Variables in Python

In this section, we will introduce variables and strings, going indepth into both. If files are included with a section, it will be mentioned.

Variables

A variable is something that holds a value that may change. Think of it like a box that you can put stuff in, look at, and swap out whenever you want. Look at the example below.

# File
some_number = 7
print someNumber
# Endfile, output below
7

The code creates a variable called someNumber and assigns it the integer value 7. When we try to print someNumber, Python finds where we declared it and prints out the value we assigned to it. Declaring/Instantiating a variable in Python follows this construct:

my_variable_name = integer/long | string | boolean/expression | float | Object | list | dictionary  # Don't worry about dictionaries yet.

As you can see, a variable can be set to many different things. The name of the variable - on the left side - can be anything as long as it starts with a letter A-Z and/or a '_'. Try to be as specific as possible when naming variables, because it makes the code easier to read. If a variable holds the number of students in a class, then call it num_of_students. If you need more than one word in a variable name, split the words with an underscore and do not capitalize.

The right side of the declaration can be any of any type. Each of the types are explained below.

Integers & Longs

• Integers - Integers are positive or negative whole numbers with no decimal point.
  • Examples: 5, 4, 7, 10, -15, 0, 100, -145
  • Creation: Either you can just put the number like 5 or if you have a string that can be turned into a number, wrap it in int(<string>) like int("5") will equal 5.
• Longs - Longs are integers of unlimited size, written like integers and followed by an "L". Use these when a number is too large for an Integer.
  • Examples: 10000000L, -100323434L, 134234245345L
  • Creation: Either you can just put the number like 10000000L or if you have a string that can be turned into a number, wrap it in long(<string>) like long("100000000") will equal 100000000L.

String

A string is a collection of characters with a length equal to the number of characters in it. A character is any alpha- numeric symbol - A-Z, 0-9, !@#$% - such as "Hello World", "c", "!dfdfwer", "I am a string!". A string can also have length zero and be equivalent to "". Strings can be single or double quoted.

• If you want the same quote type inside of the string, then you have to 'escape' it by putting a \ in front of it - example: "He said to me, \"Woah\"". If you don't want to escape it, then you can put r infront of it like so

print 'C:\\nowhere'  # Will be escaped to print 'C:\nowhere'
print r'C:\\nowhere' # Will print as is. 'C:\\nowhere'

Strings can be added together like so:

greeting = "Hello, " + "Dan, " + " you look good today!"
print greeting
# End of file, output below
Hello, Dan, you look good today!

To turn a current variable into its string representation, wrap the variable in str(value). If you gave it something like str(5) it will return "5". If you want to print a string and number together, you must wrap the number in the str() function.

Go into greeting.py and follow the directions about assigning variables and printing them out.

Boolean Expressions and Logic Operators

A Boolean value is either True or False. It is named after the British mathematician, George Boole, who first formulated Boolean algebra — some rules for reasoning about and combining these values. This is the basis of all modern computer logic. In Python, boolean values can be either True or False.

# Two different Booleans
am_i_cool = True

can_operate = False

An expression is a line of code that be evaluated to a Boolean. There are six boolean expression operators you should know:

x == y               # Produce True if ... x is equal to y
x != y               #                 ... x is not equal to y
x > y                #                 ... x is greater than y
x < y                #                 ... x is less than y
x >= y               #                 ... x is greater than or equal to y
x <= y               #                 ... x is less than or equal to y

# Some examples of boolean expressions
age = 18  # Int variable
old_enough_to_get_driving_licence = age >= 16  # True
print(old_enough_to_get_driving_licence)

# Another example
number = 7
equal_to = (6 == number)  # False

Boolean expressions get evaluated from most inward parenthesis to outward.

• Go into the 3_variables directory in this repository and run the command below to test your boolean expression knowledge!

cd 3_variables/
# Once inside of the directory
python expressions.py

Logic Operators

Coupled with Boolean expressions, Logic operators are very important to Computer Science. The three Logic Operators are and, or, and not. They are used to group multiple boolean expressions to form one boolean evaluation. They are good with telling if a number is in a range or not amongst other things. Another useful operator is using is to denote equality.

old_enough_to_drive = True
clean_record = False

# If they are old enough and have a clean record
low_insurance = old_enough_to_drive and clean_record

# If a drive is not old enough or has a bad record
bad_driver = not old_enough_to_drive or not clean_record

# Junior driver if their age is 16 or 17.
age = 16
junior_driver = age >=16 and age < 18

Floats

Floats represent real numbers and are written with a decimal point dividing the integer and fractional parts. Floats usually represent decimal numbers, but they can also represent numbers in scientific notation.

WATCH OUT: Occasionally floating point errors occur with arithmetic.

# Float examples
example1 = float(6/8)
example2 = float(7/3)
example3 = 2.323
example4 = example1 / example3

Objects

These will be covered at a later time.

Lists

The most basic data structure in Python is the list. Each element of a sequence is assigned a number - its position or index. The first index is zero, the second index is one, and so forth. The list is a most versatile datatype available in Python which can be written as a list of comma-separated values (items) between square brackets. Important thing about a list is that items in a list need not be of the same type. To create a list, follow the syntax below. To access values in lists, use the square brackets for slicing along with the index or indices to obtain value available at that index.

       # Index: 0  1  2  3  4
myNumberList = [1, 4, 6, 8, 10]
myNumber = myNumberList[0] # Get the first number; will be 1.

myStringList = ["Hello", "World"]
myString = myStringList[1]  # Equates to "World"

myMixedList = [1, "Hi", 15, True]
myMixed = myMixedList[3]  # Equates to True

To update the value at an index, you can access the value with the square brackets, and then just put that on the left side of a declaration statement.

list = ['physics', 'chemistry', 1997, 2000];

print "Value available at index 2 : "
print list[2]
list[2] = 2001;
print "New value available at index 2 : "
print list[2]

To add to an existing list, use append(some_value)

list = [1, 10, 15]
list.append(5)
print list  # Will print "[1, 10, 15, 5]"

To remove from an existing list, use remove(some_value)

list = [1, 10, 15]
list.remove(10)
print list  # Will print "[1, 15]"

If you want the value returned back, use pop(index)

list = [1, 10, 15]
my_num = list.pop(0)
print myNum  # Will print "1"

• You can mess around and assign different variables in variables.py and see how they change!

Dictionary

Dictionaries are like arrays, but allow you to map a key to a value. With lists, you use integers are indices, with maps, you can use (just about) anything.

my_dictionary = {"Dan": 19, None: "Nothing", 5: "My age.", "coolness": False}

print my_dictionary["Dan"]  # Prints 19

my_dictionary["new_key"] = "My New Value"

print my_dictionary  # Prints '{"Dan": 19, None: "Nothing", 5: "My age.", "coolness": False, "new_key": "My New Value"}'