Loops (for/while/do-while), break/continue, loop else + random / time / datetime

1. What you used in python5.py (quick scan summary)

This class code demonstrates:

• for loops (including iterating over collections)
• range() patterns (start/stop/step, reverse)
• Nested loops (pattern printing)
• continue (skip an iteration)
• break (stop the loop)
• for ... else (else runs only if the loop finishes without break)
• while loops (repeat while condition is true)
• “do-while” style using while True + break
• Measuring time using datetime.datetime.now() and subtracting timestamps
• Using standard library modules: random, time, datetime

2. for loop (iterate over a collection)

Use for when you want to run the same logic for each item in an iterable.

2.1 Syntax

for item in iterable:
    print(item)

2.2 Iterating over common collections

List

services = ["ec2", "ebs", "rds"]
for s in services:
    print(s)

String

name = "sahar"
for ch in name:
    print(ch)

Tuple

envs = ("dev", "staging", "prod")
for env in envs:
    print(env)

Set

ports = {22, 80, 443}
for p in ports:
    print(p)

3. while loop (repeat while a condition is true)

Use while when you don’t know how many iterations you need in advance.

3.1 Syntax

while condition:
    # repeat
    pass

3.2 Example: keep asking until correct PIN

pin = "1744"
guess = input("enter the pin code : ")

while guess != pin:
    guess = input("enter the pin code : ")

print("welcome")

4. “Do-While” in Python (emulation)

Python doesn’t have a real do { } while (condition) statement.

The common pattern is:

• while True: to run at least once
• then break when you want to stop

4.1 Example: stop only when user enters 100

while True:
    x = int(input("enter a number  "))
    if x == 100:
        break

4.2 Example: menu loop (calculator idea)

while True:
    print("press 1 for + or q to exit ")
    print("press 2 for - or q to exit ")
    print("press 3 for * or q to exit ")
    print("press 4 for // or q to exit ")

    choice = input(" : ")
    if choice == "q":
        break

    if choice in "1234":
        x = int(input("enter a number "))
        y = int(input("enter a number "))

    match choice:
        case "1":
            print(f"{x} + {y} = {x + y}")
        case "2":
            print(f"{x} - {y} = {x - y}")
        case "3":
            print(f"{x} * {y} = {x * y}")
        case "4":
            print(f"{x} // {y} = {x // y}")
        case _:
            print("invalid choice")

5. break vs continue

5.1 break

• Ends the loop immediately.
• Use it when you found what you need or when something failed.

services = ["ec2", "s3", "rds"]
status = [True, False, True]

offline = None

for i in range(len(services)):
    if not status[i]:
        offline = services[i]
        break

print(offline)

5.2 continue

• Skips the rest of the current iteration and moves to the next.

meals = ["starter", "salad", "main dish", "dessert"]

for meal in meals:
    if meal == "salad":
        continue
    print(meal)

6. Loop else (important)

Python has a special else on loops:

• The else block runs only if the loop finished normally
• The else block does not run if the loop ended with break

6.1 for ... else example (like your meals example)

meals = ["starter", "salad", "main dish", "dessert"]

for meal in meals:
    if meal == "salad":
        continue
    print(f"{meal} is arriving")
else:
    print("discount 30% for ordering all the menu")

6.2 for ... else as a “search” pattern

services = ["nginx", "redis", "postgres"]
target = "redis"

for s in services:
    if s == target:
        print("found")
        break
else:
    print("not found")

6.3 while ... else

Works the same idea:

i = 0

while i < 3:
    pwd = input("enter password: ")
    if pwd == "secret":
        print("welcome")
        break
    i += 1
else:
    print("locked")

7. import basics (what it means)

Python has a huge standard library. import lets you use modules that live outside your current .py file.

7.1 Basic forms

import random
import time
import datetime

Then you access things using the module name:

x = random.randint(1, 10)

7.2 Import specific objects

from datetime import datetime, timedelta

Then you can use them directly:

now = datetime.now()

8. random module (full tutorial)

random is for pseudo-random numbers (good for games/simulations/tests). It is not for cryptography.

8.1 Common functions

random.randint(a, b)

Random integer, inclusive.

import random
print(random.randint(-100, 1500))

random.random()

Random float between 0.0 and 1.0.

import random
print(random.random())

random.uniform(a, b)

Random float between a and b.

import random
print(random.uniform(10.0, 100.5))

random.choice(seq)

Pick one random item from a list/tuple/string.

import random
colors = ["red", "blue", "yellow", "black"]
print(random.choice(colors))
print(random.choice(["heads", "tails"]))

random.shuffle(list)

Shuffles a list in-place.

import random
cards = ["Ace", "King", "Queen", "Jack", "10"]
random.shuffle(cards)
print(cards)

random.randrange(start, stop, step)

Like picking a random value from a range.

import random
print(random.randrange(1, 100, 2))

8.2 random.seed()

Use seed if you want repeatable results.

import random
random.seed(42)
print(random.randint(1, 10))
print(random.randint(1, 10))

9. time module (full tutorial)

time is low-level time utilities.

9.1 time.sleep(seconds)

Pause execution.

import time
print("x")
time.sleep(3)
print("y")

9.2 time.time()

Returns Unix timestamp (seconds since 1970).

import time
print(time.time())

9.3 time.ctime([seconds])

Human-readable time string.

import time
print(time.ctime())

9.4 Measuring elapsed time (recommended)

For measuring durations, prefer time.perf_counter():

import time

start = time.perf_counter()

time.sleep(0.5)

end = time.perf_counter()
print(end - start)

10. datetime module (full tutorial)

datetime is higher-level date/time tools.

10.1 Key classes

• datetime.datetime: date + time together
• datetime.date: only date
• datetime.time: only time
• datetime.timedelta: a duration (difference)

10.2 Get current date/time

import datetime

t = datetime.datetime.now()
print(t)
print(t.date())
print(t.time())
print(t.month)
print(t.second)
print(t.microsecond)

10.3 Datetime arithmetic (duration)

import datetime

start_time = datetime.datetime.now()

input("press Enter to stop")

end_time = datetime.datetime.now()
print(end_time - start_time)

10.4 timedelta

from datetime import datetime, timedelta

now = datetime.now()
print(now)
print(now + timedelta(days=7))
print(now - timedelta(hours=2))

10.5 Formatting: strftime

from datetime import datetime

now = datetime.now()
print(now.strftime("%Y-%m-%d %H:%M:%S"))

Common format codes:

• %Y year (2026)
• %m month (01-12)
• %d day (01-31)
• %H hour (00-23)
• %M minute (00-59)
• %S second (00-59)

10.6 Parsing: strptime

from datetime import datetime

s = "2026-03-24 13:45:00"
dt = datetime.strptime(s, "%Y-%m-%d %H:%M:%S")
print(dt)

11. Practice tasks

Loops

1. Ask the user for a password. Give them 3 tries. If correct: print welcome and stop. If not: after 3 tries print locked.
2. Given a list of services and a list of booleans for status, print which services are up. If a service is down, stop checking.
3. Use a for ... else loop to search for an item in a list and print found / not found.

Modules

1. Use random.choice() to simulate coin toss 10 times and count heads vs tails.
2. Use time.sleep() to print a countdown message every second (no range required).
3. Use datetime to print:
   • current timestamp
   • timestamp after 3 days
   • elapsed time between two moments