Controlling Program Flow with Conditionals and Loops

1. Making Decisions with Conditional Statements

Conditional statements allow your program to execute certain blocks of code only if a specific condition is met. This is the primary way programs make decisions. The keywords for this are if, elif (short for "else if"), and else.

A key aspect of Python's syntax is the use of a colon (:) at the end of if, elif, and else lines, followed by an indented block of code. This indentation is not just for readability; it's how Python knows which code belongs to which statement.

The following video provides a great overview of how to use these statements. Since you are preparing for the GATE exam, you might appreciate the direct style of the "Gate Smashers" channel.

Let's break down the logic with a clear, common example: assigning a grade based on a score.

The if Statement

An if statement checks a condition. If it's True, the indented code block runs. If not, the program just skips it.

score = 95
if score > 90:
    print("Excellent work! You get an A.")

print("End of program.") 
# Output:
# Excellent work! You get an A.
# End of program.

The if-else Statement

This provides an alternative path for when the if condition is False.

num = 7
if num % 2 == 0:
    print("The number is even.")
else:
    print("The number is odd.")

# Output: The number is odd.

The if-elif-else Ladder

This is for checking multiple, mutually exclusive conditions in order. Python checks each condition from top to bottom and executes the first block where the condition is True. If none are true, the else block runs.

score = 85

if score > 90:
    grade = "A"
elif score > 80:
    grade = "B"
elif score > 70:
    grade = "C"
else:
    grade = "D"

print(f"Your grade is: {grade}") # The f-string is a handy way to format strings!
# Output: Your grade is: B

Notice that even though score > 70 is also true for a score of 85, the elif score > 80: block was executed first, and the rest of the chain was skipped. The order matters!

age_str = input("Please enter your age: ")
age = int(age_str) # Don't forget to convert the input string to an integer!

if age < 13:
    print("You are a child.")
elif age >= 13 and age <= 19: # or you could write: 13 <= age <= 19
    print("You are a teenager.")
else:
    print("You are an adult.")

2. Repeating Actions with Loops

Loops are used to execute a block of code repeatedly. This saves you from writing the same code over and over again. Python has two main types of loops: while and for.

The while Loop

A while loop repeats a block of code as long as a condition remains true. It's crucial that something inside the loop eventually makes the condition false, otherwise you'll create an infinite loop!

Here’s a typical example of a while loop that counts down:

count = 5
while count > 0:
    print(count)
    count = count - 1 # This is the crucial update step. Can also be written as count -= 1

print("Blast off!")
# Output:
# 5
# 4
# 3
# 2
# 1
# Blast off!

The for Loop

A for loop is used for iterating over a sequence, like a list, a string, or a range of numbers. It's often more convenient than a while loop when you know how many times you want to loop.

To loop a specific number of times, we use the built-in range() function.

• range(5) generates numbers from 0 up to (but not including) 5: 0, 1, 2, 3, 4.
• range(1, 5) generates numbers from 1 up to 5: 1, 2, 3, 4.
• range(1, 10, 2) generates numbers from 1 to 10, with a step of 2: 1, 3, 5, 7, 9.

Let's see how for loops work with sequences and the range() function.

Example of iterating over a list:

fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(f"I like to eat {fruit}s.")
# Output:
# I like to eat apples.
# I like to eat bananas.
# I like to eat cherrys.

Example using range():

# Print the first 3 multiples of 5
for i in range(1, 4): # Loop will run for i = 1, 2, 3
    print(f"5 * {i} = {5 * i}")
# Output:
# 5 * 1 = 5
# 5 * 2 = 10
# 5 * 3 = 15

3. Altering Loop Behavior with break and continue

Sometimes you need more control over your loops. The break and continue statements give you this power.

• break: Immediately terminates the loop entirely.
• continue: Immediately skips to the next iteration of the loop.

This video demonstrates both statements in action.

Here is a summary:

Using break: Stop the loop when a condition is met.

# Find the first number divisible by 7 in a range
for number in range(1, 20):
    if number % 7 == 0:
        print(f"Found it! The first number divisible by 7 is {number}.")
        break # Exit the loop immediately
    print(f"Checking {number}...")
# Output:
# Checking 1...
# ...
# Checking 6...
# Found it! The first number divisible by 7 is 7.

Using continue: Skip an iteration.

# Print only odd numbers
for number in range(1, 10):
    if number % 2 == 0:
        continue # If the number is even, skip to the next number
    print(number)
# Output:
# 1
# 3
# 5
# 7
# 9

Conclusion

You have now learned the essential tools for controlling program flow in Python. This is a massive step forward in your coding ability. For a quick reference of everything we've covered, this cheatsheet is an excellent resource to keep handy.

Key Takeaways:

• Conditional Statements (if, elif, else) let your program make decisions and execute different code paths based on boolean conditions.
• Loops allow you to repeat code. The while loop is for repeating as long as a condition is true, while the for loop is perfect for iterating over sequences like lists or a range of numbers.
• Indentation and colons (:) are mandatory parts of Python's syntax for defining code blocks within these structures.
• break and continue give you fine-grained control to exit a loop or skip an iteration.

In our next lesson, we will explore functions. Now that you can write more complex logic, functions will provide a way to organize that logic into reusable, named blocks, making your code cleaner, more modular, and easier to manage.