06 - Control Flow

Control flow decides which statements run and how often they run.

A simple picture

Imagine walking through a game maze:

a condition chooses one path;
a for loop visits each checkpoint;
a while loop keeps moving while a rule remains true;
break is an emergency exit.

start -> question? -> chosen path -> repeat if needed -> finish

Conditions

A condition is an expression whose answer is True or False. A branch is one possible path through the program.

score = 72
if score >= 90:
    grade = "A"
elif score >= 60:
    grade = "B"
else:
    grade = "C"
print(grade)

Python tests conditions from top to bottom and runs the first matching branch. The else branch runs when no condition matches.

Notice the colon : and four beginning spaces. The spaces show which lines belong to each branch.

Nested conditions

age = 25
has_ticket = True
if age >= 18:
    if has_ticket:
        print("Enter")
    else:
        print("Buy a ticket")
else:
    print("Not eligible")

Before nesting many levels, consider combining clear conditions with and. Functions provide another organisation tool in Chapter 10.

What is an iterable?

An iterable is a value that contains, or can produce, items one at a time. In simple words, Python can “go through” it.

Start with two iterables you already recognise:

a string produces one character at a time;
range() produces one number at a time.

A list is a group written inside square brackets, such as [10, 20, 30]. Lists receive a complete lesson in Chapter 08. The table below is a map of iterable types you will gradually learn; you do not need to master them here.

Common iterables that you will use are:

Iterable	Example items produced
List	`[10, 20, 30]` produces `10`, then `20`, then `30`
Tuple	`("red", "blue")` produces `"red"`, then `"blue"`
String	`"cat"` produces `"c"`, then `"a"`, then `"t"`
Set	`{"Python", "Java"}` produces each value; order is not guaranteed
Dictionary	`{"name": "Ravi", "age": 20}` produces its keys by default
`range()`	`range(1, 4)` produces `1`, then `2`, then `3`

Beginner examples:

for letter in "cat":
    print(letter)

for number in range(1, 4):
    print(number)

An integer, float, or boolean is not iterable because it is one value, not a group of values:

# for number in 10:  # TypeError: an integer is not iterable
#     print(number)

You do not need to memorize the technical definition. When you see for, ask: “Can Python visit the values inside this object one by one?”

`for` loops

Use for to process each item in an iterable. The name after for receives one item at a time. This name is called the loop variable:

for number in range(1, 4):
    print(number)

The loop runs three times:

number becomes 1.
number becomes 2.
number becomes 3.

The loop variable is not required to be called item; choose a name that describes each value.

for letter in "Python":
    print(letter)

Preview: looping through dictionaries from Chapter 09

Looping through a dictionary

By default, a dictionary loop visits keys:

student = {"name": "Ravi", "marks": 86}

for key in student:
    print(key)

Use .values() for values and .items() for both keys and values:

for value in student.values():
    print(value)

for key, value in student.items():
    print(f"{key}: {value}")

`range()`

range(start, stop, step) produces numbers up to but not including stop:

range(5)          # 0, 1, 2, 3, 4
range(2, 6)       # 2, 3, 4, 5
range(10, 4, -2)  # 10, 8, 6

for number in range(1, 6):
    print(number)

Accumulation

Use a variable to remember work from earlier iterations:

total = 0
for number in range(1, 6):
    total += number
print(total)  # 15

`while` loops

Use while when the number of repetitions depends on a condition:

attempts = 0
while attempts < 3:
    print("Attempt", attempts + 1)
    attempts += 1

Always ensure something inside the loop can eventually make the condition false. Otherwise the loop never ends.

Important fact: before running a while loop, point to the line that will eventually make its condition false. If no such line exists, the loop may run forever.

`break`, `continue`, and `pass`

for number in range(1, 11):
    if number == 5:
        break       # stop the loop completely
    print(number)

for number in range(1, 6):
    if number % 2 == 0:
        continue    # skip even numbers
    print(number)

pass does nothing and is useful temporarily while designing a block:

score = 80
if score >= 50:
    pass

The program above is valid, but pass produces no output. Replace it when the branch is ready.

`else` with loops

Python also allows else after a for or while loop. The else block runs when the loop finishes normally. It does not run when break stops the loop.

for number in range(2, 6):
    print(number)
else:
    print("The loop finished normally")

This is useful for searching:

numbers = [3, 7, 11]
target = 7

for number in numbers:
    if number == target:
        print("Found")
        break
else:
    print("Not found")

If target is not found, the loop reaches its end and the else block runs. If it is found, break runs and the else block is skipped.

while True:
    choice = input("Choose add, view, or exit: ").strip().lower()
    if choice == "add":
        print("Adding item")
    elif choice == "view":
        print("Showing items")
    elif choice == "exit":
        break
    else:
        print("Unknown choice")

Common mistakes

Forgetting the colon after if, for, or while.
Using range(1, 5) expecting it to include 5.
Forgetting to update a while loop’s control variable.
Putting break outside the intended loop.
Using continue without understanding which code it skips.

Bug Hunter

Bug 1 — missing colon

age = 12
if age >= 10
    print("Welcome")

Bug 2 — endless loop

count = 1
while count <= 3:
    print(count)

Bug 3 — wrong range ending

for number in range(1, 5):
    print(number)

The programmer expects 1 through 5, but range() stops before its stop value.

Show Bug Hunter fixes

# Bug 1
if age >= 10:
    print("Welcome")

# Bug 2
count = 1
while count <= 3:
    print(count)
    count += 1

# Bug 3
for number in range(1, 6):
    print(number)

Optional deeper look: how does a for loop get values?

Python asks the iterable for an iterator, an object that supplies one item at a time. The loop repeatedly asks that iterator for its next value. A special internal signal named StopIteration tells the loop that no items remain. Intermediate Python studies this protocol directly.

Practice

Try these problems on this page. Before coding, write the decision or repetition in plain English.

Problems

Check whether a number is positive, negative, or zero.
Find the larger of two numbers without max().
Find the largest of three numbers.
Print numbers from 1 to 20.
Print all even numbers in a range.
Calculate the sum from 1 to n.
Check whether a year is a leap year. A year is a leap year when it is divisible by 400, or when it is divisible by 4 but not by 100.
Build a menu-driven calculator loop.
Print a multiplication table for a chosen number.
Check whether a number is prime.

Show hints

Compare the number with zero using if and elif.
Keep the larger value in a variable.
Compare each value with the current largest value.
Use range(1, 21).
A number is even when its remainder after division by 2 is zero.
Start total at zero and add each number.
Translate each part of the written leap-year rule into remainder comparisons, then join them with and and or.
Put the menu inside a loop and provide an exit choice.
Multiply the chosen number by values from 1 to 10.
Test divisors from 2 up to the number, then improve later.

Show solution ideas

Use if number > 0, elif number < 0, otherwise zero.
Start with largest = a; use an if block to replace it with b when b is greater.
Start with largest = a, then compare b and c.
for number in range(1, 21): print(number).
Add if number % 2 == 0 inside the loop.
Add every value in range(1, n + 1).
year % 400 == 0 or (year % 4 == 0 and year % 100 != 0).
Continue until the user chooses exit.
Use for multiplier in range(1, 11).
If any number from 2 through n - 1 divides evenly, it is not prime.

Homework

Build a menu-driven quiz with five questions, a score counter, and a final result. Include an option to exit before completing all questions.

Checkpoint

Write a countdown from ten to five, explain why a while loop terminates, and find the largest of three values without using max().