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CS101 Variables and Basic Syntax

Variable Definition

In programming, variables are used to assign a name to a piece of data and to use that name to reference the data elsewhere in the program.

myName = 'Zoe'

Variable Declartion

In programming, variables are declared by giving it a name and setting it to a value using an equals sign (‘=’). Variables can later be reassigned to other values.

dogBreed = 'corgi'

Data Type Definition

In programming, data types are how computers classify different forms of information. They include numeric, string and boolean types.

For example, if using number data type, the program will know that arithmetic can be performed on it, but it can’t be capitalized.

Numbers definition

In programming, numbers are a common data type. They represent numerical values and can include numbers with and without decimal points.

-25
0
.68888
7039

String Definition

In programming, strings are a common data type. They are any sequence of characters (letters, spaces, numbers, or symbols) surrounded by single or double quotes. Strings are commonly used to represent text, speech, symbols, and other non-numerical characters.

"Hello world"
"Great work!"

Booleans Definition

In programming, booleans are a common data type. They represent the logical ideas of true and false.

AND Operator

In programming, the logical AND operator (&&) compares two values. It returns true when both values evaluate to true and false otherwise.

The following evaluates to true: grass is green AND fire is red 2 > 1 AND 6 > 5 3 == 3 AND 8 == 8

While this evaluates to false: trees are large AND ant are massive 5 < 4 AND 6 > 3 7 == 7 AND 0 == 9

Comments

A comment is a piece of text within a program that is not executed. It can be used to provide additional information to aid in understanding the code.

The # character is used to start a comment and it continues until the end of the line.

# Comment on a single line
user = "JDoe" # Comment after code

Arithmetic Operations

Python supports different types of arithmetic operations that can be performed on literal numbers, variables, or some combination. The primary arithmetic operators are:

  • + for addition
  • - for subtraction
  • * for multiplication
  • / for division
  • % for modulus (returns the remainder)
  • ** for exponentiation
# Arithmetic operations
result = 10 + 30
result = 40 - 10
result = 50 * 5
result = 16 / 4
result = 25 % 2
result = 5 ** 3

Plus-Equals Operator +=

The plus-equals operator += provides a convenient way to add a value to an existing variable and assign the new value back to the same variable. In the case where the variable and the value are strings, this operator performs string concatenation instead of addition.

The operation is performed in-place, meaning that any other variable which points to the variable being updated will also be updated.

# Plus-Equal Operator
counter = 0
counter += 10
# This is equivalent to
counter = 0
counter = counter + 10
# The operator will also perform string concatenation
message = "Part 1 of message "
message += "Part 2 of message"

Variables

A variable is used to store data that will be used by the program. This data can be a number, a string, a Boolean, a list or some other data type. Every variable has a name which can consist of letters, numbers, and the underscore character _.

The equal sign = is used to assign a value to a variable. After the initial assignment is made, the value of a variable can be updated to new values as needed.

# These are all valid variable names and assignment
user_name = "codey"
user_id = 100
verified = False
# A variable's value can be changed after assignment
points = 100
points = 120

Modulo Operator %

A modulo calculation returns the remainder of a division between the first and second number. For example:

  • The result of the expression 4 % 2 would result in the value 0, because 4 is evenly divisible by 2 leaving no remainder.
  • The result of the expression 7 % 3 would return 1, because 7 is not evenly divisible by 3, leaving a remainder of 1.
# Modulo operations
zero = 8 % 4
nonzero = 12 % 5

Integers

An integer is a number that can be written without a fractional part (no decimal). An integer can be a positive number, a negative number or the number 0 so long as there is no decimal portion.

The number 0 represents an integer value but the same number written as 0.0 would represent a floating point number.

# Example integer numbers
chairs = 4
tables = 1
broken_chairs = -2
sofas = 0
# Non-integer numbers
lights = 2.5
left_overs = 0.0

String Concatenation

Python supports the joining (concatenation) of strings together using the + operator. The + operator is also used for mathematical addition operations. If the parameters passed to the + operator are strings, then concatenation will be performed. If the parameter passed to + have different types, then Python will report an error condition. Multiple variables or literal strings can be joined together using the + operator.

# String concatenation
first = "Hello "
second = "World"
result = first + second
long_result = first + second + "!"

Errors

The Python interpreter will report errors present in your code. For most error cases, the interpreter will display the line of code where the error was detected and place a caret character ^ under the portion of the code where the error was detected.

if False ISNOTEQUAL True:
^
SyntaxError: invalid syntax

ZeroDivisionError

A ZeroDivisionError is reported by the Python interpreter when it detects a division operation is being performed and the denominator (bottom number) is 0. In mathematics, dividing a number by zero has no defined value, so Python treats this as an error condition and will report a ZeroDivisionError and display the line of code where the division occurred. This can also happen if a variable is used as the denominator and its value has been set to or changed to 0.

numerator = 100
denominator = 0
bad_results = numerator / denominator
ZeroDivisionError: division by zero

Strings

A string is a sequence of characters (letters, numbers, whitespace or punctuation) enclosed by quotation marks. It can be enclosed using either the double quotation mark " or the single quotation mark '.

If a string has to be broken into multiple lines, the backslash character \ can be used to indicate that the string continues on the next line.

user = "User Full Name"
game = 'Monopoly'
longer = "This string is broken up \
over multiple lines"

SyntaxError

A SyntaxError is reported by the Python interpreter when some portion of the code is incorrect. This can include misspelled keywords, missing or too many brackets or parenthesis, incorrect operators, missing or too many quotation marks, or other conditions.

age = 7 + 5 = 4
File "<stdin>", line 1
SyntaxError: can't assign to operator

NameError

A NameError is reported by the Python interpreter when it detects a variable that is unknown. This can occur when a variable is used before it has been assigned a value or if a variable name is spelled differently than the point at which it was defined. The Python interpreter will display the line of code where the NameError was detected and indicate which name it found that was not defined.

misspelled_variable_name
NameError: name 'misspelled_variable_name' is not defined

Floating Point Numbers

Python variables can be assigned different types of data. One supported data type is the floating point number. A floating point number is a value that contains a decimal portion. It can be used to represent numbers that have fractional quantities. For example, a = 3/5 can not be represented as an integer, so the variable a is assigned a floating point value of 0.6.

# Floating point numbers
pi = 3.14159
meal_cost = 12.99
tip_percent = 0.20

print() Function

The print() function is used to output text, numbers, or other printable information to the console.

It takes one or more arguments and will output each of the arguments to the console separated by a space. If no arguments are provided, the print() function will output a blank line.

print("Hello World!")
print(100)
pi = 3.14159
print(pi)