Solidity Integer Division – Cannot Perform Integer Division in Solidity

data-typesfixed-pointintegersmathsolidity

I'm building a new smart contract but really can't figure out how to perform integer division. I know that fixed point numbers haven't been implemented yet but it should be possible to perform 100/3 = 33, at least.

I hope someone can help me. I have seen multiple examples where they talk about rounded integer division but can't seem to be able to perform it.

Cheers,
Hugo

Best Answer

Problem is 4 & 5 fit in less that 256 bits (each). You end up with tiny uints in the constant expression, and then those aren't easily converted to the uint256, so ... cast the type explicitly.

uint x = uint(4)/uint(5);

Takeaway is caution with constants because they may be cast in unexpected types.

A sketchy idea:

contract Divide {
    
    function getDivided(uint numerator, uint denominator) public constant returns(uint quotient, uint remainder) {
        quotient  = numerator / denominator;
        remainder = numerator - denominator * quotient;
    }
}

Also check out "SafeMath" with "safeDiv()" at Zeppelin: https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/contracts/utils/math/SafeMath.sol

Also, since 4/5 obviously works out to 0 remainder 4, possibly you're really aiming for something more like 80%?

You can bump up the accuracy by working with "decimal" as is common in financial markets. You pass pairs around, the digits and the decimal offset.

So, 1.234 would be written as [1234, 3] with the 3 indicating that there is a decimal in the third spot.

In this case, you would calculate 80, with 2 decimal places (meaning .80).

4 * 10 ** 2 = 400, 400/5 = 80, we raised 4 by 10 ^ 2, so return ... 80,2 and optionally, a remainder.

Hope it helps.

Related Solutions

Solidity Fixed-Point Division – Implementing 128.128 Unsigned Fixed Point Division

Okay, so I'll post the solution here for the next guy. Shoutout to the comments for the helpful hints. The key here is one of the more obvious facts that you can break up a fraction with a common denominator into two additive parts. For example 12.525/9.5= (12/9.5)+(.525/9.5) with this in mind we have a way to break up our numbers into 2 uint256 numbers and just concatenate them with some fancy shifting.

    function div128x128 (uint256 x, uint256 y) internal pure returns (uint256) {
        unchecked {
            //Require denominator != 0
            require (y != 0);
            // xDec = x & 2**128-1 i.e 128 precision 128 bits of padding on the left
            uint256 xDec = x & 0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF;
             //xInt x *2**-128 i.e. 128 precision 128 bits of padding on the right
            uint256 xInt = x >> 128;
            //hi = xInt*2**256-1 /y ==> leave a full uint256 of bits to store the integer representation of the fractional decimal with 128.128 precision
            uint256 hi = xInt*(MAX_128x128/y);
            //xDec*2**256-1 /y ==> leave full uint256 of bits to store the integer representation of fractional decimal with 128.128 precision, right shift 128 bits since output should be the right 128 bits of precision on the output
            uint256 lo = (xDec*(MAX_128x128/y))>>128;
            /*Example: 12.525/9.5 := 12/9.5 + .525/9.5<-- legal to break up a fraction into additive pieces with common deniminator in the example above just padding to fit 128.128 output in a uint256
*/
            require (hi+lo <= MAX_128x128);
            return hi+lo;
        }
    }

This is one solution, that seems to be working so long as the require criterion are met. There are almost undoubtedly optimization improvements to be made. But I have tested this on some real data, and it seems to be accurate to 128.128 precision.

Best Answer

Related Solutions

Solidity Fixed-Point Division – Implementing 128.128 Unsigned Fixed Point Division

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