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I would like to do some basic operations in the runtime like:

let new_balance = .5 * old_balance;

Or save a percentage value in the runtime storage.

Or represent the balance of a user like: 3.141592 TOKENS

How can I do these kinds of things? Am I allowed to use floating point numbers in the runtime?

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You cannot use floating point math inside of the runtime.

Floating point numbers can lead to non-determinism in computer systems, and that would be exceptionally bad for blockchains since all of them must come to consensus.

Instead, you must make sure to use fixed-point arithmetic within the entire runtime environment, and Substrate even provides tools to help you with this in the crate sp_arithmetic.

So to answer these specific scenarios:

  • Consider dividing by 2 rather than multiplying by .5.
let new_balance = old_balance / 2;
  • Use the Percent type or some of the other per_thing types which behave like a standard percentage, but use fixed point math under the covers.

  • Use a fixed point number.

In Substrate, user balances are normally represented with a u128. However, to express these balances in "human form" a fixed point decimal is used. So while a user's real balance as a u128 would be something like 3_141_592_000_000 UNITS, we would represent it in the UI by first dividing by 10^12, and displaying it as 3.141592 TOKENS. Assuming your fixed point decimal is 12. However different networks may make different choices how many decimal places to use.

This decision is arbitrary.

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  • 1
    Is it possible to use sp_arithmetic in ink contracts?
    – forgetso
    Apr 25 at 13:53

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