> ## Documentation Index
> Fetch the complete documentation index at: https://docs.hnuicpc.tech/llms.txt
> Use this file to discover all available pages before exploring further.

# Conditions and loops

> Tutorial for Sheet 2 — Master conditional statements, loops, and control flow in C++.

<iframe width="100%" height="400" src="https://www.youtube.com/embed/s_gZEr6bH6g" title="Level 0 Session 2 Recording" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen />

## Conditional statements

### If / else if / else

```cpp theme={null}
int x;
cin >> x;

if (x > 0) {
    cout << "Positive";
} else if (x < 0) {
    cout << "Negative";
} else {
    cout << "Zero";
}
```

### Ternary operator

A shorthand for simple if/else:

```cpp theme={null}
int a = 5, b = 3;
int mx = (a > b) ? a : b;  // mx = 5
```

### Switch statement

Useful when comparing one variable against multiple values:

```cpp theme={null}
char grade;
cin >> grade;

switch (grade) {
    case 'A': cout << "Excellent"; break;
    case 'B': cout << "Good"; break;
    case 'C': cout << "Average"; break;
    default: cout << "Below average";
}
```

## Loops

### For loop

```cpp theme={null}
// Print 1 to n
for (int i = 1; i <= n; i++) {
    cout << i << " ";
}

// Print n to 1
for (int i = n; i >= 1; i--) {
    cout << i << " ";
}

// Step by 2
for (int i = 0; i < n; i += 2) {
    cout << i << " ";
}
```

### While loop

```cpp theme={null}
// Read until end of input
int x;
while (cin >> x) {
    cout << x * 2 << "\n";
}

// Count digits
int n = 12345, digits = 0;
while (n > 0) {
    digits++;
    n /= 10;
}
```

### Do-while loop

Executes at least once:

```cpp theme={null}
int n;
do {
    cin >> n;
} while (n <= 0); // Keep reading until positive
```

## Nested loops

Essential for 2D problems, pattern printing, and brute force:

```cpp theme={null}
// Print a multiplication table
for (int i = 1; i <= n; i++) {
    for (int j = 1; j <= n; j++) {
        cout << i * j << "\t";
    }
    cout << "\n";
}
```

### Pattern example: right triangle

```cpp theme={null}
// *
// **
// ***
// ****
for (int i = 1; i <= n; i++) {
    for (int j = 1; j <= i; j++) {
        cout << "*";
    }
    cout << "\n";
}
```

## Break and continue

```cpp theme={null}
// Break: exit the loop early
for (int i = 1; i <= 100; i++) {
    if (i * i > n) break;
    cout << i << " ";
}

// Continue: skip to next iteration
for (int i = 1; i <= n; i++) {
    if (i % 3 == 0) continue; // Skip multiples of 3
    cout << i << " ";
}
```

## Common patterns with conditions and loops

### Check if a number is prime

```cpp theme={null}
bool isPrime(int n) {
    if (n < 2) return false;
    for (int i = 2; i * i <= n; i++) {
        if (n % i == 0) return false;
    }
    return true;
}
```

### Sum of digits

```cpp theme={null}
int sumDigits(int n) {
    int sum = 0;
    while (n > 0) {
        sum += n % 10;
        n /= 10;
    }
    return sum;
}
```

### GCD (Euclidean algorithm)

```cpp theme={null}
int gcd(int a, int b) {
    while (b != 0) {
        int temp = b;
        b = a % b;
        a = temp;
    }
    return a;
}
```

<Tip>
  Most brute-force solutions use nested loops. If the constraints are small (n ≤ 1000), an O(n²) solution with nested loops will work. If n ≤ 10⁷, you need O(n) or O(n log n).
</Tip>

## Practice

<Card title="Sheet 2: Conditions and Loops" icon="list-check" href="/level-0/sheets/sheet-2" horizontal>
  Practice problems for this tutorial.
</Card>