Loops and Conditions: for, while, if

Why This Topic Matters

So far we have learned how to store data with variables and group it with collections. But a program is not just data sitting there. It needs to make decisions and repeat actions. That is exactly what if, for and while do.

By the end of this post you will know:

1. How to make decisions in code with if, elif and else
2. How to repeat an action with for and while
3. How to control a loop with break, continue and the loop else
4. Which loop to pick for which situation

1. Conditions: if, elif, else

1.1 What is a Condition?

A condition is a statement that runs only when something is True.

Every condition is built on a comparison, something that evaluates to True or False (a bool, as we saw in the data types post).

Simple Example

age = 20

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

# Adult

Notice the indentation. Python uses spaces (4 is standard) to decide what belongs inside the if block. This is not just style, it is a rule. Wrong indentation causes an IndentationError.

1.2 Multiple Conditions with elif

Use elif ("else if") to check more than one condition in order:

marks = 75

if marks >= 90:
    grade = "A"
elif marks >= 75:
    grade = "B"
elif marks >= 60:
    grade = "C"
else:
    grade = "D"

print(grade)   # B

Python checks each condition from top to bottom and stops at the first one that is True. Since marks is 75, it does not even look at the marks >= 90 line result, it jumps straight to marks >= 75, which is true, sets the grade, and skips the rest.

1.3 Combining Conditions

Use and, or and not to combine more than one condition:

x = 10

print(x > 5 and x < 20)   # True   both must be true
print(x > 5 or x < 2)     # True   at least one must be true
print(not (x > 5))        # False  flips True to False

Comparison operators, quick reference

2. The for Loop

2.1 What is it?

A for loop goes through each item of a collection, one at a time.

Use a for loop when you know what you are looping over, a list, a range of numbers, or any other collection.

Simple Example

fruits = ["apple", "banana", "cherry"]

for fruit in fruits:
    print(fruit)

# apple
# banana
# cherry

On each round, the loop variable (fruit here) is set to the next item, until there are no items left.

2.2 Looping with range()

To repeat something a fixed number of times, use range(). It generates a sequence of numbers without you having to write them out.

for i in range(5):
    print(i, end=" ")
# 0 1 2 3 4

print()
for i in range(2, 10, 2):     # start, stop, step
    print(i, end=" ")
# 2 4 6 8

print()
for i in range(5, 0, -1):     # counting down
    print(i, end=" ")
# 5 4 3 2 1

range(5) starts at 0 and goes up to, but not including, 5. When you give three numbers, they mean start, stop and step, and a negative step counts down.

2.3 Getting the Position Too, with enumerate()

for i, fruit in enumerate(["a", "b", "c"]):
    print(i, fruit)

# 0 a
# 1 b
# 2 c

enumerate() gives you both the position and the item, so you do not need to manage a counter variable yourself.

2.4 A Running Total

total = 0
for n in [1, 2, 3, 4, 5]:
    total += n     # same as total = total + n

print(total)   # 15

3. The while Loop

3.1 What is it?

A while loop keeps repeating as long as a condition stays True.

Use a while loop when you do not know in advance how many times you need to repeat, only the condition that should stop it.

Simple Example

count = 0
while count < 5:
    print(count, end=" ")
    count += 1

# 0 1 2 3 4

Three things must be true for a while loop to behave correctly: a starting value (count = 0), a condition (count < 5), and something inside the loop that eventually makes the condition false (count += 1). Forget the last part, and the loop never stops, this is called an infinite loop.

3.2 Example: Factorial

n = 5
fact = 1
while n > 0:
    fact *= n
    n -= 1

print(fact)   # 120

3.3 while True and break

Sometimes it is cleaner to loop forever and use break to stop when you are ready:

i = 0
while True:
    if i >= 3:
        break
    print("loop", i)
    i += 1

# loop 0
# loop 1
# loop 2

4. Controlling a Loop: break, continue, else

4.1 break, Stop the Loop Completely

for i in range(10):
    if i == 5:
        break
    print(i, end=" ")

# 0 1 2 3 4

As soon as i reaches 5, the loop stops entirely, even though range(10) had more numbers left.

4.2 continue, Skip Just This Round

for i in range(5):
    if i % 2 == 0:
        continue
    print(i, end=" ")

# 1 3

continue skips the rest of the current round and jumps straight to the next one. Here, every even number gets skipped, so only the odd numbers get printed.

4.3 The Loop else, an Underused Feature

Both for and while loops can have an else block. It runs only if the loop finished normally, without hitting a break.

for i in range(3):
    print(i)
else:
    print("done")

# 0
# 1
# 2
# done

for i in range(10):
    if i == 3:
        break
    print(i)
else:
    print("not printed")

# 0
# 1
# 2
# (else does NOT run, because break happened)

This is useful for search-style logic: "did I find what I was looking for, or did I go through the whole list without success?"

tries = ["xyz", "123", "abc"]
password = "abc"
attempts = 0
idx = 0

while attempts < 3:
    entered = tries[idx]
    idx += 1
    attempts += 1
    if entered == password:
        print("Correct")
        break
else:
    print("Locked out")

# Correct

5. Nested Loops

A loop can sit inside another loop. The inner loop finishes completely for every single round of the outer loop.

for i in range(3):
    for j in range(3):
        print(i, j, end=" | ")

# 0 0 | 0 1 | 0 2 | 1 0 | 1 1 | 1 2 | 2 0 | 2 1 | 2 2 |

This pattern is common when working with grids, tables, or comparing every item in one list against every item in another.

6. for vs while: Which One to Use

A simple way to decide: if you can say "repeat this for every item in X", reach for for. If you can only say "keep repeating until X happens", reach for while.

7. A Shortcut: List Comprehension

Once you are comfortable with for and if, Python gives you a shorter way to build a list in one line:

nums = [1, 2, 3, 4, 5, 6]

evens = [n for n in nums if n % 2 == 0]
print(evens)          # [2, 4, 6]

squares = [n * n for n in range(5)]
print(squares)         # [0, 1, 4, 9, 16]

This does exactly the same thing as a normal loop that builds a list with .append(), but in one line. It is worth knowing how to read, even before you start writing your own.

8. Practical Examples

Example 1: Grading Students

students = [
    {"name": "A", "marks": 92},
    {"name": "B", "marks": 68},
    {"name": "C", "marks": 40}
]

for s in students:
    if s["marks"] >= 90:
        grade = "A"
    elif s["marks"] >= 60:
        grade = "B"
    else:
        grade = "Fail"
    print(f"{s['name']}: {grade}")

# A: A
# B: B
# C: Fail

Example 2: Sum of Even Numbers Only

total = 0
for n in range(1, 11):
    if n % 2 != 0:
        continue
    total += n

print(total)   # 30

Example 3: Simple Login Retry

correct_password = "python123"
attempts = 0
max_attempts = 3

while attempts < max_attempts:
    entered = input("Enter password: ")
    attempts += 1
    if entered == correct_password:
        print("Login successful")
        break
    print(f"Wrong password. {max_attempts - attempts} attempts left")
else:
    print("Account locked")

9. Common Mistakes

Quick Recap

if, for and while are the building blocks that make a program actually do something with the data it holds. Almost every program you write, from a small script to a full ML pipeline, is built on some combination of these three.

Practice Questions

1. What does range(3, 12, 3) produce?
2. What is the difference between break and continue?
3. Write the condition that checks if a number n is between 10 and 20, including both ends.
4. Why can a while True loop be dangerous if you forget the break?
5. Does the loop else run if the loop finishes normally, with no break?