6. Conditionals & Loops

💡 Why this matters for DSA Every algorithm uses conditionals and loops. Most DSA solutions are a loop body with a few if-conditions inside.

6.1 if / elif / else

python

x = 10

if x > 0:
    print("positive")
elif x == 0:
    print("zero")
else:
    print("negative")

Ternary (inline if-else)

python

result = "even" if x % 2 == 0 else "odd"

✅ Tip — Common DSA conditional patterns:

python

# Bounds check
if 0 <= i < len(arr):   # chained comparison

# None guard
if result is not None:

# Truthy check (prefer explicit for clarity)
if arr:         # True if non-empty list
if not arr:     # True if empty list

6.2 for loop

python

# Over a range
for i in range(5):           # 0, 1, 2, 3, 4
for i in range(2, 7):        # 2, 3, 4, 5, 6
for i in range(0, 10, 2):    # 0, 2, 4, 6, 8
for i in range(5, 0, -1):    # 5, 4, 3, 2, 1

# Over a list
for x in [1, 2, 3]:
for ch in "hello":
for row in matrix:

range() details

Call	Result
`range(5)`	0 to 4
`range(1, 5)`	1 to 4
`range(1, 10, 2)`	1, 3, 5, 7, 9
`range(5, 0, -1)`	5, 4, 3, 2, 1

6.3 enumerate() — index + value together

python

arr = ["a", "b", "c"]

for i, val in enumerate(arr):          # i=0,val='a'; i=1,val='b'; ...
for i, val in enumerate(arr, start=1):  # start index at 1

✅ Tip — Essential for DSA: Whenever you need both index and element, use enumerate() — cleaner than range(len(arr)).

6.4 zip() — iterate multiple lists together

python

names = ["a", "b", "c"]
scores = [85, 92, 78]

for name, score in zip(names, scores):
    print(f"{name}: {score}")

# Unzip
pairs = [(1, 'a'), (2, 'b'), (3, 'c')]
nums, letters = zip(*pairs)

# Zip to dict
keys = ["name", "age"]
vals = ["Manik", 25]
d = dict(zip(keys, vals))  # {"name": "Manik", "age": 25}

6.5 while loop

python

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

When to use while vs for

Scenario	Use
Known number of iterations	`for`
Until a condition changes	`while`
Binary search	`while left <= right`
Two-pointer technique	`while left < right`

✅ Tip — Common DSA while patterns:

python

# Two pointers
left, right = 0, len(arr) - 1
while left < right:
    # ...
    left += 1
    right -= 1

# Binary search
while left <= right:
    mid = (left + right) // 2
    if arr[mid] == target: break

# BFS/DFS with queue/stack
while queue:
    node = queue.popleft()

6.6 break, continue, pass

python

# break — exit loop immediately
for x in arr:
    if x == target:
        break

# continue — skip to next iteration
for x in arr:
    if x % 2 == 0:
        continue
    print(x)        # only odd numbers

# pass — do nothing (placeholder)
if condition:
    pass  # TODO: handle later

Practice Drill

python

# 1. Print all even numbers from 1 to 20
# 2. Given a list, find the first negative number (use break)
# 3. Count vowels in a string (using for loop)
# 4. Print a multiplication table (nested loops)

💡 Click for Solutions

python

# 1. Even numbers
for i in range(2, 21, 2):
    print(i)

# 2. First negative number
arr = [3, 7, -1, 5, -8]
for x in arr:
    if x < 0:
        print(f"First negative: {x}")
        break

# 3. Count vowels
s = "hello world"
vowels = "aeiou"
count = 0
for ch in s:
    if ch in vowels:
        count += 1
print(count)  # 3

# 4. Multiplication table
for i in range(1, 11):
    for j in range(1, 11):
        print(f"{i*j:4}", end="")
    print()

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