3 Control Flow

A loop is a tool that allows us to instruct Python to repeat a task several times, usually with slight variations each time.
We will consider two types of loops in this course: for loops and while loops.

Important Note on Python Syntax

Indentation, not braces
- Unlike R, Python does not use {} braces to structure code blocks.
- Instead, Python relies on indentation (whitespace at the beginning of a line).
Colons (:)
- A colon at the end of a statement (if, for, while, def, etc.) indicates the start of a new block.
- The code that follows must be indented consistently.
You may use spaces or tabs, but be consistent within a file.
The convention is 4 spaces per indentation level.
Incorrect indentation will raise an IndentationError.

Example: `if` in R vs Python

x <- 5
if (x > 0) {
  print("Positive")
} else {
  print("Non-positive")
}

Python:

x = 5
if x > 0:
    print("Positive")
else:
    print("Non-positive")

Example: `for` loop in R vs Python

for (i in 1:5) {
  print(i^2)
}

Python:

for i in range(1, 6):
    print(i**2)

For Loops

The syntax for constructing a for loop is as follows:

for i in range(a,b): 
    # body of the loop

The variable i is referred to as a loop counter

Looping over Lists

names_list = ['Alex', 'Beth', 'Chad', 'Drew', 'Emma']

for name in names_list:
    print(f'Hello, {name}!')

Hello, Alex!
Hello, Beth!
Hello, Chad!
Hello, Drew!
Hello, Emma!

Looping over Ranges

for i in range(0,5):
    print('The value of i is', i)

The value of i is 0
The value of i is 1
The value of i is 2
The value of i is 3
The value of i is 4

Example: A python function to calculate the sum of the squares of the first 100 integers

# Sum the first 100 positive integers
total = 0
for i in range(1,101):
    total += i**2

print(total)

Other Iterables

A for loop can work with any iterable object.
An iterable is anything that can return its elements one at a time (for example, in a loop).
Common iterables in Python:
- Lists → [1, 2, 3]
- Tuples → (4, 5, 6)
- Strings → "hello"
- Ranges → range(5)
- Sets → {1, 2, 3}
- Dictionaries → {"a": 1, "b": 2} (iterates over keys by default)

for char in "isrt, du":
    print(char)

i
s
r
t
,
 
d
u

for key in {"x": 1, "y": 2}:
    print(key)

x
y

Exercise 3.1

Write a python function to:

calculate the sum of squares of elements in a list.
Print the multiplication table of 7 using a for loop.
Count the number of vowels in a given string.
Given a list of numbers, print only the even numbers.

While Loops

The syntax for creating a while loop is as follows:

while condition:
    # code to be executed each iteration

Example: Print out the squares of the first five positive integers.

n = 1
while n <= 5:
    print(n**2)
    n += 1

A for loop is best when the number of iterations is known in advance, making the code shorter and easier to read. In contrast, a while loop is more flexible and useful when the number of iterations cannot be predetermined, such as when it depends on external conditions like user input.

Break and Continue

The break statement immediately exits the loop when a condition is met.
- Useful when you want to stop once a desired value is found.

i = 1
while i < 6:
    print(i)
    if i == 3:
        break
    i += 1

1
2
3

for i in range(1, 6):
    if i == 3:
        break
    print(i)

1
2

The continue statement skips the current iteration and moves to the next cycle of the loop.
- Useful to skip specific values during iteration without stopping the loop.

i = 1
while i < 6:
    i += 1
    if i == 3:
        continue
    print(i)

for i in range(1, 6):
    if i == 3:
        continue
    print(i)

Conditional Statements

Conditional statements allow us to control whether or how certain parts of our code will execute, based on logical conditions we specify.
The most basic form is the if statement. Its syntax is:

if condition:
    # code to execute when the condition is True

If-Else Statements

Sometimes, we need to choose between two alternatives: one block if the condition is true, and another if it is false.
This is achieved using an if-else statement.

Syntax:

if condition:
    # code to execute if condition is True
else:
    # code to execute if condition is False

grade = 42

if grade >= 40:
    print("You passed the exam.")
    print("Congratulations!")
else:
    print("You failed the exam.")
    print("Better luck next time.")

You passed the exam.
Congratulations!

If-Elif-Else Statements

Sometimes, we need to choose from more than two alternatives.
In such cases, we use an if-elif-else statement.

Syntax:

if condition1:
    # code to execute if condition1 is True
elif condition2:
    # code to execute if condition1 is False, but condition2 is True
elif condition3:
    # code to execute if all above are False, but condition3 is True
elif condition4:
    # code to execute if all above are False, but condition4 is True
...

else:
    # code to execute if none of the above conditions are True

grade = 75

if grade >= 80:
    print("You got an 'A+' on the exam.")
elif grade >= 75:
    print("You got a 'A' on the exam.")
elif grade >= 70:
    print("You got a 'A-' on the exam.")
elif grade >= 65:
    print("You got a 'B' on the exam.")
else:
    print("You got an 'F' on the exam.")

You got a 'A' on the exam.

Ternary (Conditional) Expressions

Now that we understand full if-else blocks, Python also provides a shorthand form called ternary expressions.
A ternary expression is a shorthand way to write an if–else in a single line.
General syntax:

value_if_true if condition else value_if_false

Example 1:

age = 20
status = "Adult" if age >= 18 else "Minor"
print(status)

Adult

Example 2:

x = -5
sign = "Positive" if x > 0 else "Zero" if x == 0 else "Negative"
print(sign)

Negative

We can break it into multiple lines for readability:

sign = (
    "Positive" if x > 0
    else "Zero" if x == 0
    else "Negative"
)

Use ternary expressions for simple decisions.
For more complex logic, use regular if–else blocks for readability.

Summary

For loop → predetermined number of iterations
While loop → runs until condition is False
Break → exit loop, Continue → skip iteration
If-Else → choose between two paths
If-Elif-Else → choose between multiple paths
Ternary expression → compact one-line if–else for simple cases

Exercises

3.A — Warm-ups (For loops)

Write a for loop that prints the squares of all integers from 1 to 10 (inclusive).
Given the list nums = [3, 10, -4, 7, 0, 9], use a loop to print only the positive numbers.
Write a program that asks for a string and counts how many of its characters are vowels (a, e, i, o, u), ignoring case.

3.B — Ranges & Aggregation

Use a loop to compute the sum of the cubes of integers from 1 to 50.
Compute the product of all integers from 1 to 8 (that is, calculate 8!).
Print the multiplication table for 6, showing results from 6 × 1 up to 6 × 10.

3.C — Looping over Iterables

Loop over the string "ISRT, DU" and print each character, skipping commas and spaces.
Given the dictionary d = {"x": 4, "y": 9, "z": 16}, print each key along with the square root of its value.

3.D — While Loops

Using a while loop, print the first 7 powers of 2: 1, 2, 4, 8, ....
Write a program that processes numbers in a list one by one. Stop when a negative number is found, and print the sum of all the numbers before that negative number.

3.E — Break & Continue

Given nums = [5, 12, 7, 0, 13, 9], use a loop to print each number until you reach 0. At that point, stop the loop immediately (use break).
Write a loop that prints all numbers from 1 to 20, but skips the multiples of 3 (use continue).

3.F — If / Elif / Else

Write a program that reads a student’s percentage score (an integer between 0 and 100) and prints their letter grade according to the following rules:

A+ if score ≥ 80
A if 75 ≤ score ≤ 79
A- if 70 ≤ score ≤ 74
B if 65 ≤ score ≤ 69
F otherwise

3.G — Ternary Expressions

Write a one-line expression that sets the variable parity to "Even" if n is even, otherwise to "Odd".
Write a one-line expression that sets the variable status to "Pass" if grade >= 40, otherwise to "Fail".