Chapter Two

Note

If you are totally new to programming, please make sure you've completed Chapter One.

Dealing with information?

You might be asking this question "What do you mean by dealing with information and why do we bother talking about it"? Well, if we take a closer look at the world of software, everything is about information. Programs are nothing but dealers with information. We use them to manage information. With Notepad we store information; with websites we share information with the world; even playing videos games deal with information-which character are we playing? How much health of the player is remaining? Where is our character now? Did the enemy get damaged? Everything is about information. That is why there is a special term for the world of computer: Information Technology or in short, IT.

Think about the program we wrote. What did it do? It displayed the text Hello world! to the user or in other words, it displayed a piece of information to the user on the screen. All the programs we will write deal with information. In our programs we will be handling information, storing it, sharing it, creating it, generating it, transforming it and more.

Variables

Before we discuss the word Variable. Let's write a simple program that will lay the groundwork for us to explore the idea of variables.

Follow the steps in the first chapter to write this code in a new file named variables.py:

message = "Hello world!"
print(message)

Run the program using python variables.py command in the Terminal.

Output of the program:

Hello world!

What did this program do?

Well, this program does the same thing as the one we wrote in the previous chapter. It simply displays the text Hello world! on the screen to the user.

So, what is the difference then? The difference is quite obvious: this program is written differently, but it does the same thing.

Explanation:

This program consists of two lines of code.

message = "Hello world!"
print(message)

The first line of the code:

message = "Hello world!"

This line tells the computer to store the text Hello world! in the container named message. This container is called variable in programming. It is like a labeled box that stores things inside it. To store something in these boxes, we write its name first, then an assignment operator, (the equal symbol = from our keyboard), and then the thing we want to store in our container which is the text Hello world in this case. This text we want to store inside the variable is a piece of information. So basically our containers in programs, which are variables, store information inside them. The text you want to store in a variable should be inside double quotation marks (""). The complete syntax is as follow:

variable_name = information_you_want_to_store_in

The second line of code:

print(message)

The second line of our code is quite similar. The only difference that instead of writing our text directly inside print() function, we have written the name of the container we created. This tells the print function to print the content of the container, which is the text Hello world!. This leads to the same result as the the program in the previous chapter.

Why variables?

You might be wondering, if both of our programs, up to this point do the same thing, why bother creating a container to store our text inside it and then give the name of the container to the print() function to display its content? You are right, there is no need for an extra line of code just to display a text. We can directly print them using one line of code. However, programs are not always about printing some text on the screen. Programs can be much more complicated. What our programs do can be something huge with many tasks to perform. As we keep writing programs we will face situations where we must use these containers. In fact, all the programs we see and use daily have lots and lots of containers (variables) inside them.

Therefore, the concept of variables is one of the most fundamental part programming to learn. We need to learn how to crete variables, when to create variables and how to use variables, what kinds of variables we can create and much more.

Some words to remember

Take a look at these containers (variables). You don't have to write or run it, just check them out.

name = "John Dove"
roadName = "Second Ave"
label = "Click me"
someVariable = "" # Containers can be empty

Comments: Comments are some extra text we write inside our code. It start with a hash symbol # and is not part of our code. Just some extra text.

Now let's become a bit more technical and learn some terms.

Term	Meaning of the term
Variable	These are the containers we talked about.
Declaration of variable	When create a variable you declare them by writing their name.
Initialization of variable	When storing a piece of information inside a variable.
Data	It has different uses, but we can use them instead of the word information here.
Value	We value of a variable, refers to the things inside a container

We will be using these terms from time to time throughout the course. Make sure to remember them.

In the coming sections we are going to learn about types of variables and how to use them.

Basic Types of Variables

Before we start let's write and run this program:

message = "Hello world!"
print(message)

name = "Jade Khan"
phone_number = 123456789
weight = 84.4

print(name)
print(phone_number)
print(weight)

Run the program using python variables.py command in the Terminal.

Output:

Hello world!
Jade Khan
123456789
84.4

Explanation: So, in this program we created some more variables. The first variable, message, is the same as the before containing a text that we printed it on the screen. Next, we created a variable called name which holds the value Jade Khan. Then, we created another variable named phoneNumber, which hold a phone number notice this isn't enclosed in double quotation marks. Lastly, we create a variable called weight which also hold a number but with a decimal point.

When we run the program, it goes thought the following steps:

1. Create variable named message which hold the text Hello world!. It is a text variable.
2. Print the content of the variable message.
3. Create a variable named name which holds the value Jade Khan. It is a text variable.
4. Create a variable named phone_number which holds the value 123456789. It is a number variable.
5. Create a variable named weight which hold the value 84.4. It is a decimal number.
6. Print the content of variable name
7. Print the content of variable phone_number
8. Print the content of variable weight

Remember: The programs run in order, starting from the first line up until the last line in a the file.

In Python we have three types of variables.

1. Number
2. String (text)
3. Boolean (It is very easy, we will get to it once we explored the other two types)

Numbers

The first common basic type of information we store in variables are numbers. So our variables can hold any number as its value.

variable_name=number_you_want_to_store

Examples:

marks = 74
number_of_students = 310
height_of_room = 3.8
temperature = 34
player_health = 88
products_in_stock = 40
net_worth = 28440181
radius = 0.3

You don't have to write and run this, just check it out and notice how we have captured different types of information to be stored in variables.

The real power of programming

The real power of using variables lies in how we can can manipulate them, do math with them and so much more.

Create a new file named var_math.py and write this code and run it:

message = "This is my bill in US Dollar: "

apples = 2.50
bananas = 1.20
bread = 2.0
milk = 3.0
eggs = 2.50
cheese = 4.00
chicken = 7.50

total = apples + bananas + bread + milk + eggs + cheese + chicken

print(message)
print(apples)
print(bananas)
print(milk)
print(eggs)
print(cheese)
print(chicken)

print("Total cost: ")
print(total)

Run the program using python var_math.py command in the Terminal.

Output:

This is my bill in US Dollar:
2.5
1.2
3.0
2.5
4.0
7.5
Total cost:
22.7

Explanation: Our program stores prices of different items in a bill, sum them up and displays it to the user.

The process of applying math operation on our number variables is quite straight forward. Take a look at these examples, you will figure it out that how easy it is.

Examples:

You don't have to write and run this code just observe and learn. However you are free to practice with them.

a = 10
b = 20

c = a + b # c is 30 now, store result of 10 + 20
d = c - 5 # d is 30 - 5, which is 25

var1 = 40
var2 = 10
var3 = var1 / var2 # Dividing 40 over 10 which is 4

var4 = 2 * 3
var5 = a * var2 # 10 * 10

num1 = 13
num2 = 15
num3 = 18

avg = (num1 + num2 + num3) / 3 # taking average of three numbers

print(avg)

Strings

The second common piece of information we store in variables is text which referred to as string in programming.

Syntax of storing text in variables is pretty much the same, the only difference is that we need to put it inside double quotation mark.

Take a look at these examples:

country = "Turkey"
place_of_birth = "Dubai, UAE"

# Storing a longer text
home_address = "House No 43, Street 89, G10/4, Islamabad, Pakistan"

school_name = "Stand International School"
father_name = "Mohammad Khan"

Things we can do with strings

You might be thinking, "Can we do math with string variables?". The answer isn't a simple yes or no. We cannot do math with text since it doesn't make sense, but there is one mathematical operation we can perform on strings: addition. This isn't actually math in the traditional sense, but rather a way of to join multiple strings into one, which is called string concatenation in programming.

example:

first_name = "Michel"
last_name = "Jackson"

# make one them one
full_name = first_name + last_name # Becomes "MichelJackson"

Now let's do some fun things with strings. Create a new file named strings.py and write the following code:

str1 = "Greetings, everyone!"

# Print the string
print(str1)

# Print in uppercase
print(str1.upper())

# Print in lowercase
print(str1.lower())

# Replace a word in the string
print(str1.replace("everyone", "Jack"))

# Joining two strings
first_name = "Michel"
last_name = "Jackson"
print(first_name + last_name)

No run the code by entering python strings.py command in the Terminal.

Output:

Greetings, everyone!
GREETINGS, EVERYONE!
greetings, everyone!
Greetings, Jack!
MichelJackson

Explanation Our programs do multiple things with our string Greetings, everyone!. Convert it to uppercase letters and print it then convert it to lowercase letters and print it and replacing the word everyone with the word Jack and print it. Lastly it create two string and join them together to become one string.

Boolean

Boolean variable are also containers that hold information but the type of information it hold is very simple. Boolean variable can only store one of these two value, a True or a False.

Examples:

is_happy = True
test_passed = False
is_empty = True
is_player_alive = True
in_stock = False

You might be asking what is the use of containers that only hold one of two predefined values. It has its own special use. When the time comes you will know how important they are, I promise. Just keep in mind that we have a type of variables that can only a True or a False at a time.

Note: Keep in mind that we cannot perform mathematical operations on boolean values.

Collection of Information

The types of data we discussed form the foundation of information handling within our application. There are other types of data that are built on top of these basic data types, but they are more like collections of information stored in one container.

Create a new file named collections.py and write this code:

# Collection of data in one container. In Python called List
student_marks = [67, 81, 83]
print(student_marks)

# Collection of more complicated data in one container. In Python called Dictionary.
student_marks_names = {"John": 67, "Khan": 81, "Michel": 83}
print(student_marks_names)

Run the code using the command python collections.py.

Output:

[67, 81, 83]
{'John': 67, 'Khan': 81, 'Michel': 83}

Explanation Our program creates two variables but they are not as simple as our string or number variables that hold only one value. These variables hold multiple values inside them, like grouped pieces of information. The variable student_marks is called a list in Python which, hold multiple values and each value is a basic type like number, string or boolean. The variable student_marks_names also hold multiple values but each value is a pair separated with comma. This data type is called dictionary in Python. Each pair consist of a key and value. In our case the first pair is the name of the student, John and his marks which are 67. Notice the difference: the list holds only marks of the students, separated by comma while the dictionary holds the names and the marks of the students, also separated comma.

We have many types of collections with varying complexities, but the most common data types that hold collections of data are these two:

1. Lists
2. Dictionaries

Lists

Let's explore a couple of examples of list and explore what kind of things can we do with lists.

Create file named lists.py and write this code (you don't have write the comments starting with # unless if you want):

# First example, using data in the list
student_marks = [67, 81, 83, 62]
print(student_marks) # print the whole list
print(student_marks[0]) # print item number 1, lists start with zero
var2 = student_marks[2] # taking third element and assigning it to  a variable var2
print(var2) # var2 is now 83

# Second example, changing data in the list
print(student_marks[3]) # student_marks[3] is 62
student_marks[3] = 65 # change last item from 62 to 65
print(student_marks[3]) # student_marks[3] has changed to 65 now

# Third example, Adding and removing items to the list and from the list
student_marks.append(77) # adding 77 to the end of the list
print(student_marks) # list is now [67, 81, 83, 62, 77]

student_marks.remove(81) # remove 81 from the list
print(student_marks) # list is now [67, 83, 62, 77], 81 has been removed

Run the program by typing python lists.py in the Terminal.

Output:

[67, 81, 83, 62]
67
83
62
65
[67, 81, 83, 65, 77]
[67, 83, 65, 77]

Explanation:

The first example: it shows how we can use list. We can access each item in the list separately. By using list_name[index] where index is a number indicating which item in the list to access. We can assign this item to other variable, do math with them or print them on the screen.

The second example: it shows how we can change the items in the list. We can access items by their indices and assign them new values. Just like we changed the last item to 65.

The third example: shows that how we can remove an item from a list or add a new item to the list.

Note: In Python, list indexing start from 0. So, the first item is at index 0, the second item is at index 1, the third element is at index 2 and so on.

Dictionary

Dictionaries are totally the same a list that stores collection of information but the inside a dictionary is accessed by a name instead of index numbers.

Create a new file named dictionaries.py and write this code (you don't have write the comments starting with # unless if you want):

# First example, using the data inside a dictionary
student_marks_names = {"John": 67, "Khan": 81, "Michel": 83, "Ali": 62}
print(student_marks_names) # Print the whole dictionary
print(student_marks_names["John"]) # print John's score, which is 67
var2 = student_marks_names["Michel"] # Assign Michel's score to var2
print(var2) # var2 is now 83

# Second example, changing data in the dictionary
print(student_marks_names["Ali"]) # Ali's score is 62
student_marks_names["Ali"] = 65 # Changing Ali's score from 62 to 65
print(student_marks_names["Ali"]) # Ali's score is 65 now

# Third example, adding or removing a pair of data from, dictionaries
student_marks_names["Peter"] = 77 # adding a new student's score to the end of dictionary
print(student_marks_names) # now: {"John": 67, "Khan": 81, "Michel": 83, "Ali": 62, "Peter": 77}

del student_marks_names["Khan"] # deleting Khan and his score from the dictionary
print(student_marks_names) # now: {"John": 67, "Michel": 83, "Ali": 62, "Peter": 77}

Run the program by typing python dictionaries.py in the Terminal.

Output:

{'John': 67, 'Khan': 81, 'Michel': 83, 'Ali': 62}
67
83
62
65
{'John': 67, 'Khan': 81, 'Michel': 83, 'Ali': 65, 'Peter': 77}
{'John': 67, 'Michel': 83, 'Ali': 65, 'Peter': 77}

Explanation:

We access the value of an item in a dictionary by its name, unlike lists where we access the value of an item by its index number. When a adding a new data pair to our dictionary we use dictionary_name["new_data_name"] = new_data_value syntax. To delete an item from a dictionary, we use del dictionary_name["data_name_to_be_deleted"]. The keyword del, tells our program to delete a specific item from our dictionary unlike list where we use list_name.remove(item_index).

Note: Remember that the pair inside a dictionary is called a key value pair where key is the name of the data and value is value of the data and each pair is separated by comma. A complete syntax of how to create dictionary in Python is as follow:

variable_name = {key:value}

Let the user enter some data into the program

Can we allow the user who runs our program to store some data in variables inside our program? Of course! This is called input in programming, and most of the programs we use daily deal with input. In our examples, we printed some data to the screen so the user can see it which is called output in programming. Let's write a program where the user enters his name, and then the program will print it for them on the screen.

Create a file named input.py and write this code:

user_name = input("Enter your name: ") # assign the entered value to user_name

capt_user_name = user_name.upper() # we treat user_name just like other variables

print("Your name is: ")
print(user_name) # user_name, now contains the value user entered

Explanation: In this program we ask the user to enter their name. Then program converts it to uppercase letter and displays it on the screen just like we displayed other things in previous examples.

To let user the enter some data in the program, we use input() function. Inside the parenthesis, we can enter the message that we want to show to the user first before they enter something. This message is optional, you can leave empty if you prefer.

So the whole idea of programs is to get something from the user, process it, and then provide an output. This is exactly what our programs does: it takes the user's as input, processes it by converting it to uppercase letters, and then displays it on the screen. The name is input, converting it to uppercase is processing, and printing it on the screen is output.

What we've learned so far?

In the previous chapter, we learned how to setup the environment and write our first program. In this chapter, we first talked about what variables are and why we need them. Then, we learned about the basic types of variables, which are numbers, strings and booleans and did some fun things with them. After that, we explored more complex variables types like dictionaries and lists. We learned how to use these data types and how to modify them. Now we are ready to dive even deeper and do some more interesting things with our programs. In the next chapter, we will learn to do things conditionally where we will look at some conditions and, based on those condition, we will tell our program what to do such as whether to print something or not.

Exercise

1. Write a program that creates all the variables types we have learned up to this point. Initialize them with data of your choice and then print them all.
2. In the same program, perform some operations on the variables, such as
   • Math on basic number variables
   • String transformation
   • Modifying data in the lists and dictionaries