How to set byte in fixed-size byte array?

Question

  function computePairId(bytes20 base, bytes20 quote) public view returns (bytes4) {

    bytes32 pairHash = sha256(abi.encodePacked(base ^ quote));

    bytes4 pairId;
    pairId[0] = pairHash[28];
    pairId[1] = pairHash[29];
    pairId[2] = pairHash[30];
    pairId[3] = pairHash[31];
    return pairId;
  }

However I get

SolcCompilationError: Zocr.sol:31:5: TypeError: Expression has to be an lvalue.
    pairId[0] = pairHash[28];
    ^-------^

in Solidity ^0.4.24;

Answer 1

  function computePairId(bytes20 base, bytes20 quote) public pure returns (bytes4) {

    bytes32 pairHash = sha256(abi.encodePacked(base ^ quote));

    bytes4 pairId;
    bytes4 x = bytes4(0xff000000);

    pairId ^= (x & pairHash[28]);
    pairId ^= (x & pairHash[29]) >> 8;
    pairId ^= (x & pairHash[30]) >> 16;
    pairId ^= (x & pairHash[31]) >> 24;

    return pairId;
  }