Conditions in C++

C++ Conditional Statements Notes (From Simple to Complex)

1. If - Else

Basic if

if (x > 0) {
    // code
}

if-else

if (x > 0) {
    // positive
} else {
    // zero or negative
}

if-else if-else Ladder

if (x > 0) {
    // positive
} else if (x == 0) {
    // zero
} else {
    // negative
}

Nested if

if (x > 0) {
    if (x % 2 == 0) {
        // positive even
    } else {
        // positive odd
    }
}

2. Ternary Operator (?:) ⭐

int res = (x > 0) ? x : -x;  // absolute value

3. Switch-case (when value checking only) ⭐⭐

1. Integer constant

switch(3) {
    case 1: cout << "One"; break;
    case 2: cout << "Two"; break;
    default: cout << "Other";
}

2. Character constant

switch('z') {
    case 'a': cout << "Letter a"; break;
    case 'b': cout << "Letter b"; break;
    default: cout << "Other letter";
}

3. Enum constant

enum Meal { breakfast, lunch }; // breakfast = 0, lunch = 1
switch(breakfast) {
    case breakfast: cout << "B"; break;
    case 1: cout << "L"; break;
    default: cout << "Invalid";
}

Allowed in case

• Integer constants → case 1:, case 100:
• Character constants → case 'a':, case '\n':
• enum constants → case breakfast: (from enum Meal { breakfast, lunch, dinner };)
• constexpr / const values known at compile time

Not allowed - Variables

int x = 5;
switch(n) {
    case x:   // ❌ ERROR: x is a variable (runtime value)
        ...
}

4. Use Case & Safe Coding Practices

Logical Conditions

if (x > 0 && y > 0) {
    // both positive
}

if (x == 0 || y == 0) {
    // at least one is zero
}

Use with auto, const, or references

for (const auto &x : vec) {
    if (x > 0) ...
}

Short-circuiting ⭐

if (x != 0 && y / x > 1) {
    // safe, prevents division by zero
}

• Prevents unnecessary computation and runtime errors.
• Stops evaluating further conditions if the first one is false.
• Useful in DSA for safe operations like division, null-pointer checks, or bounds checking.

Avoiding dangerous access (extreme cases)

if (!st.empty() && st.top() < x) {
    // Safe access to st.top()
}

• Ensures container is not empty before accessing its elements.
• Prevents runtime errors like segmentation faults or undefined behavior.
• Common in stacks, queues, vectors, and other containers in DSA.

Guard Clauses

if (vec.empty()) return;

if (i < 0 || i >= vec.size()) return;

• Immediately exits the function when invalid or edge-case conditions are met.
• Keeps the main logic clean and unindented.
• Prevents runtime errors like out-of-bounds access.
• Useful in DSA for vectors, arrays, stacks, or recursive functions.

Common Pitfalls

• Using = instead of ==

  if (x = 0) // ❌ assigns 0 to x, condition always false

• Accessing container without check

  vec[0] // ❌ unsafe if vec.size() == 0

Note: Always verify conditions and container size before operations.

Summary Table:

Type	Usage	Example
Simple if	One condition	if (x > 0)
if-else	Two paths	if (x) else
Ladder	Multiple conditions	if-else if-else
Ternary	Short inline condition	int x = a > b ? a : b;
Nested if	Condition inside another	if (x) { if (y) ... }
Logical ops	Combine conditions	if (x && y)
Short-circuit	Prevent runtime errors	if (ptr && ptr->val)
Guard clause	Early exit on invalid conditions	if (!ok) return;

Use conditionals carefully to avoid undefined behavior and keep logic readable.