Representing data: Variables and Expressions

Learning Goals

• describe the statement and expression
• introduce print and assignment statements
• provide opportunities to work with documentation
• experiment with changing the default parameters

Key terminology

• statement: a line of code, an instruction; usually doesn’t return anything
  • a print statement: https://docs.python.org/3/library/functions.html#print
  • an assignment statement: uses = to create a new variable/object
• expression: computes/evaluates a value
• A literal is a specific value, such as “cat” or 42.
• An operator is a symbol that performs a built-in calculation, e.g., / performs division; subtraction operator - is also the operator for negation (e.g., -val + rate).

See additional bolded terms below.

Objects, Variables, and Assignments

Python creates a new variable, which is actually a new Python object, each time you use an assignment statement.

= is not “equals”! Thus, num = 11 is read as “num is assigned with 11” (or “11 is assigned to num”) instead of “num equals 11”.

A print statement allows you to display the value of an object/variable or a result of running an expression. Take a look at the documentation to see the optional parameters sep and end and their default values: https://docs.python.org/3/library/functions.html#print. Play around with them to get a sense of what they do and how to change them.

Expressions

Boolean (True / False)

1 == True
0 == False

Technically, anything that’s != 0 is True.

Comparison Operators

> , >= , < , <=
and , or , not
==	# equal
!=  # not equal

Comparison Statements

42 == 42  # True
“cat” == “cat” # True
“cat” == “dog” # False
“cat” != “dog” # True

Arithmetic

+	-	*	/
** # power, e.g., 2^3 is 2**3 in Python

Increasing a variable’s value by 1, as in num = num + 1, is known as incrementing the variable. A shorthand for it is num += 1 (same for the other basic operators). These are special, compound operators and include -=, *=, /=, and %=.

Play around with them and test the order of operations:

print(6 * 7)  # 42
print(2**3)  # 8

Which one of the following lines is this statement print(2**3**4) equivalent to?

print((2**3)**4)
print(2**(3**4))

Division and modulo

The division operator / performs division and returns a floating-point number.

The floored division operator // rounds down the result of a floating-point division to the closest whole number. The result is an integer if both operands are integers; otherwise, if either operand is a float, a float is returned.

% - modulo, i.e., remainder The modulo operator is commonly used to determine if the number is even or odd. (If you have a variable num, how would you determine if it’s even or odd?)

print(11 / 8) 
print(11 // 8) 
print(11 % 8) # 3

Given a credit card number stored as an integer, % and // can be used to get any part, such as the prefix.

• / by a power of 10 shifts a value right. E.g., 789 // 10 is 78 and 789 // 100 is …
• % by a power of 10 gets the rightmost digits. E.g., 789 % 10 is … and 789 % 100 is …

Operator Precedence

An expression is evaluated using the order of standard mathematics, known in programming as precedence rules. The PEMDAS (pronounced “pem-dass”) acronym stands for the following standard mathematical order of operations:

• Parentheses
• Exponents
• Multiplication and Division
• Addition and Subtraction Note that the unary minus (negation) comes after the exponents, and modulo (%, which is the remainder of a division) has equal precedence with “MD” :-)

PEMDAS applies in Python programming as well and if more than one operator of equal precedence could be evaluated, evaluation occurs left to right (except that the ** operator is evaluated from right-to-left).

Good practice is to use parentheses to make order of evaluation explicit, rather than relying on precedence rules.

Check your understanding

• What is the difference between a statement and expression?
• What is the purpose of variables? Are they the same as “objects”?
• Can assignments have the same variable on left and right of =?
• What is the difference between the sep and end?
• What is the difference between the /, //, and %?
• Print multiple items (text, variable values, and new lines) with one statement.
• Would this statement generate an error print('Name' user_name)? Why?