Question

Preparing for exam and got stuck at this question:
Allowed operators are <<,+,& no loops allowed and minimum temp variables.
Write a function in C, that gets 4-bit number (char) and returns mirrored (relative to center) bits.
Example: given b4,b3,b2,b1 return b1,b2,b3,b4

O_o thanks!

it might be not clear, but general language tools are allowed (‘==’,if,>,< etc..)

Answer 1

This is not possible given the constraints of only the operators <<, +, & and no other constructs besides return.

To move b3 from the 3rd position to the 2nd position, you will need a way to shift to the right which requires something like >> or /. Of the operators provided, none can be used with b3 to set the 2nd or 1st bit position.

if you can use if statements and the assignment operator =, it is possible. You can then write a messy solution such as

char flip(char c)
{
  char f;
  f = (c & 1) << 3 + (c & 2) << 1;
  if (c & 4)
    f = f + 2;
  if (c & 8)
    f = f + 1;
  return f;
}

A more ugly but shorter one liner if you can use the similar to if ? operator.

char flip(char c)
{
  return (c & 1) << 3 + (c & 2) << 1 + ((c & 4) ? 2 : 0) + ((c & 8) ? 1 : 0);
}