Control Flow in Python¶

Ahmed Moustafa

Please scan...¶

Definition of Control Flow¶

  • Control flow is the order in which statements and instructions are executed in a program
  • Control flow can be affected by decision-making statements, loops, and function calls.

Code blocks and indentation¶

  • Code blocks are a group of statements that are executed together.
  • In Python, code blocks are defined by their indentation level.
In [4]:
%%script echo
if condition:
    # This code is indented, so it's part of the if block
    print('The condition is true.')
    
# This code is not indented, so it's not part of the if block
print('This code runs regardless of the condition.')




















If-else statements:¶
    

  • If-else statements are used to execute code based on a condition.
    • 

  • If the condition is true, the code within the if block is executed.
    • 

  • If the condition is false, the code within the else block is executed.
    • 



Syntax:











In [5]:



     


%%script echo
if condition:
    # Code to execute if the condition is true
else:
    # Code to execute if the condition is false



     


















    
    

























If-else statements - Example¶










In [6]:



     


age = 25
if age >= 18:
    print("You are an adult")
else:
    print("You are a minor")



     


















    
    






You are an adult




















You can also use a shorthand notation for assigning a value to a variable based on a condition:











In [7]:



     


x = 10
y = 20
max_value = x if x > y else y
print(max_value)



     


















    
    






20




















Boolean Operators¶
    

  • Boolean operators are used to combine multiple conditions in an if-statement
    • 

  • Python provides the operators and, or, and not
    • 












In [8]:



     


x = 5
y = 10
if x > 0 and y < 20:
    print("Both conditions are true")
if x > 0 or y < 20:
    print("At least one condition is true")
if not x == y:
    print("x is not equal to y")



     


















    
    






Both conditions are true
At least one condition is true
x is not equal to y




















Truth Table¶
















    
    



























Source [Princeton's Introduction to CS: Boolean Logic](https://introcs.cs.princeton.edu/java/71boolean/)











Boolean Operators with Non-boolean Values¶










Boolean operators can also be used with non-boolean values. In Python, any non-zero or non-empty value is considered true, while zero or empty values are considered false:











In [10]:



     


name = ""
if not name:
    print("The name is not set")



     


















    
    






The name is not set




















Nested If Statements¶
    

  • Nested if statements allow for more complex conditions to be checked
    • 

  • Multiple conditions can be checked using if, elif, and else statements inside of each other
    • 



Syntax:











In [11]:



     


%%script echo
if condition1:
    if condition2:
        # Code to execute if both conditions are true
    else:
        # Code to execute if condition1 is true and condition2 is false
else:
    # Code to execute if condition1 is false



     


















    
    

























Nested If Statements - Example¶










In [12]:



     


grade = 85
if grade >= 90:
    print("You got an A")
elif grade >= 80:
    print("You got a B")
elif grade >= 70:
    print("You got a C")
else:
    print("You failed the course")



     


















    
    






You got a B




















Loops: for¶
for loops are used to iterate over a sequence of items.
Syntax:











In [13]:



     


%%script echo
for item in sequence:
    # Code to execute for each item in the sequence



     


















    
    

























In [14]:



     


fruits = ['apple', 'banana', 'cherry']
for fruit in fruits:
    print(fruit)



     


















    
    






apple
banana
cherry




















Loops: while¶
    

  • while loops are used to repeat a block of code while a certain condition is true.
    • 



Syntax:











In [15]:



     


%%script echo
while condition:
    # Code to execute as long as the condition is true



     


















    
    

























Loops: while - Example¶










In [16]:



     


x = 0
while x < 5:
    print(x)
    x += 1



     


















    
    






0
1
2
3
4




















continue and break Statements¶
    

  • continue statements are used to skip to the next iteration of a loop.
    • 

  • break statements are used to exit a loop early.
    • 












In [17]:



     


%%script echo
for item in sequence:
    if condition:
        continue # Skip to the next iteration
    if other_condition:
        break # Exit the loop early
    # Code to execute for each item in the sequence



     


















    
    

























continue and break Statements - Example¶










In [18]:



     


fruits = ['apple', 'banana', 'cherry']
for fruit in fruits:
    if fruit == 'banana':
        continue # Skip printing "banana"
    if fruit == 'cherry':
        break # Exit the loop
    print(fruit)



     


















    
    






apple




















Exercise: Fibonacci Sequence¶
Write a program that generates the first 20 numbers in the Fibonacci sequence.


Hints:


    

  • The Fibonacci sequence is a sequence of numbers where each number is the sum of the two preceding numbers.
    • 

  • The first two numbers in the sequence are 0 and 1.
    • 

  • Use a for loop to generate the sequence.
    • 



Example output:


0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, 233, 377, 610, 987, 1597, 2584, 4181

















    
    



























Exercise: Fibonacci Sequence (GitHub)¶
















    
    



























Summary¶
Python provides several control flow statements to manage the order of execution of code, including if-else statements, nested if statements, for loops, while loops, break statements, and continue statements. These statements allow data scientists to write more efficient and effective code.