lab_encoding/questions.md

Boolean questions

Create the following variables.

a = Bits("11110000")
b = Bits("10101010")

For each of the following bytes, give an equivalent expression which uses only a, b, and bit operators. The answers to the first two questions are given.

1. 01010101

~b

2. 00000101

~a & ~b

3. 00000001

a>>7

4. 10000000

(a>>7)<<7

5. 01010000

~b<<4

6. 00001010

b>>4

7. 01010000

~b<<4

8. 10101011

b|(~a>>3)

Integer questions

These questions are difficult! Try exploring ideas with Bits in Terminal, a paper and pencil, and a whiteboard. And definitely talk with others.

9. If a represents a positive integer, and one = Bits(1, length=len(a)), give an expression equivalent to -a, but which does not use negation.

(~a) + one

10. It is extremely easy to double a binary number: just shift all the bits to the left. (a << 1 is twice a.) Explain why this trick works.

a << 1 doubles a because shifting left moves all the bits one place, so each bit’s value doubles.

11. Consider the following:

>>> hundred = Bits(100, 8)
>>> hundred
01100100
>>> (hundred + hundred)
11001000
>>> (hundred + hundred).int
-56

Apparently 100 + 100 = -56. What's going on here?

100 + 100 = 200, but we only have 8 bits, which can’t hold 200, so it becomes a negative number.

12. What is the bit representation of negative zero? Explain your answer.

Negative zero is 00000000 because flipping zero and adding 1 is still zero.

13. What's the largest integer that can be represented in a single byte? Explain your reasoning.

The largest integer that can be represented in a single byte is 127 because the first bit is 0 (positive) and the other 7 bits are all 1, which makes the biggest number possible.I also checked this in the terminal using largest = Bits("01111111"), print(largest.int).

14. What's the smallest integer that can be represented in a single byte? Explain your reasoning.

The smallest integer that can be represented in a single byte is -128 because the first bit is 1 (negative) and the other 7 bits are all 0.

15. What's the largest integer that can be represented in n bits? Explain your reasoning.

    The largest integer that can be represented in n bits is when the first bit is 0 (positive) and all the other bits are 1. This makes the biggest number possible with n bits.

Text questions

16. Look at the bits for a few different characters using the utf8 encoding. You will notice they have different bit lengths:

    >>> Bits('a', encoding='utf8')
    01100001
    >>> Bits('ñ', encoding='utf8')
    1100001110110001
    >>> Bits('♣', encoding='utf8')
    111000101001100110100011
    >>> Bits('😍', encoding='utf8')
    11110000100111111001100010001101
    

    When it's time to decode a sequence of utf8-encoded bits, the decoder somehow needs to decide when it has read enough bits to decode a character, and when it needs to keep reading. For example, the decoder will produce 'a' after reading 8 bits but after reading the first 8 bits of 'ñ', the decoder realizes it needs to read 8 more bits.

    Make a hypothesis about how this could work.

The decoder knows how many bits to read by looking at the first bits. The pattern at the start tells it if the character is 1, 2, 3, or 4 bytes long.