Bioinformatics Laboratory at BiUH.
Core Values: Respect, Reflection, Communication, Commitment.
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Section 2. Conditionals in Python
Section 3. Loop Structures in Python
Section 4. Control Flow - Practical Examples
Section 5. Special Considerations
Control flow determines which code statements run first, which run next, whether to repeat them, and whether to skip them.
Q: Why is control flow fundamental to programming?
Draw Control Flow: https://mermaid.live/edit
Section 1. Control Flow
1.1. Conditional Statements
Traffic Light Example
light_color = input("Traffic light color: ")
if light_color == "red":
print("STOP")
elif light_color == "yellow":
print("SLOW DOWN")
elif light_color == "green":
print("GO")
else:
print("INVALID COLOR")
1.2. Flowchart - Decision Making
graph TD
A[Start] --> B{Light Color?}
B -->|Red| C[Stop]
B -->|Yellow| D[Slow Down]
B -->|Green| E[Go]
C --> F[End]
D --> F
E --> F
1.3. While Loops
Morning Run Example
import time
runs_remaining = 3
while runs_remaining > 0:
time.sleep(1) # suppose you are a superman, 400m in 1 second
print("Morning run #"+str(4 - runs_remaining)+" completed!")
runs_remaining = runs_remaining - 1
print("All runs finished!")
1.4. Flowchart - Loop Execution
graph TD
A[Start] --> B{Runs Left?}
B -->|Yes| C[Run]
C --> D[Reduce Count]
D --> B
B -->|No| E[End]
1.5. Why Control Flow Matters
- Programs need to make decisions
- Handle different situations
- Repeat tasks efficiently
- Makes code adaptable
Automatic Door Control Example
# since we don't have a sensor, we just pretend to be a sensor.
import time
def object_detected():
# get signal from the sensor
signal = input('detected(y/n):')
if signal=='y':
result=True
print('detected!')
else:
result=False
print('no object!')
time.sleep(1)
return(result)
def keep_open():
print('open door!')
# send signal to the door
time.sleep(1)
def close_door():
print('close door!')
# send signal to the door
time.sleep(1)
sensor_active = True
while sensor_active:
if object_detected():
keep_open()
else:
close_door()
1.6. Vending Machine Example
coins = 0
while coins < 5:
insert_coins = int(input("Insert coin (1-3): "))
coins = coins + insert_coins
print("Total inserted: "+str(coins))
print("Here's your drink!")
Question: how can we limit the number of inserted coins each time
Question: how can we get the extra coins
1.7. Common Patterns
Flag-controlled loops
running = True
while running:
user_input = input("Continue? (y/n): ")
if user_input == "n":
running = False
Counter-controlled loops
attempts = 3
while attempts > 0:
# code here
attempts=attempts-1
attempts = 0
while attempts < 10:
# code here
attempts=attempts+1
1.8. Take-home message
if-elif-elsefor decisionswhilefor repeating tasks- Condition evaluation (True/False)
- Loop control variables (i, counter)
- Avoid infinite loops!
# Always update condition!
i = 0
while i < 10:
print(i)
i = i+1 # Don't forget this!
Section 2. Conditionals in Python
2.1. if Statement Basics
# Basic structure
age = 18
if age >= 18:
print("You can vote!")
if= Program’s “what if” question- Colon
:is mandatory (show error without colon) - Indentation = 4 spaces (never mix with Tab)
2.2. Indentation Demo
# Correct vs Wrong
if True:
print("Right") # ← 4 spaces
if True:
print("Wrong") # ← No indentation
2.3. Comparison Operators
height = 175
print(height > 160) # True
print(height == 170) # False
# Real-life example
bus_height_limit = 200
luggage_height = 210
if luggage_height > bus_height_limit:
print("Cannot take bus")
2.4. Logical Operators
# Weather check
is_raining = True
has_umbrella = False
if is_raining and has_umbrella:
print("Go outside")
else:
print("Stay home")
and: Both must be Trueor: At least one Truenot: Reverse boolean
2.5. Common Mistake Fix
# Wrong
if temperature = 30:
print("Hot")
# Correct
if temperature == 30:
print("Hot")
- Single
=is assignment - Double
==is comparison
2.6. Multi-Condition Flow
age = 15
if age < 13:
print("Child ticket")
elif age < 18:
print("Teen ticket")
else:
print("Adult ticket")
2.7. Nested Conditions
age = 20
is_student = True
if age < 18:
print("50% discount")
else:
if is_student:
print("30% discount")
else:
print("No discount")
- Each level needs 4-space indent
- Max 3 nesting levels recommended
2.8. Ticket System Flowchart
# Full example
age = 25
is_student = False
has_membership = True
if age < 5:
print("Free ticket")
elif age < 18:
print("Child ticket")
elif is_student:
print("Student discount")
elif has_membership:
print("Membership discount")
else:
print("Regular ticket")
Question: what if you are a 19 years old student without membership.
Question: what if you are a 19 years old student with membership.
Question: how can we get both the Student discount and the Membership discount
2.9. Truth Value Testing
# Common False values:
- False
- None
- 0
- "" (empty string)
- [] (empty list)
name = ""
if not name:
print("Name is required!")
2.10. Short-form Conditional Expression (not recommended for beginners)
# Short form
temperature = 28
message = "Hot" if temperature > 25 else "OK"
print(message)
# preferred
temperature = 28
if temperature >25:
message='Hot'
else:
message='OK'
print(message)
- Use for simple two-way choices
- Not recommended for complex logic
Section 3. Loop Structures in Python
3.1. For Loop Basics
# Like checking a shopping list
groceries = ["apple", "milk", "bread"]
for item in groceries:
print("Don't forget to buy:", item)
flowchart TD
A[Start] --> B{Items left?}
B -->|Yes| C[Process item]
C --> B
B -->|No| D[End]
3.2. The range() Function
Three usages:
# Single argument
for num in range(5):
print(num) # 0-4
# Two arguments
for num in range(2, 5):
print(num) # 2-4
# Three arguments
for num in range(0, 10, 2):
print(num) # 0,2,4,6,8
3.3. String Traversal with Index
word = "hello"
for index, letter in enumerate(word):
print("Character at position "+str(index)+": "+letter)
Output:
Character at position 0: h
Character at position 1: e
...
3.4. While Loop Mechanics
# Like a persistent alarm clock
countdown = 3
while countdown > 0:
print(countdown)
countdown = countdown-1
print("Go!")
3.5. Termination Condition Design
# Dangerous version (for demonstration)
# temperature = 25
# while temperature > 20:
# print("AC running")
# Forgot to decrease temperature!
3.6. Safe Loop Writing Technique
Always write termination first:
# 1. Set initial value
counter = 0
# 2. Define termination
MAX_ATTEMPTS = 3
while counter < MAX_ATTEMPTS:
print("Attempt:", counter+1)
counter += 1
3.7. Break Statement
# Emergency exit example
passwords = ["123", "admin", "letmein"]
for pwd in passwords:
if pwd == "admin":
print("Security alert!")
break
print("Checking:", pwd)
3.8. Continue Statement
# Skip odd numbers
for num in range(10):
if num % 2 != 0:
continue
print(num, "is even")
3.9. Else Clause in Loops
# Only executes if loop completes normally
numbers = [2, 4, 6]
for n in numbers:
if n % 2 != 0:
break
else:
print("All numbers are even!")
3.10. Nested Loops: Multiplication Table
for i in range(1, 4): # Rows
for j in range(1, 4): # Columns
print(str(i)+'x'+str(j)+'='+str(i*j), end="\t")
print() # New line after each row
Output:
1x1=1 1x2=2 1x3=3
2x1=2 2x2=4 2x3=6
3x1=3 3x2=6 3x3=9
3.11. Understanding Indentation Levels
# Outer loop (level 1)
for x in range(2):
# Inner loop (level 2)
for y in range(2):
# Nested block (level 3)
if x == y:
print("Match at "+str(x)+','+str(y))
# Back to level 1
print("----")
Section 4. Control Flow - Practical Examples
4.1. Smart Grade Rating System
Basic Structure
score = float(input("Enter score (0-100): "))
if score >= 90:
print("Grade A")
elif score >= 80:
print("Grade B")
elif score >= 70:
print("Grade C")
else:
print("Grade D")
flowchart TD
A[Start] --> B[Input Score]
B --> C{Score >=90?}
C -->|Yes| D[Grade A]
C -->|No| E{Score >=80?}
E -->|Yes| F[Grade B]
E -->|No| G{Score >=70?}
G -->|Yes| H[Grade C]
G -->|No| I[Grade D]
4.2. Adding Loop for Multiple Inputs
Continuous Processing
while True:
score = input("Enter score (q to quit): ")
if score.lower() == 'q':
break
# Grade logic here
print("Grade: " + grade)
flowchart TD
A[Start] --> B{Input Score}
B -->|q| C[Exit]
B -->|Number| D[Process Grade]
D --> B
4.3. Number Guessing Game
Core Game Loop
import random
target = random.randint(1,100)
while True:
guess = int(input("Guess (1-100): "))
if guess == target:
print("Correct!")
break
elif guess > target:
print("Too high")
else:
print("Too low")
flowchart TD
A[Generate Random Number] --> B{Guess?}
B -->|Correct| C[Win]
B -->|High| D[Prompt Lower]
B -->|Low| E[Prompt Higher]
D --> B
E --> B
4.4. Tracking Attempts
Counter Implementation
attempts = 0
while attempts < 5:
# Game logic
attempts += 1
if attempts == 5:
print("Game Over! Number was", target)
4.5. Victory/Failure Branches
Final Conditions
if guess == target:
print("Victory in " + str(attempts) + " tries!")
else:
print("Failed! Number was " + str(target))
4.6. Key Concepts Summary
if-elif-elsefor decision makingwhileloops for repeated actionstry-exceptfor basic error handling- Counter variables for tracking state
- Random module for game mechanics
4.7. Final Complete Code Demo
Grade System
while True:
# quit
score = input("Enter q to quit: ")
if score.lower() == 'q':
break
# grade
score = float(input("Enter score (0-100): "))
if score >= 90:
grade = 'A'
elif score >= 80:
grade = 'B'
elif score >= 70:
grade = 'C'
else:
grade = 'D'
print("Grade:"+grade)
Guessing Game
import random
target = random.randint(1,100)
attempts = 0
while attempts < 5:
guess = int(input("Guess (1-100): "))
if guess == target:
print("Correct!")
break
elif guess > target:
print("Too high")
else:
print("Too low")
attempts=attempts+1
if guess == target:
print("Victory in"+str(attempts)+" tries!")
else:
print("Failed! Number was "+str(target))
Section 5. Special Considerations
5.1. Multi-line Conditional Statements
# Recommended style
if (temperature > 100
and humidity < 70
and is_sunny == True):
print("Heat warning!")
flowchart TD
A[Check Conditions] --> B{All True?}
B -->|Yes| C[Execute Code]
B -->|No| D[Skip Block]
5.2. Splitting Long Logical Expressions
# Before
if (num > 0 and num%2 == 0 and num < 100 and name != "admin"):
# After
is_positive_even = num > 0 and num%2 == 0
within_limit = num < 100
valid_user = name != "admin"
if is_positive_even and within_limit and valid_user:
5.3. Debugging with Print Statements
counter = 0
while counter < 3:
print("DEBUG: Current counter - " + str(counter)) # Debug line
counter += 1
flowchart TD
S[Start] --> I[counter=0]
I --> C{counter < 3?}
C -->|Yes| P[Print counter]
P --> U[counter +=1]
U --> C
C -->|No| E[End]
5.4. Using TODO Comments
def calculate_discount(price):
# TODO: Add seasonal discount logic
# TODO: Implement user tier system
return price * 0.9 # Temporary 10% discount
5.5. Missing Colon Error
# Incorrect
while x < 5
print(x)
x += 1
# Correct
while x < 5:
print(x)
x += 1
5.6. Indentation Error Examples
# Wrong mixing of spaces/tabs
def test():
while n > 0: # Missing indentation
print(n) # Wrong level
n -= 1
# Correct
def count_down(n):
while n > 0:
print(n)
n -= 1
5.7. Floating Point Comparison
# Unreliable
result = 0.1 + 0.2
if result == 0.3:
print("Equal")
# Safe approach
tolerance = 1e-9
if abs(result - 0.3) < tolerance:
print("Effectively equal")
5.8. While Loop Flow Control
# Safe loop pattern
max_retries = 3
attempt = 0
while attempt < max_retries:
print("Attempt " + str(attempt+1))
# Your code here
attempt = attempt + 1
flowchart TD
S[Start] --> I[attempt=0]
I --> C{attempt < 3?}
C -->|Yes| L[Execute loop]
L --> U[attempt +=1]
U --> C
C -->|No| E[End]
End