How to compare msg.data (calldata) data type to an integer data type uint256?

Question

As per my question here:

I need to compare msg.data (specifically, numbers that are sent with the sendTransaction data object) to a data type of uint256 within my contract.

How is this possible?

Answer 1

Byte-per-byte comparisons. Conversions needed. reddit link is dead. Why do you need to access msg.data directly?

EDIT: Convert a uint256 from calldata bytes to a proper uint256 in Solidity: https://gist.github.com/anonymous/a734a5d299ffa7b5a834

EDIT: The typing system and posting rules on this site are super weird. Anyways that works. It should always return true.

Answer 2

The specification for msg.data is the Ethereum Contract ABI.

Here's the code from the answer provided by Andreas:

contract ReadConvertUint256Bytes {

    function equal(uint a) constant returns (bool) {
        uint x = 0;
        for (uint i = 0; i < 32; i++) {
            uint b = uint(msg.data[35 - i]);
            x += b * 256**i;
        }
        return a == x;
    }

}

It starts reading from the byte indexed at 35, because the first 4 bytes (at index 0 to 3) is the Method ID (see ABI above), and everything in Ethereum internally is big-endian.

Answer 3

Whilst other answers may provide the tools required to answer this question as verbatim, my problem was solved because I was not aware of other ways to send parameters to contracts on Ethereum. Namely: eth.sendTransaction(2{from: eth.accounts[0], value: web3.toWei(5, 'ether'), to: '0xXXXX', gas: 200000, data: web3.fromAscii('MinedBlock')})

With '2' being the parameter being passed which can then easily be compared inside the contract to other variables. Thanks for the input.