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contracts...lottery.sol

// SPDX-License-Identifier: GPL-3.0

pragma solidity >=0.5.0 <0.9.0;

contract lottery {
    address public manager;
    address payable[] public participants;

    constructor (){
        manager=msg.sender;           // glob_var
    }

    receive() external payable
    { 
        require(msg.value==1 ether);     // only once we can use this func it is always external or payable
        participants.push(payable(msg.sender));
    }

    function getbalance() public view returns(uint)
    {
        require(msg.sender==manager);
        return address(this).balance;
    }
    function random() public view returns(uint)
    {
        // keccak256() creates random no in string so we can convert into uint
     return uint(keccak256(abi.encodePacked(block.difficulty,block.timestamp,participants.length)));
    }

    function selectwinner() public 
    {
        require(msg.sender==manager);
        require(participants.length>=3);
        uint r=random();
        address payable winner;
        uint index=r%participants.length;  // it converts big no into participents.length
        winner=participants[index];        // random participents
        winner.transfer(getbalance());
        participants=new address payable[](0);    // clear data from participents array

    }
}