Ethereum is a blockchain platform. It has its own native cryptocurrency, ether, but in this challenge, we'll be focusing on its ability to run smart contracts.

The ERC-20 standard describes an implementation of fungible tokens within a smart contract. Some examples of this implementation are EOS, OmiseGo, and the Golem Network Token. In this challenge, your job is to find the amount of tokens owned by a certain address using the fewest bytes possible.

The standard defines a balanceOf(address) contract method which is simple enough to use; however, it returns it in terms of a much smaller unit. To get the actual balance, you must divide the result by 10 to the power of whatever the decimals() method returns. For example, calling balanceOf(0x4530d03744d7ef769025623d31966da1e8485eef) returns 1.500000000000015e+21, and dividing it by 10^decimals() (10^18) results in the true balance of 1500.000000000015.


  • Assume that the computer that your program/function/etc. will be running on is running a fully-synced Geth node on the Ethereum mainnet with the JSON-RPC endpoint listening on the default port of localhost:8545.
  • Your program should take two inputs: the token contract address (e.g. 0xea097a2b1db00627b2fa17460ad260c016016977) and the wallet address (e.g. 0x4530d03744d7ef769025623d31966da1e8485eef). Input can be taken in any reasonable format, such as a string, an integer, or an array of bytes (NB: Ethereum addresses are 20 bytes long).
  • It should have one output: the token balance of the address in any reasonable format. Cases where the balance is zero must still be handled.
  • Loss of fine floating-point precision in the final balance (e.g. the 17th or 18th decimal after dividing the balance by 10^decimals()) is okay.
  • The ERC-20 standard technically says that the decimals() method might not be implemented; however, you may assume that it is.
  • The use of libraries like web3.js is permitted, but discouraged.
  • Standard loopholes are obviously not allowed.

Test Cases

token, wallet -> balance
0xea097a2b1db00627b2fa17460ad260c016016977, 0x4530d03744d7ef769025623d31966da1e8485eef -> 1500.000000000015
0xab95e915c123fded5bdfb6325e35ef5515f1ea69, 0x4530d03744d7ef769025623d31966da1e8485eef -> 317.18737335873794694
0x6810e776880c02933d47db1b9fc05908e5386b96, 0x3257bde8cf067ae6f1ddc0e4b140fe02e3c5e44f -> 8495410
0xb64ef51c888972c908cfacf59b47c1afbc0ab8ac, 0x0be3dead1ba261560362361838dd9671a34b63bb -> 314873.06011271

Reference Implementation

A reference implementation written in Crystal can be found here.

  • \$\begingroup\$ Sandbox link here. \$\endgroup\$ Jan 8, 2018 at 23:31


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Browse other questions tagged or ask your own question.