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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How to print the value of a Struct ?(mapping)

If the set function receives two inputs 1.["0xca35b7d915458ef540ade6068dfe2f44e8fa733c",50,"ROC","park"] 2.["0x14723a09acff6d2a60dcdf7aa4aff308fddc160c",60,"AU","school"] How do I display the ...
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About send Ethereum Classic (ETC) coin

Need guideline for send ETC coin using web3js,As per documented flow I understand we can access all methods same as ETH but I have query regarding set provider for ETC after that I can connect to ETC ...
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Migrate Database

I want to migrate my no-SQL dB on-chain. There are two options as far as I know: Store the objects in the form of string using a simple smart contract. Create a structure to implement the dB schema. ...
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Contract address is not shown in rinkeby testnet etherscan

I am using Getch with truffle framework and i created an account. I have Migrations and Election.sol. After i can migrate, i couldnt see it in etherscan website. Although truffle migrate --reset --...
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Are the timestamps of blocks for Ethereum only accepted when they are monotonically increasing?

Are the timestamps of blocks for Ethereum only accepted when they are monotonically increasing? If yes, why? And why other blockchains like Bitcoin and other blockchains with shorter block time can ...
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How can I find the block where a user enacted a certain event on a contract?

I'm trying to build a web3.py program to take in a given user's address, and return the amount of ETH traded since they added liquidity to a Uniswap exchange. Uniswap is a smart contract on the ...
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22 views

Code:const web3 = new Web3(window.web3.currentProvider) Error: window.web3 is undefined

I am going to connect my contract with the website.However,there was something wrong with the web3? I'm not sure. import Web3 from "web3"; const web3 = new Web3(window.web3.currentProvider); const ...
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11 views

Which Protocol is suitable if I want a validated person to authenticate the block?

I want to create a system in which Every user will have some specific authorities. Like Examination will validate the block containnig Student Marks. Finance should have authority to add block ...
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Error: Keystore: Must include seedPhrase in createVault inputs. How to fix?

i'm studying blockchain development, and i'm trying to create wallet with seed words using eth-lightwallet and vuejs. But i receive this error: Keystore: Must include seedPhrase in createVault inputs, ...
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How to copy the data from a running private network to another

I have two seperately running private ethereum networks,i want to copy the data from the first private network to the second one without stopping.I have tried multiple ways but was not successful.I ...
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Can I Create Node.js API for each function in Smart Contract but sign a Transaction from frontend via Metamask.?

I am trying to develop a Dapp with nodejs backend but I cannot get accounts from Metamask which resides on my frontend. I tried to create an endpoint for each function in Smart Contract. And it seems ...
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Data encryption

Since all data on the Blockchain is public. Presume I have multiple users submitting data which I want to hide, is there a way I can encrypt that data on the Blockchain ? Or do I have to encrypt off-...
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Get Submitted Data from Blockchain

I'm using a html code connected to my smart contract via testrpc node, where users submit temperature data. How can I view that data from the smart contract, each time I view from the contract I see ...
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2answers
26 views

do users should be aware of smart contract of the dApp they are using?

So dApps are frontend + contract. Users interact with smart contracts via frontend. Do users i.e. people should be aware of how the contract is designed while using any dApp? are their any risks of ...
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console.log(App.ticTacToeInstance.events); in createNewgame function getting undefined

import Web3 from "web3"; import { default as contract } from 'truffle-contract' import ticTacToeArtifact from "../../build/contracts/TicTacToe.json"; import $ from "jquery"; let TicTacToe = ...
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1answer
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html:82 Uncaught ReferenceError: App is not defined at HTMLButtonElement.onclick (html:82)

It's showing me uncaught reference error in my index.html file <div id="candidate-box"></div> <button class="btn btn-primary" onclick="App.vote()">Vote</button> ...
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19 views

How I can use blockchain in my current Mobile wallet application?

I want to apply the blockchain concept for my Fiat currency wallet. I have android app and use the nodejs in backend(API). I want to apply the blockchain in payment module to secure the payments. ...
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I can see my ethereum but its not going to my wallet

My ethereum has been confirmed delivered but not in my wallet,fairly new but I have made successful transactions before,I can see my ethereum in etherscan.io 0x2ecc74c6e9c42eea14863fbc1e1d9656b339a6ec ...
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26 views

Interaction smart contract and third-party app : How to make sure the request comes from the original contract?

I'm working on a blockchain project where I need to access data stored outside the smart contract. The smart contract own a RSA keypair (one public and one private key). We want the private key to ...
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20 views

Block-chain data retrieval

I'm using a testrpc to test my sensor contract, where many account get to submit data to the block-chain whether using an html client side application or using the smart contract is self. How can I ...
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Getting TxnHash but not recipt web3j android

I dont know if question is asked before but I did not find any. I am facing this issue since yesterday. I am able to get balance and txnHash but it never shows on etherScan.io , nor it gives me txn. ...
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Can anybody help me with protocol development?

I have seen a lot of resources for dApp development but I don't know how to start learning more about protocol development. I have read whitepapers, some articles and started reading go-ethereum's ...
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1answer
32 views

Create multiple tokens for a unique property

here is the idea: we assign a set of unique tokens to a company and then we allow people to buy some tokens and get partial ownership of that company. How can I achieve this? I was thinking of ...
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26 views

Geth Sync failed

I started to sync Ethereum using command geth --syncmode "fast" on my windows system. It was taking so much time so i have stopped sync and used the following command. geth --syncmode "fast" --cache=...
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1answer
45 views

totalSupply doesn't pass test

I'm starting programming. I wanted to do my first contract and I assigned totalSupply to 1000000. pragma solidity ^0.5.8; /** * The DappToken contract Constructor Set the total number of tokens ...
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36 views

Hosting pet-shop DApp on IPFS - JSON Files problem

I am trying to host DApp "pet-shop" which one of the truffle boxes on IPFS. rsyncing of src, build/contracts and ipfs add files went very well and there are plenty of peers as seeders on the network ...
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1answer
18 views

can't get struct in return

I am trying to execute unit tests for a solidity smart contract: I have: mapping (bytes16=>Device) public Device_Table struct Device { bytes16 UUID; bytes16 hardware_UUID; string ...
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32 views

Cannot read property 'deployed' of undefined

Hello i try with web3 to run frontend with remix. The chrome shows the error from title for this line> App.contracts.Creation.deployed().then(function(instance) { I would love to help me!
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Unhandled Rejection (TypeError): Election.methods.addCandidate is not a function

//solidity code function addCandidate(string memory _name) public { candidatesCount++; candidates[candidatesCount] = Candidate(candidatesCount, _name, 0); } //App....
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Time in blockchain and HTML

I'm trying to get time once data is submitted off-chain, since only way to get on-chain is via now to get the block time. Off-chain I'm using "document.getElementById("demo").innerHTML = Date()" to ...
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How to compile solidity code in Kali Linux

I have tried to install the remix IDE for compile my codes in solidity, since for some reason the remix IDE don't open online. I executed the codes bellow and give that errors. Sorry anything ...
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Can't call contract function in Remix in Web3 provider mode, but it works in Javascript VM

A simple contract for addition, an error is reported when calling the function add in Web3 provider mode: "Gas estimation errored with the following message (see below). The transaction execution ...
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Why this error occurring when i running my dapp?

I create a dapp using Solidity,Web3Js,Javascript and Html with metamask i always getting this error when i am trying to submit my transition i am getting this error at c (inpage.js:1) at ...
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138 views

Error: The data field must be HEX encoded data

I am trying to make a simple dApp by using a tutorial. I am using the following command ; npm install ganache-cli web3 so that we can use web3 to get fake ethereum nodes/accounts by connecting it ...
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1answer
96 views

Can't connect to localhost:8545 while using ethereumjs-testrpc web3?

I am developing a simple Voting dApp by following a tutorial. In this tutorial we are using npm install ethereumjs-testrpc web3 so that we can get the fake nodes of Ethereum. For that i first ...
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How can I combine Quorum and Zero Knowledge Range Proofs

How can I combine Quorum and Zero Knowledge Range Proofs I have 20 years of experience at java JEE coding, and 10 years in devops as well, but no experience with blockchains, or the Go language. I ...
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What repo in loom do I fork to create my own sidechain?

I am trying to create a sidechain. I believe I need the code for the node that supports the sidechain. How many repos will I need to collect to properly start my own sidechain? Ideally a proof of ...
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How to get the value from the frontend form and save value to the blockchain using +web3 1.0.0-beta.37 + reactjs

I am writing my first Dapp and want to save the value of the form to the blockchain. I am Using React and web3 js 1.0.0-beta.37 on the frontend. Also I am new to react. Following is my App.js ...
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How to handle password for unlock Account in web3?

In web3 there is function web3.eth.personal.unlockAccount(address [, callback]) to unlock the account so whenever we call the smart contract function we need to unlock the account in geth client ...
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Where I can find the response in the blockchain (transaction inputs) or storage file in smart contract (resultat of transaction)?

when a user sends a tarnsaction to define the access control of a resource to a device s1,then it will be executed by smart contract and the result of the transaction is stored in the storage file of ...
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Want to understand token migration

Can anyone please explain the process of Token migration from one network to another network, Like omesi go and tron has done. they first lauch there own ERC20 token over ethereum and later migrated ...
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How merkle tree is used to verify transaction in blockchain

May any one give example how merkle tree is used in blockchain to verify the transaction? I confused because validation of data in ethereum is done through the proof of work where you find the nonce(...
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How to prepare for blockchain interview?

I am learning blockchain and want to pursue my career on blockchain, so what will be the best way to do that? What certification I must do and what think I should cover? appreciate all your career ...
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14 views

Parity synced at not existing block 4839212

I have problem with parity, cant get synchronization in archive mode to end, anything, changing arguments, max-, min-, snapshot-peers, parity verisions do not help. I have blockchain in archive mode ...
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Automating user data storage on a blockchain

I have a smart contract that contains a function to store data onto the blockchain. The function stores two integer variables to the blockchain. Now, I can manually store the values from the remix-ide ...
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1answer
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When is chain data (length) compared between nodes? Is there any way to control this?

I am trying to use an ethereum blockchain in a multiagent network. I would like to control when information is exchanged between nodes to keep the network synchronous, so I am trying to figure out how ...
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Decrease the time between new blocks in Ethereum Development

I'm trying to create a DAPP (with docker-compose) which will require many transactions, and I am currently experiencing nonce problems with my Transaction Manager. However, I'm not looking for help ...
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How can I store the hash of a set of data in the ethereum and make the chain validated always?

I am working on a University Management System and I want to develop the system using Centralized Database like Mysql and to keep the data safe I want to use blockchain to store the hash of the data ...
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How to decode SHA256 hash value and retrieve data in Solidity

I am working on a project in solidity where i am encode a string to SHA256 hash value, and now i want to decode SHA256 hash value and retrieve the actual data. please help hoe to do it. Below is the ...
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Store data on ethereum blockchain from a text file

I have a simple smart contract running over a local Ethereum node. It has two functions: one for sending data and another for retrieving it back. I have a text file and want to send data from it to ...