Python Loops and Logic

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Python loops are control flow structures used to repeatedly execute a block of code.

• two types:
  • for
    • used to iterate over a sequence or other iterable object
  • while
    • used to repeatedly execute a block of code as long as a certain condition is True

Constructing while Loops

subnet = "192.168.1."
ip_addresses = []
i = 1
 
while i < 256:
    ip_addresses.append(subnet + str(i))
    i += 1
 
for ip in ip_addresses:
    print(ip)

Incrementing in while Loop

• incrementing increases a counter variable
• need to prevent the loop from running indefinitely

i = 0
while i < 5:
    print(i)
    i += 1  # Increment

break Statement

The break statement in Python is used to exit a loop prematurely.

• used when there’s a need to stop the loop before the condition is False

subnet = "192.168.1."
ip_addresses = []
i = 1
 
while True:
    if i > 255:
        break
    ip_addresses.append(subnet + str(i))
    i += 1
 
for ip in ip_addresses:
    print(ip)

continue Statement

The continue statement in Python is used in loops to skip the rest of the current iteration and move directly to the next one.

subnet = "192.168.1."
ip_addresses = []
i = 0
 
while i < 256:
    i += 1
    if i == 100:  # Let's say we want to skip this IP
        continue
    ip_addresses.append(subnet + str(i))
 
for ip in ip_addresses:
    print(ip)

else Statement

The else statement in a Python while loop specifies a block of code to be executed when the loop condition becomes False.

• only executes after the loop finishes, not when exited with break

subnet = "192.168.1."
ip_addresses = []
i = 0
 
while i < 256:
    i += 1
    ip_addresses.append(subnet + str(i))
else:
    print("All IP addresses have been generated.")
 
for ip in ip_addresses:
    print(ip)

Constructing for Loops

A for loop in Python is a control flow statement that allows code to be executed repeatedly.

for variable in iterable:
    # code to be executed

•  iterable is a collection of objects
•  variable takes the value of the next item in the iterable each time through the loop

for Loops vs while Loops

• both are control flow statements
  • differ in usage and control conditions
  • for loop
    • is used for iterating over a sequence
    • the set of statements is executed once for each item in the list
    • loop continues until it has gone through each item in the sequence
    • typically used when number of iterations is known or finite
  • while loop
    • used when a set of statements needs to be executed until a condition is false
    • condition is checked before each iteration
    • used when number of iterations is unknown or infinite

Looping Through a String

• Python has built-in support for string iteration

for character in string:
	# code to be executed

• string is the string that needs to be iterated over
• character is the variable that takes the value of each character in the string for each iteration

Example

ip_address = "192.168.1.1"
octets = ip_address.split(".")
octet_count = 0
 
for octet in octets:
    if octet.isdigit() and 0 <= int(octet) <= 255:
        octet_count += 1
 
print(f"The IP address {ip_address} has {octet_count} octets.")

range() Function

The range() function in Python is used to generate a sequence of numbers within a given range.

• used in for loops when there’s a need to repeat an action a specific number of times
• Syntax: range(start, stop, step)
  • start: An integer number specifying at which position to start. Default is 0
    • Optional
  • stop: An integer number specifying at which position to stop (not included)
  • step: An integer number specifying the incrementation. Default is 1
    • Optional

subnet = "192.168.1."
ip_addresses = [subnet + str(i) for i in range(1, 256)]
 
for ip in ip_addresses:
    print(ip)

pass Statement

The pass statement in Python is a placeholder statement that is used when the syntax requires a statement, but no action needs to be taken.

for variable in iterable:
    # code to be executed
    pass  # no action taken