Data and Expressions I
CS 160: Computer Science Orientation
Today’s Schedule
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Numeric Literals
String Literals
Encoding
Formatting
Joining
Live Coding
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July 24, 2016
Numeric Literals
• Literal – sequence of one or more characters
that stands for itself
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July 24, 2016
Numeric Literals
• Contains only:
• Digits: 0 – 9
• Sign: + or • Exponential notation: e
• Commas CANNOT be used!!!
• Explicit + for positive number rarely used
3
July 24, 2016
Numeric Literals
• Integer
• Positive or negative whole numbers
• E.g., 10
• NO SIZE LIMIT!
• Floating Point
• Decimal number
• E.g., 10.24
• Has limited range and precision...
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July 24, 2016
Numeric Literals
• Floating points have limited range:
• “only” 10-308 to 10308
• ... and limited precision:
• “only” 16-17 digits of precision (after the decimal)
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July 24, 2016
Numeric Literals
• Watch out for overflow! E.g.,
>>> 1.5e200 * 2.0e210
>>> Inf
• Watch out for underflow! E.g.,
>>> 1.0e-300 / 1.0e100
>>> 0.0
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July 24, 2016
Numeric Literals
• Computers are discrete
• ==> Only approximations of infinite series can be stored in memory
• Python limits floating point values to 16 decimal points
>>> 1.0 / 3.0
>>> 0.3333333333333333
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July 24, 2016
Numeric Literals
• Watch out for integer division!!
>>> 3 / 2
>>> ???
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July 24, 2016
Numeric Literals
• Watch out for integer division!!
>>> 3 / 2
>>> 1
Wha???
Shouldn’t this be 1.5?
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July 24, 2016
Numeric Literals
• Solution? explicitly declare value as floating point number
• (I.e., use a decimal)
>>> 3 / 2
>>> 1
>>> 3.0 / 2.0
>>> 1.5
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July 24, 2016
Today’s Schedule
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Numeric Literals
String Literals
Encoding
Formatting
Joining
Live Coding
11
July 24, 2016
String Literals
• Delimited by single OR double quotes
• Strings must be contained on a single line of code
• (Except when using triple quotes: ‘’’ – more later)
>>> print ‘hello’
hello
>>> print “hello”
hello
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July 24, 2016
String Literals
• Python convention: use single quotes
• Unless double quotes are required, e.g.,
>>> print ‘you’re awesome at programming’
SYNTAX ERROR!!!
>>> print “you’re awesome at programming”
you’re awesome at programming
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July 24, 2016
String Literals
• Strings can be empty or contain whitespace, e.g.,
>>> print ‘’
>>> print ‘
>>>
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July 24, 2016
‘
Today’s Schedule
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Numeric Literals
String Literals
Encoding
Formatting
Joining
Live Coding
15
July 24, 2016
Encoding
• Computers only deal with 0s and 1s
• So how do they represent numbers and strings?
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July 24, 2016
Encoding
• Computers only deal with 0s and 1s
• So how do they represent numbers and strings?
binary encoding!!
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July 24, 2016
Encoding
• 8-bit ASCII encoding standard
• Sequence of eight 0s and 1s determine character, e.g.,
‘A’ encoded as 0100 0001
‘B’ encoded as 0100 0010
...
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July 24, 2016
Encoding
0100 0001 is ‘A’ according to ASCII encoding
and
FYI, 0100 0001 is binary for 65
= 26 + 20
= 64 + 1
= 65
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July 24, 2016
Encoding (ASCII Table)
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July 24, 2016
Encoding
• Python has built-in functions for ASCII encoding
• ord( ) function returns decimal value of character
>>> ord( ‘A’ )
65
>>> ord( ‘E’ )
69
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July 24, 2016
Encoding
• Python has built-in functions for ASCII encoding
• chr( ) function returns character of encoded value
>>> chr( 65)
‘A’
>>> chr( 69 )
‘E’
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July 24, 2016
Encoding
• Unicode is an international encoding standard
• Intended to be universal
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8 to 32 bits per character
Currently over 100,000 Unicode defined characters
Capable of defining 4,000,000,000+ more
Encode characters for ALL past and present languages
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July 24, 2016
Encoding
• Difference between numeric and string representation...
Numeric: value 124 is 01111100 in binary
String: “124” is 00110001 00110010 00110100 in binary!
(contains characters ‘1’, ‘2’, and ‘4’)
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July 24, 2016
Encoding
• Aside: binary numbers
Numeric: value 124 is 01111100 in binary
= 26 + 25 + 24 + 23 + 22
= 64 + 32 + 16 + 8 + 4
= 124
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July 24, 2016
Encoding
• Control characters – special chars not displayed on screen
• Control display of output (among other things)
• Represented by escape sequence
• Escape sequence• Begins with escape character (i.e., backslash \ )
• Escape sequence allows character to escape its original meaning
• E.g., \n is “newline”
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July 24, 2016
Encoding
• Example: \n escape sequence => newline control character
>>> print “Hello there!!!\nHow are you doing?\n”
Hello there!!!
How are you doing?
>>>
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July 24, 2016
Today’s Schedule
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Numeric Literals
String Literals
Encoding
Formatting
Joining
Live Coding
28
July 24, 2016
Formatting
• Python’s format function controls display of strings
• format( value, format_specifier )
• Round floating point to 2 decimal places:
>>> format( 22.0 / 7.0, ‘.2f’)
‘3.14’
NOTE: format implicitly converts float to string...
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July 24, 2016
Formatting
• Python’s format function controls display of strings
• format( value, format_specifier )
• Right justify string, set width to 20 characters
>>> format(‘hello’, ‘>20’)
'
hello’
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July 24, 2016
Formatting
• Python’s format function controls display of strings
• format( value, format_specifier )
• Center string, set width to 50 characters
>>> format(‘center?’, ‘^50’)
'
center?
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July 24, 2016
'
Today’s Schedule
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Numeric Literals
String Literals
Encoding
Formatting
Joining
Live Coding
32
July 24, 2016
Joining
• Python syntax is based on whitespace
• More on this later...
• Long lines of code may need to span multiple lines for
increased readability
• Lines may be joined explicitly using backslash \
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July 24, 2016
Joining
>>> x = 10 + \
... 20 + \
... 30 + \
... 40
>>> print x
100
NOTE: Python interpreter automatically adds the ellipses
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July 24, 2016
Today’s Schedule
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Numeric Literals
String Literals
Encoding
Formatting
Joining
Live Coding
35
July 24, 2016
Live Coding
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Integer division
Floating point range and precision
‘Hello World’ ASCII encoding
Joining
36
July 24, 2016