Questions tagged [blockchain]

The Ethereum blockchain is a distributed database based on the Ethereum whitepaper specification and stores both the full transaction list and the most recent state.

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Duplicate data in “returnValues” field of event emitted in solidity script

I am using quoram distribution through Azure blockchain service.. we are emitting few events according to our logic and these events are being used to get specific data from the transaction.. though ...
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How to find the source code of my contract which is not yet verified

I had created a token using eth based blockchain via remix.ethereum.org But I need to verify the source code ,but I am not having the backup copy of source code How can I see the source code of the ...
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Error: Returned error: gas required exceeds allowance (89459648255865) or always failing transaction

I am running a contract on a private network in geth, and using web3js. I have the following error when I run a transaction: web3.min.js:1 Uncaught (in promise) Error: Returned error: gas required ...
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1answer
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parse error: expected type name [closed]

pragma solidity >=0.4.0 < 0.7.0; contract Doctor { mapping (address => doctor) internal doctors; mapping (address => mapping(address => uint)) internal doctorToPatient; ...
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ParserError pragma solidity ^0.4.21

thank you so much for your help I really appreciated this contract give me error in the line with (length) "ParserError: Expected primary expression. returns candidate.length" pragma solidity ^0....
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Getting HTTP/1.1 403 Forbidden when tyring to get account balance [closed]

some months ago, i can get the balance of my account using the same code (i run the code in Unity 2018 and get the code from tutorial in github). but, yesterday i can't get the balance. would anybody ...
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2answers
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mapp values do not upadte when calling other functions after populating the map

I am using react as my front end to call functions from smart contract . this is the front end where i call the functions. However, after calling the first function which populates the map I then ...
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Connect to any ethereum node remotely

Is it possible to connect to any synced ethereum node remotely and query their blockchain? Also, if the former is possible then are there any known remote nodes that offer this functionality for free? ...
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How to deploy blockchain app in heroku

I want to deploy my blockchain app in heroku and i just want to use ganache as my blockchain network. is that possible?
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Is it possible to use ganache blockchain when I deploy my web app in heroku? or should I deploy the contract first to a test network?

const web3 = new Web3( new Web3.providers.HttpProvider( "https://kovan.infura.io/v3/INFURA_KEY" ) ); const abi = JSON.parse( '[ { "constant": false, "inputs": [ { "internalType": "string", "...
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1answer
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Value of emitted events in App.js

Actually in my App.js I have called my contract function like this: [ this.state.swapcontract.methods.sendFunds(address).send({ from: this.state.account , value:amount}) ] The function is called ...
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solidity declarationerror identifier not found or not unique for voting

I have this code for a voting system but when I compile it give me error in here ( boolean if_voted; ) can any one help me please. thank you this is the code : pragma solidity >=0.4.22 <0.7....
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Question about transaction confirmation

My question is that when a miner broadcast some transactions and other nodes don't think these transactions are valid, what will happen? Fork?
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Will Ethereum Continues to Grow in Use and Price in 2020? [closed]

The Ethereum blockchain has been a platform enabling new tokens' initial life in cryptocurrency. Many developers use Ethereum as the go-to chain to create their business and tokenonomy (Token economy) ...
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1answer
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How to read blockheaders of Ethereum Blockchain?

TO be specific, how to read block headers of Rinkeby on Ropsten blockchain? Since both the blockchains are public test networks, reading data of one blockchain on another should be possible using ...
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1answer
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Is it possible to create this with Ethereum?

I am building a mobile application and I want to create a cryptocurrency around it. My problem is that I am new to crypto and to be honest I am not even sure if it’s possible what I want to do. To ...
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when iam runing npm run dev it showing me this error, how to solve this [closed]

[![shiva@shiva-VirtualBox:~/sta$ npm run dev StarNotary@1.0.0 dev /home/shiva/sta lite-server ** browser-sync config ** { injectChanges: false, files: [ './**/*.{html,htm,css,js}' ], ...
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ipfs with ethereum

Module not found: Can't resolve 'ipfs-api' in 'C:\Users.......\src' I have used this code //using the infura.io node, otherwise ipfs requires you to run a //daemon on your own computer/server. const ...
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Error: VM Exception while processing transaction: revert at PromiEvent When calling variable function from the deployed contract variable

I am new to blockchain and development and learning from the dapp university tutorial. I have created a contract Todolist.sol pragma solidity ^0.5.0; contract TodoList{ uint public ...
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ATTENTION!This page does not represent the entire state of the ethereum network - listing a node on this page is a voluntary process [closed]

I am trying to attach my private blockchain to ethstats. When i run the npm, it gives me this error.
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1answer
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How to connect two nodes on two different networks

I'm trying to create my own chain so I have a VM which is the main node that will mine all the time, so that I don't lose the chain if all the nodes are off. I want also to make my friends able to ...
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Can Blockchain Be Effectively used for Freelancing industry? [closed]

As more people are joining the work from home force, I want to ask if the blockchain can be put to use for freelancers and something really effective like Upwork can be created based on this amazing ...
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Confidential data encryption on blockchain

I already built a supply chain that is able to collect product information on Ethereum. Now, that data can be confidential, but I still want to do Matrix calculations on it. I came across homomorphic ...
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3answers
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Address as name of an array in solidity?

silly question. But can i make an array with its name as address of user? For example string[] public 0x7v77wfr349C9864De0119E94c3f8d something like that?
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1answer
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Is there something special about using blockchain technology in the public construction industry? [closed]

The German federal parliament "Deutscher Bundestag" is discussing* a proposal to use blockchain technology in the public construction industry. I see blockchain is a technology, something to solve ...
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Block Chain Polling/Voting using Honest/Costly Signalling [Why is this wrong, can this system work?]

This Poll measures (Support for X) not (People who support X) You signal support for a candidate by forfeiting any amount of value to a candidate specific address in return you get... A public and ...
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1answer
23 views

Difference between ERC 20 and ERC 721 transaction receipt

I want to fetch certain transaction logs from a local ethereum node. For that, I need to identify how to differentiate between ERC 20 and ERC 721 transaction receipts. From what I saw on Etherscan, i ...
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1answer
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Which is the best mobile platform to develop a blockchain based mobile application

I am new to the blockchain technology. For my semester porject i have to debelop a blockchain based healthdata management system. That includes a blockchain developed using ethereum to store the data ...
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How can I test a consensus protocol that I created?

Currently, I am designing my own blockchain consensus protocol and I want to test its performance. Is there any available tool or well-known framework that gives me this ability?
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RE: Universal Wallet

I would like to create a multi currency wallet as a POC. I am currently able to make a wallet for Ethereum (Like balance, tx and all) but would like to create it for other as well (like Bitcoin). I ...
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1answer
23 views

Why Do I need to deploy contract each time I start a geth node

I have a simple smart contract that I deploy to my own geth node and it works fine but the problem is when I close the geth console and start it again I have to redeply the smart contracts again Here ...
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PORT ERROR!! Set up a Private Ethereum instance with Geth on MacOS

Hello I am trying to set up a private ethereum instance with geth on MacOS I have a startnode.sh file using ATOM But every time I try to start the node with "./startnode.sh", the http access account ...
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What are “if” and “this” in this function?

function() public { if(delegate.delegatecall(msg.data)) { this; What are the requirements for "if" to execute "this" and what does "this" refer to in this function?
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How to pass a selected radio button value in reactjs to ethereum blockchain

I am new to react and ethereum. I am working on voting dApp where I want to pass the selected candidate with a radio button in react to the blockchain. How do i go about it? here is my code. I have ...
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2answers
39 views

Setting a value for ERC20 token on private blockchain?

Background: Currently we have an Dapp running on Ropsten. We've created an ERC20 token on Ropsten. Our employees are awarded these tokens over time. We want to use Dappos to allow our employees to ...
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16 views

How to create blockchain nodes locally

For educational purposes I am implementing a blockchain from scratch without the use of any frameworks. How would I go about testing my blockchain using multiple nodes when I only have access to one ...
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24 views

Migrating 2 contracts in one Contract.sol script using 'truffle migrate'

I have 2 contracts in 1 .sol file and when I run 'truffle migrate', it tells me that it has deployed 1 of the contracts and given me an additional error: Blockquote Invalid number of parameters ...
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1answer
55 views

How to kill a contract in ethereum?

I'm trying to kill this contract /** *Submitted for verification at Etherscan.io on 2016-08-31 */ //sol Wallet // Multi-sig, daily-limited account proxy/wallet. // @authors: // Gav Wood <g@...
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2answers
31 views

Token Sale Smart Contract

I am new to Ethereium Block Chain and have been trying some code written in solidity as follows. pragma solidity ^0.5.0; import "./MyERC20Token.sol"; import "./ERC20Burnable.sol"; contract ...
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2answers
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Etherscan says there is not enough ether, why?

Etherscan says there is not enough ether, why? Arguments, which i enter => requestLoan(3,31,3) and after change account, provideGuarantee(0,3) I have created dApp and how it looks in my code you can ...
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1answer
22 views

Smart contracts, tokens and ether

I am sending some ethers and tokens to my smart contract. So how will the contract differentiate between ethers and tokens? Code will be helpful
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Are there more important “impossible triangles” aside from CAP theorem, FLP impossibility and Zamfir's triangle?

I have come across CAP theorem, FLP impossibility, and Zamfir's triangle ever since I began to study blockchain. As I interpret CAP theorem as the tradeoff between consistency, Availability, and ...
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Where's a contract storage stored exactly and how does it interact with the blockchain? [duplicate]

From what I've understood a contract is a special account that gets initialized with a 256 bit array and this array is stored as the root hash of a merkle tree, is that correct? But where is this ...
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1answer
29 views

What happens in a 50% Attack

I thik it is kind of clear to me that of 50%+1 attack, but now i am thinking about something that occurred when i was testing a DAPP in a two node network. Each node had a blockchain (the same ...
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2answers
42 views

how transaction in block-chain is validated

Please explain complete procedure/journey how transaction in block-chain get validated by miners, From broadcast of transaction to become part of new block.
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1answer
27 views

Understanding Transaction in Bitcoin [closed]

I read a lecture about Bitcoin Blockchain. As shown in the slide below, a user Alice sends a transaction to BoB. Alice should 1-) Hash(Hash of the transaction of the coin || Bob Public Key || Alice ...
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1answer
36 views

Solidity error message on compilation (DeclarationError: Identifier not found or not unique. using Address for address;)

I'm trying to compile a code for a crowdsale contract (Timed and with allowance) from openzeppelin library that is based on a deployed token. However during compilation on remix i get a syntax error ...
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2answers
26 views

Sending data outside the blockchain

Once a transaction is committed to the blockchain is there a way to let an entity outside the blockchain (i.e. An entity that is not a node in the blockchain) know that a particular transaction has ...
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0answers
38 views

Error: gas required exceeds allowance (8000000) or always failing transaction [duplicate]

I deployed a test ERC20 to a private ethereum blockchain. It deployed fine because I can get the transaction receipt, see the logs and everything looks ok as far as I can tell. However, when I try to ...
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1answer
29 views

Architectural blockchain adoption possibilities

I am working on a new shipments management project which will need the blockchain technology to certify the app-generated documents. I have no prior experience in blockchain and after a deep browsing ...

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