Numeric Expressions, Data type, Type Conversions

Numeric Expressions

• Becuase of the lack of mathematical symbols on computer keyboards - we use “computer-speak” to express the classic math operations
• Asterisk is multiplication
• Exponentiation (raise to a power) looks different than in math

Operator	Operation
+	Addition
-	Subtraction
*	Multiplication
/	Division
**	Power
%	Remainder

Order of Evaluation

• When we string operators together - Python must know which one to do first
• This is called operator precedence
• Which operator takes precedence over the others?

Parenthesis → Power → Multiplication → Addition → Left to Right

Operator Precedence

• Remember the rules top to bottom
• When writing code - use parentheses
• When writing code - keep mathematical expressions simple enough that they are easy to understand
• Break long series of mathematical operations up to make them more clear

What does “Type” Mean?

• In Python variables, literals, and constants have a “type”
• Python knows the difference between an integer number and a string
• For example “+” means “addition” if something is a number and “concatenate” if something is a string

>>> ddd = 1 + 4
>>> print(ddd)
5
>>> eee = 'hello ' + 'there'
>>> print(eee)
hello there

Type Matters

• Python knows that “type” everything is
• Some operations are prohibited
• You cannot “add 1” to a string
• We can ask Python what type something is by using the type() function

Type Conversions

• When you put an integer and floating point in an expression, the integer is implicitly converted to a float
• You can control this with the built-in functions int() and float()

>>> print(float(99) + 100)
199.0
>>> i = 42
>>> type(i)
<class 'int'>
>>> f = float(i)
>>> print(f)
42.0
>>> type(f)
<class 'float'>

Integer Division

Integer division produces a floating point result

>>> print(10/2)
5.0
>>> print(9/2)
4.5

String conversions

• You can also use int() and float() to convert between strings and integers
• You will get an error if the string does not contain numeric characters

User Input

• We can instruct Python to pause and read data from the user using the input() function
• The input() function returns a string

nam = input('Who are you? ')
print('Welcome', nam)

Converting User Input

• If we want to read a number from the user, we must convert it from a string to a number using a type conversiong function
• Later we will deal with bad input data

inp = input('Europe floor?')
usf = int(inp) + 1
print('US floor', usf)

Comments in Python

• Anything after a # is ignored by Python
• Why comment?

1. Describe what is going to happen in a sequence of code
2. Document who wrote the code or other ancillary information
3. Turn off a ling of code - perhaps temporarily