Lecture 4

strings and integers

MCS 260 Fall 2020
David Dumas

Reminders

• Quiz 1 due today at 6pm Central
  • Excuse requests must be sent to TA before deadline
• Python 3 and editor working?
  • If not, tell me immediately
• Worksheet 2 available, Quiz 2 will be posted soon

Storage units

A byte (B) is a sequence of 8 bits, equivalently, an 8-digit binary number or a 2-digit hex number. It can represent an integer between 0=$\texttt{0x00}$ and 255=$\texttt{0xff}$.

A word is a longer sequence of bits of a length fixed by the hardware or operating system. Today, a word usually means 16 bits = 2 bytes.

Computers store information as sequences of bytes.

Counting bytes to measure the size of data often leads to large numbers.

Coarser units based on SI prefixes:

• kilobyte (KB) = 1,000 bytes
• megabyte (MB) = 1,000,000 bytes
• gigabyte (GB) = 1,000,000,000 bytes

• kibibyte (KiB) = $2^{10}$ bytes = 1024 bytes
• mebibyte (MiB) = 1024 KiB = 1,048,576 bytes
• gibibyte (GiB) = 1024 MiB = 1,073,741,824 bytes

• Occasionally, SI abbreviations are used for IEC units; in Windows, "GB" means GiB.
• Very often, IEC units are read aloud using SI names; e.g. write 16GiB and read aloud as "16 gigabytes"

Code point numbers are always written $\texttt{U+}$ followed by hexadecimal digits.

The first 127 code points, U+0 to U+7F, include all the printable characters on an "en-us" keyboard, numbered exactly as in the older ASCII code (1969).

strings

A string literal is a way of writing a str in code.

Several syntaxes are supported:

        'Hello world'  # single quotes
        "Hello world"  # double quotes
        
        # multi-line string with triple single quote
        '''This is a string
        that contains line breaks'''
        
        # multi-line string with triple double quote
        """François: How is MCS 260?
        Binali: It's going ok, I guess.
        François: [shrugs]"""
    

Escape sequences

• $\texttt{\\n}$ - the newline character
• $\texttt{\\'}$ - a single quote
• $\texttt{\\"}$ - a double quote
• $\texttt{\\\\}$ - a backslash
• $\texttt{\\u0107}$ - Code point $\texttt{U+107}$
• $\texttt{\\U0001f612}$ - Code point $\texttt{U+1f612}$

(There is a full list of escape sequences.)

>>> print("I \"like\":\n\u0050\u0079\u0074\u0068\u006f\u006e")
I "like":
Python
>>> 

Operations on strings

$\texttt{+}$ is allowed between two strings. It concatenates the strings (meaning joins them).

$\texttt{*}$ is allowed with a string and an int. It concatenates $n$ copies of the string, where $n$ is the int argument.

>>> "Hello" + " " + "world!"
'Hello world!'
>>> "Hello" - "llo"
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for -: 'str' and 'str'
>>> "Ha" * 4
'HaHaHaHa'
>>> prefix = "Dr. "
>>> fullname = "Ramanujan"
>>> prefix+fullname
'Dr. Ramanujan'

len and indexing

>>> len("MCS 260")
7

>>> s = "lorem ipsum"
>>> s[2]
'r'

int

>>> int("1001",2)
9
>>> int("3e",16)
62
    

>>> int("0b1001",0)
9
>>> int("0x3e",0)
62
>>> int("77",0)
77
    

Bitwise operators

$\texttt{a << b}$ moves the bits of $\texttt{a}$ left by $\texttt{b}$ positions.

$\texttt{a >> b}$ moves the bits of $\texttt{a}$ right by $\texttt{b}$ positions.
(This detroys the lowest $\texttt{b}$ bits of $\texttt{a}$.)

>>> 9 << 3  # 9 = 0b1001 becomes 0b1001000 = 72
72
>>> 7 << 1  # 7 = 0b111 becomes 0b1110 = 14
14
>>> 9 >> 2  # 9 = 0b1001 becomes 0b10
2

>>> 9 & 5  # 9 = 0b1001,  5 = 0b0101
1

    >>> 9 | 5  # 9 = 0b1001,  5 = 0b0101
    13
    

    >>> 9 ^ 5  # 9 = 0b1001,  5 = 0b0101
    12
    

Logic gates

Circuits that perform logic operations on bits, logic gates, are fundamental building blocks of computers.

Thus the Python operators $\texttt{<<}$,$\texttt{>>}$,$\texttt{&}$,$\texttt{|}$,$\texttt{^}$ are especially low-level operations.

This chip (or integrated circuit / IC) contains four AND gates built from about $50$ transistors. The processor in an iPhone 11 has about $8,\!500,\!000,\!000$ transistors.