Solidity – Math Operations Between int and uint Explained

contract-developmentsolidity

How is it possible to either convert uint to int or do simple math comparison operations?

Something like:

uint a;
uint b;
int c;


if (a - b < c) doSomething();

What I'm trying to do is to see if the subtraction of two integers are negative or not. however one of those values comes from msg.value so it should be uint, and I'm not sure if we do uint – uint the result could smaller than 0 to check it on if condition.

Best Answer

To convert one type to another, use uint(x) or int(x). The rules are basically the same as in C, where the first bit of the uint becomes the sign.

For example:

if (int(a - b) < c) doSomething();

is likely what you want, as the numbers will be subtracted, then converted.

The type(x) syntax is used for all kinds of casting, such as truncating a number (uint8(someUint)), converting to and from odd types (bytes32(someUint)), or even casting a given address to a specific contract type (someContract(someAddress)).

Related Solutions

Solidity Math – Performing Logarithm Operations

Though there's no current implementation (and I couldn't see one in the dapp-bin either), you could implement your own using a Taylor Series, as suggested by Vitalik in this old Reddit thread.

For the example in the thread, it effectively comes down to something like the following:

 x = msg.data[0]
 LOG = 0
 while x >= 1500000:
     LOG = LOG + 405465
     x = x * 2 / 3
 x = x - 1000000
 y = x
 i = 1
 while i < 10:
      LOG = LOG + (y / i)
      i = i + 1
      y = y * x / 1000000
      LOG = LOG - (y / i)
      i = i + 1
      y = y * x / 1000000
return(LOG)

Bear in mind the number of steps and the associated gas cost...

Delegatecall in Transactions – Usage and Best Practices

DELEGATECALL is an instruction interpreted by EVM. So to execute DELEGATECALL you have to have a program (contract) that gets executed on EVM.

DELEGATECALL takes six operands, one of them is the address of the calee. So it can only call deployed contracts. Note that so called library is just a special contract which operations are called with DELEGATECALL.

So to answer your question directly as I understand it: No - it is not possible to execute arbitrary code from an external account.

What you can do though is you can create a contract that implements the logic as you want it and uses DELEGATECALL to preserve sender and value when calling other contracts.

Best Answer

Related Solutions

Solidity Math – Performing Logarithm Operations

Delegatecall in Transactions – Usage and Best Practices

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