In our betting application, two people can choose to bet on a football match with one person supporting the home team and the other person supporting the away team. They both should bet the same amount of money, and the winner takes all the money. If the match is a draw, then they both will take back their money.

We will use the FastestLiveScores API to find out the result of matches. It provides a free API, which lets us make 100 requests per hour for free. First, go ahead and create an account and then generate an API key. To create an account, visit https://customer.fastestlivescores.com/register, and once the account is created, you will have the API key visible at https://customer.fastestlivescores.com/. You can find the API documentation at https://docs.crowdscores.com/.

For every bet between two people in our application, a betting contract will be deployed. The contract will contain the match ID retrieved from the FastestLiveScores API, the amount of wei each of the parties need to invest, and the addresses of the parties. Once both parties have invested in the contract, they will find out the result of the match. If the match is not yet finished, then they will try to check the result after every 24 hours.

Here is the code for the contract:

pragma Solidity ^0.4.0; 

import "github.com/Oraclize/Ethereum-api/oraclizeAPI.sol"; 
import "github.com/Arachnid/Solidity-stringutils/strings.sol"; 

contract Betting is usingOraclize 
{ 
    using strings for *; 

    string public matchId; 
    uint public amount; 
    string public url; 

    address public homeBet; 
    address public awayBet; 

    function Betting(string _matchId, uint _amount, string _url)  
    { 
        matchId = _matchId; 
        amount = _amount; 
        url = _url; 

        oraclize_setProof(proofType_TLSNotary | proofStorage_IPFS); 
    } 

    //1 indicates home team 
    //2 indicates away team 
    function betOnTeam(uint team) payable 
    { 

        if(team == 1) 
        { 
            if(homeBet == 0) 
            { 
                if(msg.value == amount) 
                { 
                    homeBet = msg.sender;    
                    if(homeBet != 0 && awayBet != 0) 
                    { 
                        oraclize_query("URL", url); 
                    } 
                } 
                else 
                { 
                    throw; 
                } 
            } 
            else 
            { 
                throw; 
            } 
        } 
        else if(team == 2) 
        { 
            if(awayBet == 0) 
            { 
                if(msg.value == amount) 
                { 
                    awayBet = msg.sender;           

                    if(homeBet != 0 && awayBet != 0) 
                    { 
                        oraclize_query("URL", url); 
                    } 
                } 
                else 
                { 
                    throw; 
                } 
            } 
            else 
            { 
                throw; 
            } 
        } 
        else 
        { 
            throw; 
        } 
    } 

    function __callback(bytes32 myid, string result, bytes proof) { 
        if (msg.sender != oraclize_cbAddress()) 
        { 
            throw;     
        } 
        else 
        { 
            if(result.toSlice().equals("home".toSlice())) 
            { 
                homeBet.send(this.balance); 
            } 
            else if(result.toSlice().equals("away".toSlice())) 
            { 
                awayBet.send(this.balance); 
            } 
            else if(result.toSlice().equals("draw".toSlice())) 
            { 
                homeBet.send(this.balance / 2); 
                awayBet.send(this.balance / 2); 
            } 
            else 
            { 
                if (Oraclize.getPrice("URL") < this.balance)  
                { 
                    oraclize_query(86400, "URL", url); 
                } 
            } 
        } 
    } 
}

The contract code is self-explanatory. Now compile the preceding code using solc.js or browser Solidity depending on whatever you are comfortable with. You will not need to link the strings library because all the functions in it are set to the internal visibility.