You can't, since only owner of ether have the private keys and able to sign transaction, the only way is to get user's private keys. Nobody will give you the private keys.
You can for example create a holder address, user send a deposit to address you own and after that you able to sent some ether from that address.
That's exactly what Etherscan does - they have a list of tokens that they track. You'll then have to do a balanceOf query on each token with the address you want to get balance for.
There is no central repository of token balances, since they are all their own individual contracts that just contain user balances.
Kovan has its own etherscan at: https://kovan.etherscan.io
You can search for any token e.g. DAI: https://kovan.etherscan.io/token/0xc707fd5a456eec2609463f7fea79756356f0a754
Check this project out as well: https://github.com/bokkypoobah/WeenusTokenFaucet - It has a list of a few ERC20's that you can mint from their faucet. You can then add these to a Uniswap ...
No, I don't know how you imagined this, but as long as a user has
the private key
the ETH still on the account
Nothing can stop him from spending his ETH. So whatever you do, locking ETH can only happen if ETH is transferred, be it to an escrow contract or maybe an off-chain multisig EOA.
No, they use a different algorithms to generate addresses.
No, you cannot use a bitcoin address to receive ethers, or any other cryptocurrency.
Yes, there are "Hierarchical Deterministic Wallets".
For Ethereum see this: How are ethereum addresses generated?
Please note, you can get started with a single centralized way to pull data from an API, but for your contract to be decentralized you'd want to pull data from multiple sources via multiple nodes.
You can, for example, pull data from a single Chainlink node just to start developing, however for a production grade system you'd want ...
The most Ethereum friendly solution would be
Create and deploy a controller smart contract. This co-ordinates that we have deterministic addresses for payment forwarded smart contracts.
For each new deposit address deploy a payment forwarded smart contract using CREATE2 from depositor controller. You need deploy a batch of these smart contracts upfront and ...
The Etherchain.org website has pages that show a list of recent blocks and transactions. Looking under the hood, it looks like they use a REST API which fetches the latest blocks and transactions. Limiting the length parameter in the API call should give you the latest block and latest transaction.
The reason you got that error because there is no function name payable that take an integer value and return payable address. The parser doesn't recognise it and give you an error like that.
I don't know your complainMap struct but i can guess that in it contain contrivutor variable and it's type is address. In solidity version greater than 0.5.0 they add ...