Programming in Python

4.1 Input/Output Statements (I/O)

Think of Input/Output (I/O) statements as the “mouth” and “ears” of your program. They are the primary ways your code interacts with the person using it.

1. Print Statement (The “Mouth”)

Whenever you want your program to “speak” or display a result on the screen, you use the print() function. For example, print("Hello, Python!") will simply display: Hello, Python! on the screen.

2. Input Function (The “Ears”)

When your program needs to ask the user for information, you use the input() function. By default, anything the user types is captured as a Text String. For example, when you use number = input("Enter a number: "), the program pauses, displays the prompt “Enter a number: “, and waits for the user to type something and press Enter.

4.2 Data Types, Identifiers, and Variables

To process information correctly, Python needs to know what kind of data it is looking at.

1. Core Data Types

Python offers several built-in data types to handle different categories of information.

2. Identifiers

An Identifier is simply a name you give to a program unit, such as a variable or a function. A good rule of thumb is that an identifier should make sense. user_age is a great identifier, while x123 is confusing!

3. Variables

Think of Variables as labeled storage boxes or containers that hold information in the computer’s memory. A variable is created the moment you assign a value to it using the equals sign (=). For example, in the code a = 15 and name = "Aarosh", a is the box holding the integer 15, and name is the box holding the string “Aarosh”.

4.3 Operators and Expressions

Operators are special mathematical and logical symbols. They allow us to manipulate our variables and perform specific actions.

1. Arithmetic Operators

Used for performing standard mathematical calculations.

2. Relational Operators

Relational operators compare two values. They act like questions and always answer with a Boolean (True or False).

3. Logical Operators

Logical operators combine multiple conditions to make complex decisions.

4.4 Conditional Statements

In programming, we don’t always want every line of code to run. Sometimes, we want code to run only if a specific condition is met. These are the decision-makers!

1. The if Statement

This is the simplest decision. If the condition is true, execute the code. If not, do nothing.

number = int(input("Enter a number: "))
if number > 0:
    print("The number is positive.")

2. The if-else Statement

This gives us a “Plan B.” If the condition is true, do the first thing; otherwise, do the second thing.

user_age = int(input("How old are you? "))
if user_age >= 18:
    print("You are an adult!")
else:
    print("You are a teenager or a kid.")

3. The if-elif-else Statement

Used when there are multiple distinct possibilities to check in sequence. elif stands for “else if”.

user_number = int(input("Enter a number: "))
if user_number > 0:
    print("The number is positive.")
elif user_number == 0:
    print("The number is zero.")
else:
    print("The number is negative.")

4. Nested if (if inside if)

Sometimes a decision requires a follow-up decision. A nested if is an if statement placed completely inside another.

age = int(input("Enter your age: "))
if age >= 16:
    print("You are eligible for citizenship.")
    if age >= 18:
        print("You are eligible to cast a vote.")
    else:
        print("You are not eligible to cast a vote.")
else:
    print("You are a minor.")

4.5 Iteration (Loops)

Iteration means doing things repeatedly. It is the process of repeating a specific task automatically until a condition is met, saving us from writing the same code over and over again.

1. The for loop

Use a for loop when you know exactly how many times you want to repeat a block of code.

for x in range(5):
    print("Programming")

The range(5) function tells the loop to run exactly 5 times.

2. The while loop

Use a while loop when you want the code to repeat as long as a condition remains true.

count = 1
while count <= 5:
    print(count)
    count += 1

Important: count += 1 adds 1 to ‘count’ each time, preventing an infinite loop!

4.6 Python Lists

A list is a built-in data structure in Python that acts like a highly flexible filing cabinet. It allows you to store multiple items (even of different data types) inside a single variable using square brackets [ ].

thislist = ["Computer", "Science", 20, True]
print(thislist)