command line. If you don't know how to use the command line we recommend you skip this tutorial and instead build a Custom token using the graphical user interface. Smart contracts are account holding objects on the ethereum blockchain. They contain code functions and can interact with other contracts, make decisions, store data, and send ether to others.

Contracts are defined by their creators, but their execution, and by extension the services they offer, is provided by the ethereum network itself. They will exist and be executable as long as the whole network exists, and will only disappear if they were programmed to self destruct.

What can you do with contracts? Well, you can do almost anything really, but for our getting started guide let's do some simple things: To start you will create a classic "Hello World" contract, then you can build your own crypto token to send to whomever you like. Once you've mastered that then you will raise funds through a crowdfunding that, if successful, will supply a radically transparent and democratic organization that will only obey its own citizens, will never swerve away from its constitution and cannot be censored or shut down.

And all that in less than lines of code. Please confirm that the GUI is closed before entering the geth console. Run geth to begin the sync process this may take a while on the first run.

The Frontier is a big open territory and sometimes you might feel lonely, so our first order of business will be to create a little automatic companion to greet you whenever you feel lonely. The Greeter is an intelligent digital entity that lives on the blockchain and is able to have conversations with anyone who interacts with it, based on its input. Here is its code:. You'll notice that there are two different contracts in this code: This is because Solidity the high level contract language we are using has inheritance , meaning that one contract can inherit characteristics of another.

This is very useful to simplify coding as common traits of contracts don't need to be rewritten every time, and all contracts can be written in smaller, more readable chunks. So by just declaring that greeter is mortal you inherited all characteristics from the "mortal" contract and kept the greeter code simple and easy to read.

The inherited characteristic "mortal" simply means that the greeter contract can be killed by its owner, to clean up the blockchain and recover funds locked into it when the contract is no longer needed. Contracts in ethereum are, by default, immortal and have no owner, meaning that once deployed the author has no special privileges anymore.

Consider this before deploying. You can get both of these by using a Solidity compiler. If you have not installed a compiler, you can either:. If you installed the compiler on your machine, you need to compile the contract to acquire the compiled code and Application Binary Interface. This will create two files, one file containing the compiled code and one file creating the Application Binary Interface in a directory called target. You will see that there are files created for both contracts; but because Greeter includes Mortal you do not need to deploy Mortal to deploy Greeter.

You have now compiled your code and made it available to Geth. Now you need to get it ready for deployment, this includes setting some variables up, like what greeting you want to use. Edit the first line below to something more interesting than "Hello World! If you don't have Solc installed, you can simply use the online IDE. Copy the source code at the top of this page to Remix and it should automatically compile your code. You can safely ignore any yellow warning boxes on the right plane.

To access the compiled code, ensure that the dropdown menu on the right pane has greeter selected. Then click on the Details button directly to the right of the dropdown. Create a temporary text file on your computer and paste that code.

Make sure to change the first line to look like the following:. Now you can paste the resulting text on your geth window, or import the file with loadScript "yourFilename. Wait up to thirty seconds and you'll see a message like this:. You may have to "unlock" the account that is sending the transaction using the password you picked in the beginning, because you need to pay for the gas costs to deploying your contract: There are many useful stats, including the latest gas prices at the network stats page.

Notice that the cost is not paid to the ethereum developers , instead it goes to the Miners , those peers whose computers are working to find new blocks and keep the network secure. Gas price is set by the market of the current supply and demand of computation.

If the gas prices are too high, you can become a miner and lower your asking price. Within less than a minute, you should have a log with the contract address, this means you've successfully deployed your contract. You can verify the deployed code which will be compiled by using this command:. If it returns anything other than "0x" then congratulations! Your little Greeter is live! If the contract is created again by performing another eth.

Since this call changes nothing on the blockchain, it returns instantly and without any gas cost. You should see it return your greeting:. If you compiled the code using Remix , the last line of code above won't work for you!

On the right pane, click on the Details button and scroll down to the ABI textbox. Click on the copy button to copy the entire ABI, then paste it in a temporary text document. Then you can instantiate a JavaScript object which can be used to call the contract on any machine connected to the network. Of course, greeterAddress must be replaced with your contract's unique address.

You must be very excited to have your first contract live, but this excitement wears off sometimes, when the owners go on to write further contracts, leading to the unpleasant sight of abandoned contracts on the blockchain. In the future, blockchain rent might be implemented in order to increase the scalability of the blockchain but for now, be a good citizen and humanely put down your abandoned bots.

A transaction will need to be sent to the network and a fee to be paid for the changes made to the blockchain after the code below is run. The self-destruct is subsidized by the network so it will cost much less than a usual transaction. This can only be triggered by a transaction sent from the contracts owner.

You can verify that the deed is done simply seeing if this returns Notice that every contract has to implement its own kill clause. In this particular case only the account that created the contract can kill it.

If you don't add any kill clause it could potentially live forever independently of you and any earthly borders, so before you put it live check what your local laws say about it, including any possible limitation on technology export, restrictions on speech and maybe any legislation on the civil rights of sentient digital beings.

Treat your bots humanely. Create a tradeable digital token that can be used as a currency, a representation of an asset, a virtual share, a proof of membership or anything at all. These tokens use a standard coin API so your contract will be automatically compatible with any wallet, other contract or exchange also using this standard.

The total amount of tokens in circulation can be set to a simple fixed amount or fluctuate based on any programmed ruleset. Building a smart contract using the command line This page will help you build a Hello, World contract on the ethereum command line.

So let's start now. Here is its code: Compiling your contract using the Solc Compiler Before you are able to deploy your contract, you'll need two things: The compiled code The Application Binary Interface, which is a JavaScript Object that defines how to interact with the contract You can get both of these by using a Solidity compiler.

If you have not installed a compiler, you can either: You can use these two files to create and deploy the contract. Make sure to change the first line to look like the following: Wait up to thirty seconds and you'll see a message like this: You can verify the deployed code which will be compiled by using this command: Run the Greeter In order to call your bot, just type the following command in your terminal: You should see it return your greeting: The Address where the contract is located The ABI Application Binary Interface , which is a sort of user manual describing the name of the contract's functions and how to call them to your JavaScript console To get the Address , run this command: Cleaning up after yourself: You can verify that the deed is done simply seeing if this returns 0: Design and issue your own cryptocurrency Create a tradeable digital token that can be used as a currency, a representation of an asset, a virtual share, a proof of membership or anything at all.

A tradeable token with a fixed supply A central bank that can issue money A puzzle-based cryptocurrency.