C++ Loops Interview Questions

All Types of Loops: for, while, do-while, nested loops

What are the three main types of loops in C++?
C++ provides three main loop constructs:
Loop TypeSyntaxWhen to Use
for loopfor(init; condition; increment)Known number of iterations
while loopwhile(condition)Unknown iterations, condition check first
do-while loopdo {...} while(condition);Execute at least once, condition check last
What is the basic syntax and flow of a for loop?
// for loop syntax
for (initialization; condition; increment/decrement) {
  // Loop body - executes while condition is true
}

// Example: Print numbers 1 to 5
for (int i = 1; i <= 5; i++) {
  cout << i << " ";
}
// Output: 1 2 3 4 5
Flow: Initialization → Condition check → Body execution → Increment → Repeat
What is the difference between while and do-while loops?
Key difference: When condition is checked.
// while - condition check FIRST
int count = 5;
while (count < 5) {
  cout << count << " ";
  count++;
}
// No output - condition false initially

// do-while - condition check LAST
count = 5;
do {
  cout << count << " ";
  count++;
} while (count < 5);
// Output: 5 - executes at least once
What are infinite loops and how to create/avoid them?
Infinite loops run forever (until program is terminated):
// Common infinite loop patterns
// 1. Missing increment
for (int i = 0; i < 10; ) { // i never increments
  cout << "Infinite!";
}

// 2. Condition always true
while (true) {
  cout << "Running forever";
}

// 3. No condition change
int x = 5;
while (x > 0) {
  cout << x; // x never changes
}
Use break or ensure condition eventually becomes false.
What are nested loops and provide an example?
Nested loops: loop inside another loop:
// Print multiplication table (nested for loops)
for (int i = 1; i <= 10; i++) { // Outer loop
  for (int j = 1; j <= 10; j++) { // Inner loop
    cout << i * j << "\t";
  }
  cout << endl;
}

// Total iterations: outer * inner = 10 * 10 = 100
What are loop control statements: break, continue, goto?
// break - exit loop immediately
for (int i = 1; i <= 10; i++) {
  if (i == 5) {
    break; // Exit loop when i == 5
  }
  cout << i << " ";
}
// Output: 1 2 3 4

// continue - skip current iteration
for (int i = 1; i <= 5; i++) {
  if (i == 3) {
    continue; // Skip iteration when i == 3
  }
  cout << i << " ";
}
// Output: 1 2 4 5

// goto - jump to label (avoid when possible)
int i = 0;
start:
  cout << i++ << " ";
  if (i < 5) goto start;
When to use for vs while vs do-while loops?
Loop TypeBest Use CaseExample
forKnown number of iterationsArray traversal, fixed count
whileCondition-based, may be zero iterationsReading until EOF, user input validation
do-whileMust execute at least onceMenu-driven programs, retry logic
What is the range-based for loop in C++11?
Range-based for loop (C++11) simplifies iteration over containers:
// Traditional for loop
int arr[] = {1, 2, 3, 4, 5};
for (int i = 0; i < 5; i++) {
  cout << arr[i] << " ";
}

// Range-based for loop (C++11)
for (int num : arr) {
  cout << num << " ";
}

// With auto keyword
for (auto num : arr) {
  cout << num << " ";
}

// Reference to modify elements
for (int &num : arr) {
  num *= 2; // Double each element
}
How to calculate factorial using different loops?
// Factorial using for loop
int n = 5, factorial = 1;
for (int i = 1; i <= n; i++) {
  factorial *= i;
}
cout << "Factorial of " << n << " = " << factorial;

// Factorial using while loop
factorial = 1;
int i = 1;
while (i <= n) {
  factorial *= i;
  i++;
}

// Factorial using do-while loop
factorial = 1;
i = 1;
do {
  factorial *= i;
  i++;
} while (i <= n);
What are common loop errors and how to avoid them?
Common errors:
// 1. Off-by-one errors
// Wrong: Starts at 0, ends at 10 (11 iterations)
for (int i = 0; i <= 10; i++) {
  cout << i << " ";
}

// Correct: 10 iterations (0-9 or 1-10)
for (int i = 1; i <= 10; i++) {
  cout << i << " ";
}

// 2. Modifying loop variable in body
for (int i = 0; i < 10; i++) {
  cout << i << " ";
  i++; // Double increment!
}

// 3. Floating point counters
// Avoid: May cause infinite loops due to precision
for (double d = 0.0; d != 1.0; d += 0.1) {
  cout << d << " ";
}
How to implement Fibonacci series using loops?
// Fibonacci series: 0, 1, 1, 2, 3, 5, 8, 13...
int n = 10;
int t1 = 0, t2 = 1, nextTerm;

cout << "Fibonacci Series: " << t1 << ", " << t2;

// Using for loop
for (int i = 3; i <= n; i++) {
  nextTerm = t1 + t2;
  cout << ", " << nextTerm;
  t1 = t2;
  t2 = nextTerm;
}

// Using while loop
t1 = 0; t2 = 1;
int count = 2;
cout << "\nFibonacci: " << t1 << ", " << t2;
while (count < n) {
  nextTerm = t1 + t2;
  cout << ", " << nextTerm;
  t1 = t2;
  t2 = nextTerm;
  count++;
}
What is loop optimization and best practices?
Optimization techniques:
// 1. Move invariant code out of loop
// Bad: Calculation repeated each iteration
for (int i = 0; i < n; i++) {
  int result = expensiveCalculation() * i;
}

// Good: Calculate once outside loop
int base = expensiveCalculation();
for (int i = 0; i < n; i++) {
  int result = base * i;
}

// 2. Use prefix increment (++i) vs postfix (i++)
// ++i is generally more efficient for objects
for (int i = 0; i < n; ++i) { // Use prefix
  // loop body
}

// 3. Minimize loop overhead
// Bad: Function call in condition
for (int i = 0; i < getSize(); i++) {
  // getSize() called each iteration
}

// Good: Cache value
int size = getSize();
for (int i = 0; i < size; i++) {
  // loop body
}
How to print patterns using nested loops?
// Right triangle pattern
int rows = 5;
for (int i = 1; i <= rows; i++) {
  for (int j = 1; j <= i; j++) {
    cout << "* ";
  }
  cout << endl;
}
/* Output:
*
* *
* * *
* * * *
* * * * *

// Pyramid pattern
for (int i = 1; i <= rows; i++) {
  // Print spaces
  for (int space = 1; space <= rows - i; space++) {
    cout << " ";
  }
  // Print stars
  for (int j = 1; j <= 2*i-1; j++) {
    cout << "* ";
  }
  cout << endl;
}
What are loop variables and their scope?
// Loop variable declared inside for - limited scope
for (int i = 0; i < 5; i++) {
  cout << i << " ";
}
// i is NOT accessible here (out of scope)

// Loop variable declared outside - broader scope
int j;
for (j = 0; j < 5; j++) {
  cout << j << " ";
}
// j IS accessible here (scope: whole function)
cout << "\nFinal j = " << j;

// C++17: Initialization in condition
for (int i = 0, j = 10; i < j; i++, j--) {
  cout << i << " " << j << endl;
}
How to implement prime number check using loops?
// Check if number is prime
int num;
cout << "Enter a number: ";
cin >> num;

bool isPrime = true;

if (num <= 1) {
  isPrime = false;
} else {
  // Check divisors from 2 to sqrt(num)
  for (int i = 2; i * i <= num; i++) {
    if (num % i == 0) {
      isPrime = false;
      break; // No need to check further
    }
  }
}

if (isPrime) {
  cout << num << " is prime";
} else {
  cout << num << " is not prime";
}
What is the difference between entry-controlled and exit-controlled loops?
TypeLoopsCondition CheckMinimum Executions
Entry-controlledfor, whileBefore loop body0 (may not execute at all)
Exit-controlleddo-whileAfter loop body1 (always executes at least once)
// Entry-controlled: condition checked FIRST
int x = 10;
while (x < 5) { // Condition false initially
  cout << "This won't execute";
}

// Exit-controlled: condition checked LAST
do {
  cout << "This executes once";
} while (x < 5); // Condition false, but body ran once
How to implement bubble sort using nested loops?
// Bubble sort algorithm
int arr[] = {64, 34, 25, 12, 22, 11, 90};
int n = sizeof(arr)/sizeof(arr[0]);

// Outer loop: passes through array
for (int i = 0; i < n-1; i++) {
  // Inner loop: compare adjacent elements
  for (int j = 0; j < n-i-1; j++) {
    // Swap if in wrong order
    if (arr[j] > arr[j+1]) {
      int temp = arr[j];
      arr[j] = arr[j+1];
      arr[j+1] = temp;
    }
  }
}

// Print sorted array
cout << "Sorted array: ";
for (int i = 0; i < n; i++) {
  cout << arr[i] << " ";
}
What are loop variants and invariants?
// Loop invariant: true before and after each iteration
int sum = 0;
// Invariant: sum = sum of processed elements
for (int i = 1; i <= n; i++) {
  // Before iteration: sum = sum of 1..(i-1)
  sum += i;
  // After iteration: sum = sum of 1..i
}

// Loop variant: changes each iteration, ensures termination
for (int i = n; i > 0; i--) {
  // Variant: i decreases by 1 each iteration
  // Ensures loop terminates when i == 0
  cout << i << " ";
}
Loop invariants help prove correctness, variants help prove termination.
How to handle user input validation with loops?
// Input validation with do-while loop
int age;
bool validInput = false;

do {
  cout << "Enter age (0-120): ";
  cin >> age;

  if (cin.fail()) {
    cout << "Invalid input! Please enter a number." << endl;
    cin.clear();
    cin.ignore(1000, '\n');
  } else if (age < 0 || age > 120) {
    cout << "Age must be between 0 and 120!" << endl;
  } else {
    validInput = true;
  }
} while (!validInput);

cout << "Valid age entered: " << age;
What are the performance considerations for nested loops?
// Time complexity examples
// O(n) - Single loop
for (int i = 0; i < n; i++) {
  // n iterations
}

// O(n²) - Nested loops (worst case)
for (int i = 0; i < n; i++) {
  for (int j = 0; j < n; j++) {
    // n * n = n² iterations
  }
}

// O(n³) - Triple nested loops
for (int i = 0; i < n; i++) {
  for (int j = 0; j < n; j++) {
    for (int k = 0; k < n; k++) {
      // n * n * n = n³ iterations
    }
  }
}

// Optimization: Reduce nesting when possible
// Break early when condition met
// Use more efficient algorithms
Loop Selection Guidelines:
  1. Use for loop when you know exact number of iterations
  2. Use while loop when iterations depend on condition, may be zero
  3. Use do-while loop when you need at least one execution
  4. Avoid deep nesting (more than 2-3 levels) for readability
  5. Use break/continue judiciously - can make code harder to follow
  6. Consider range-based for for container iteration (C++11+)
  7. Always test edge cases: empty loop, single iteration, maximum iterations
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