Data: Values, types and expressions and Variables

Values and Types

Every value has a type

• Value (data) types in python:
  - Integers (type int)
  - Floating-point numbers (type float) - Strings (type str)
  - Truth values (type bool)
  - …and so on.
• Types determine what we can do with values
• The type() function tells us the type of a value:

Numeric types
int

• Integers (type int) represent positive and negative whole numbers (0, 1, 2, −1, −17, 4096, . . .).
• Values of type int have no inherent size limit.
• Note: Can’t use commas to “format” integers (must write 1282736, not 1,282,736).

float

• Floating-point numbers (type float) represent decimal numbers.
• Values of type float have limited range and limited precision.
  - Min/max value: ±1.79e308.
  - Smallest non-zero value: 2.22e−308.
  - Smallest value > 1: 1 + 2.22e−16.
• Type float also has special values and (not a number).
• Every decimal number is a float:
  >>> type(1.5 - 0.5)
<class ’float’>
• The result of division is always a float:
  >>> type(4 / 2)
<class ’float’>

Strings

• Strings (type str) represent text.
• A string literal is enclosed in single or double quote marks:
  >>> "Hello world"
’Hello world’
  - A string can contain other types of quote mark, but not the one used to delimit it.
  >>> ’4" long’
’4" long’

Type conversion

• Explicit conversions use the type name like a function:
  >>> int(2.0)
>>> float(" -1.05")
>>> str(0.75 * 1.75)
• Conversion from str to number only works if the string contains (only) a numeric literal.
  >>> float("12s")
ValueError: could not convert string to float: '12s'
• Conversion from int to float is automatic.
  - E.g., int times float becomes a float.

Numeric operators in python

Precedence

• There is an order of precedence on operators, that determines how an expression is read:
  - -1 ** 5 means -(1 ** 5), not (-1) ** 5.

Math functions

• The math module provides standard math functions, such as square root, logarithm, trigonometric functions, etc.
• Almost all math functions take and return values of type float.

Comparison operators

• Can compare two values of the same type (for almost any type).
• Comparisons return a truth value (type bool), which is either True or False.
• Conversion from any type to type bool happens automatically, but the result may not be what you expect.

Expressions

• ((523 - 485) / 485) * 100 is an expression

Variables

• A variable is a name that is associated with a value in the program.
  - The python interpreter stores name–value associations in a namespace.

Valid names in python (reminder)

• A (function or variable) name in python may contain letters, numbers and underscores ( ), but must begin with a letter or undescore.
• Reserved words cannot be used as names.
• Names are case sensitive: upper and lower case letters are not the same.
  - Length_Of_Ropeand length_of_rope are different names.