Question

so we need to write a method

byteSwap – swaps the nth byte and the mth byte

• Examples: byteSwap(0x12345678, 1, 3) = 0x56341278
• byteSwap(0xDEADBEEF, 0, 2) = 0xDEEFBEAD
• You may assume that 0 <= n <= 3, 0 <= m <= 3
• Legal ops: ! ~ & ^ | + << >>

also we cannot use loops/recursion

but my following code fails with this test case :

Test byteSwap(-2147483648[0x80000000],0[0x0],3[0x3]) failed…
…Gives -128[0xffffff80]. Should be 128[0x80]

not sure why though this is my code so far

int byteSwap(int x, int n, int m) { 
int nBitShift = n << 3;  
int mBitShift = m << 3;
int nByte = x & (0xFF << nBitShift);  //gets the byte at position n
int mByte = x & (0XFF << mBitShift);  //gets the byte at position m 
int newX = x + ~(nByte + mByte) + 1; //make 0's in the nth and mth byte
nByte = nByte >> nBitShift; //shift back    
nByte = nByte << mBitShift; //shift to the mth spot
mByte = mByte >> mBitShift;  //shift back   
mByte = mByte << nBitShift; //shift to the nth spot

return nByte + mByte + newX;

}

edit:yes this is hw, but i need help

Answer 1

Arithmetic shift of a signed value sign-extends the operand. If you switch the type of your temporary variables to unsigned, your solution would avoid this problem.