Motivating example: point quadrant

Given nonzero numbers x and y, determine the quadrant of the point (x, y) on the Cartesian plane.

x = int(input())
y = int(input())

if x > 0:
    if y > 0:               # x > 0, y > 0
        print("First quadrant")
    else:                   # x > 0, y < 0
        print("Fourth quadrant")
else:
    if y > 0:               # x < 0, y > 0
        print("Second quadrant")
    else:                   # x < 0, y < 0
        print("Third quadrant")

Note: A comment starts with # and continues to the end of the line. Python ignores comments.

Why (and when) to nest

• Use nesting when the second decision depends on the first being true (or false).
• Keep nesting shallow—deep nesting is harder to read. Often you can flatten with elif or combine conditions.

Flattening nested conditionals

Same logic, but clearer with elif and combined conditions:

x = int(input())
y = int(input())

if  x > 0 and y > 0:
    print("First quadrant")
elif x < 0 and y > 0:
    print("Second quadrant")
elif x < 0 and y < 0:
    print("Third quadrant")
else:  # x > 0 and y < 0
    print("Fourth quadrant")

Guard clauses (early exits)

Handle exceptional or edge cases first, then proceed. This reduces nesting.

x = int(input())
y = int(input())

# Guard: ensure neither coordinate is zero (outside quadrant definition)
if x == 0 or y == 0:
    print("Point lies on an axis, no quadrant")
else:
    if  x > 0 and y > 0:
        print("First quadrant")
    elif x < 0 and y > 0:
        print("Second quadrant")
    elif x < 0 and y < 0:
        print("Third quadrant")
    else:
        print("Fourth quadrant")

Indentation rules recap

• Each nested block increases indentation (recommended: 4 spaces per level).
• All lines in the same block must align exactly.
• Do not mix tabs and spaces (configure your editor to insert spaces on Tab).

Alternative patterns

1) Mapping from signs to labels

Compute the sign pair once, then look up the result. Great for avoiding branching repetition.

x = int(input())
y = int(input())

if x == 0 or y == 0:
    print("On axis")
else:
    sign = (1 if x > 0 else -1, 1 if y > 0 else -1)
    quadrant = {
        ( 1,  1): "First quadrant",
        (-1,  1): "Second quadrant",
        (-1, -1): "Third quadrant",
        ( 1, -1): "Fourth quadrant",
    }[sign]
    print(quadrant)

2) Conditional expression for tiny branches

Prefer only for very short alternatives—don’t cram complex logic into one line.

n = int(input())
parity = "even" if n % 2 == 0 else "odd"
print(parity)

Common mistakes

• Missing colons after if/elif/else.
• Inconsistent indentation (e.g., 4 spaces on one line, 2 on the next).
• Over-nesting when elif or combined conditions would be clearer.
• Unreachable branches (e.g., an elif that can never be true given previous checks).

Quick practice

1. Read an integer temperature t. Print cold if t < 0, warm if 0 ≤ t < 25, else hot. (Start with nested ifs, then flatten to elif.)
2. Given three integers, print the median (the middle value). Try a nested approach first.
3. Given x and y, print axis if the point lies on an axis, else the quadrant name.

# 1) Temperature with elif
t = int(input())
if t < 0:
    print("cold")
elif t < 25:
    print("warm")
else:
    print("hot")

# 2) Median of three (nested approach)
a = int(input()); b = int(input()); c = int(input())
if a <= b:
    if b <= c:
        print(b)       # a <= b <= c
    elif a <= c:
        print(c)       # a <= c < b
    else:
        print(a)       # c < a < b
else:
    if a <= c:
        print(a)       # b <= a <= c
    elif b <= c:
        print(c)       # b <= c < a
    else:
        print(b)       # c < b < a

# 3) Axis vs quadrant
x = int(input()); y = int(input())
if x == 0 or y == 0:
    print("axis")
elif x > 0 and y > 0:
    print("First quadrant")
elif x < 0 and y > 0:
    print("Second quadrant")
elif x < 0 and y < 0:
    print("Third quadrant")
else:
    print("Fourth quadrant")