Control Flow and Loops

Most DSA solutions are built from a small set of control-flow patterns.

Conditions

value = 7

if value % 2 == 0:
    print("even")
elif value % 3 == 0:
    print("divisible by 3")
else:
    print("other")

for loops

Use for when you know the range or want to iterate directly over values.

nums = [4, 8, 15, 16]

for x in nums:
    print("value:", x)

for i in range(len(nums)):
    print("index:", i, "value:", nums[i])

while loops

Use while when the loop condition changes dynamically, such as with binary search or fast/slow pointers.

left = 0
right = 5

while left < right:
    mid = (left + right) // 2
    print(left, mid, right)
    left += 1

Loop helpers

enumerate and zip keep code cleaner than manual indexing.

letters = ["a", "b", "c"]
scores = [10, 20, 30]

for i, ch in enumerate(letters):
    print(i, ch)

for ch, score in zip(letters, scores):
    print(ch, score)

Pattern intuition

• Traversal of an array: for i in range(n)
• Two pointers: while left < right
• Repeated processing until empty: while stack
• BFS queue processing: while queue