Programming Essentials in Python Lecture 5

Control Structures in Python

Control structures determine the flow of execution in a program. In Python, the key control structures include if/else statements and loops.

If/Else Statements

General Form

An if/else statement determines which block of code to execute based on a Boolean condition.

• if Statement: Executes a block if the condition is true.
• else Statement: Executes if the if condition is false.

# General structure of if/else
if condition:  # The condition must evaluate to True or False
  # Block of statements executed if condition is True
else:
  # Block of statements executed if condition is False

Nested If/Else

If/else statements can be nested to evaluate multiple conditions.

value = int(input("Enter an integer value in the range 0-10: "))

if value >= 0:
  if value <= 10:
    print(value, "is in range")
  else:
    print(value, "is too large")
else:
  print(value, "is too small")

print("Done")

Elif (Multiple Decision-Making)

The elif keyword allows checking multiple conditions in sequence.

value = int(input("Enter an integer in the range 0-3: "))

if value < 0:
  print("Too small")
elif value == 0:
  print("Zero")
elif value == 1:
  print("One")
elif value == 2:
  print("Two")
elif value == 3:
  print("Three")
else:
  print("Too large")

print("Done")

Ternary Operator (Conditional Expression)

The ternary operator is a shorthand for if/else conditions.

n = int(input("Enter a number: "))

# Using a ternary operator
print("|", n, "| =", (-n if n < 0 else n))

# Equivalent if/else block
if n < 0:
  print("|", n, "| =", -n)
else:
  print("|", n, "| =", n)

Loops

Introduction

Loops are used to repeat a block of code as long as a condition is met. Each repetition is called an iteration.

---

While Loop

The while loop repeats as long as the condition is true.

# Example: Count from 1 to 5
count = 1
while count <= 5:
  print(count)
  count += 1

While with Else

The else block executes when the loop condition becomes false.

i = 1
while i < 6:
  print(i)
  i += 1
else:
  print("i is no longer less than 6")

Continue Statement

Skips the current iteration and moves to the next.

# Skip printing 3
i = 0
while i < 6:
  i += 1
  if i == 3:
    continue
  print(i)

Break Statement

Terminates the loop immediately.

# Stop when i equals 3
i = 1
while i < 6:
  print(i)
  if i == 3:
    break
  i += 1

---

For Loop

The for loop iterates over a sequence (like a range or list).

# Example: Loop through numbers 1 to 5
for n in range(1, 6):
  print(n)

print("n contains:", n)

For with Else

The else block executes after the loop finishes normally (without a break).

for x in range(6):
  print(x)
else:
  print("Finally finished!")

For with Break

If a break occurs, the else block is skipped.

for x in range(6):
  if x == 3:
    break
  print(x)
else:
  print("Finally finished!")

For with Continue

Skips the current iteration and moves to the next.

# Skip printing 3
for x in range(6):
  if x == 3:
    continue
  print(x)

---

Infinite Loops

Loops can run indefinitely if the condition is always true. Use them with caution and ensure there's an exit mechanism.

# Example of an infinite loop (not recommended)
while True:
  val = input("Type 'exit' to quit: ")
  if val == "exit":
    break

---

Practical Example: Summing Values with While

sum = 0
done = False

while not done:
  val = int(input("Enter positive integer (type 999 to exit): "))

  if val < 0:
    print("Negative value", val, "ignored")
    continue  # Skip to the next iteration

  if val == 999:
    done = True  # Exit the loop
  else:
    print("Adding", val)
    sum += val

print("Sum =", sum)

Summary

• If/Else: Conditional execution.
• Loops: Repeated execution.
• Break: Terminates the loop.
• Continue: Skips to the next iteration.

These control structures form the foundation of Python's flow control and are essential for writing efficient and readable code.