Nested Conditions

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

<h3 style="margin:12px 0 6px; font-size:1.05rem;">1) Mapping from signs to labels</h3>
<p>Compute the sign pair once, then look up the result. Great for avoiding branching repetition.</p>
<pre style="background:#f6f8fa; padding:12px; border-radius:8px; overflow:auto;"><code class="language-python">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)

<h3 style="margin:12px 0 6px; font-size:1.05rem;">2) Conditional expression for tiny branches</h3>
<p>Prefer only for very short alternatives—don’t cram complex logic into one line.</p>
<pre style="background:#f6f8fa; padding:12px; border-radius:8px; overflow:auto;"><code class="language-python">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")