Loops (for, while, do-while) and Patterns

What Is It?

What Are Loops?

A loop is a control flow structure that repeats a block of code multiple times until a specified condition becomes false. Instead of writing the same statement 100 times, you write it once inside a loop and let the loop handle the repetition.

Consider printing numbers 1 to 1000. Without loops, you need 1000 cout statements. With a loop, you need 3 lines. Loops are one of the three fundamental building blocks of all programs, alongside sequences and conditionals.

C++ provides the following loop constructs:

1. for loop -- when you know the number of iterations in advance
2. while loop -- when you want to repeat while a condition is true (condition checked before each iteration)
3. do-while loop -- like while, but the body executes at least once (condition checked after each iteration)
4. Range-based for loop (C++11) -- for iterating over arrays and containers without managing an index

Why Does It Matter?

Why Are Loops Essential?

1. Eliminating Repetition

Any task involving repetition requires loops. Printing a multiplication table, processing each element of an array, reading lines from a file, handling multiple requests in a server -- all are impossible without loops. They are the foundation of automation in programming.

2. Processing Collections of Data

Real programs work with collections: arrays, vectors, database rows, API responses. Loops let you iterate over each element, apply transformations, filter results, compute aggregates (sum, average, min, max), and search for specific values. Without loops, you cannot work with data of variable size.

3. Algorithm Implementation

Almost every algorithm uses loops. Searching (linear search, binary search), sorting (bubble sort, selection sort), matrix operations, string processing, mathematical computations (factorial, Fibonacci, prime checking) -- all require loops. For competitive programming and placements, loop mastery is non-negotiable.

4. Pattern Printing

Pattern printing problems are a staple of coding interviews and college exams. They train you to think systematically about nested loops: which row you are on, how many spaces to print, how many stars to print. Mastering patterns means mastering nested loop control flow.

Detailed Explanation

Detailed Explanation

1. The for Loop

The most commonly used loop when you know (or can calculate) the number of iterations. It has three parts in the header: initialization, condition, and update.

for (initialization; condition; update) {
    // body
}

Execution flow:

1. Initialization executes once, at the very beginning.
2. Condition is checked before each iteration. If true, the body executes. If false, the loop ends.
3. Update executes after each iteration of the body.

for (int i = 1; i <= 5; i++) {
    cout << i << " ";
}
// Output: 1 2 3 4 5

Variations of for Loop

• Counting down: for (int i = 10; i >= 1; i--)
• Step by 2: for (int i = 0; i <= 20; i += 2)
• Multiple variables: for (int i = 0, j = 10; i < j; i++, j--)
• Empty parts: Any of the three parts can be omitted: for (;;) is an infinite loop.

2. The while Loop

The while loop repeats as long as its condition is true. The condition is checked before each iteration, so if the condition is false from the start, the body never executes.

while (condition) {
    // body
}

int count = 1;
while (count <= 5) {
    cout << count << " ";
    count++;
}
// Output: 1 2 3 4 5

If you forget count++, the condition is always true, causing an infinite loop.

3. The do-while Loop

The condition is checked after the body executes, guaranteeing at least one execution.

do {
    // body
} while (condition);

Note the semicolon after the while condition. It is required.

int num = 10;
do {
    cout << num << " ";
    num++;
} while (num <= 5);
// Output: 10

Even though 10 <= 5 is false, the body runs once and prints 10.

4. Range-based for Loop (C++11)

A cleaner way to iterate over arrays and containers:

int arr[] = {10, 20, 30, 40, 50};
for (auto x : arr) {
    cout << x << " ";
}
// Output: 10 20 30 40 50

The variable x takes each element's value in order. To modify elements, use a reference: for (auto& x : arr). The auto keyword lets the compiler deduce the type.

5. break and continue

break exits the loop entirely. continue skips the rest of the current iteration and moves to the next.

for (int i = 1; i <= 10; i++) {
    if (i == 5) break;
    cout << i << " ";
}
// Output: 1 2 3 4

for (int i = 1; i <= 10; i++) {
    if (i % 3 == 0) continue;
    cout << i << " ";
}
// Output: 1 2 4 5 7 8 10

In nested loops, break and continue apply only to the innermost enclosing loop.

6. Nested Loops

A loop inside another loop. The inner loop completes all its iterations for each single iteration of the outer loop. If the outer loop runs N times and the inner runs M times, the innermost body executes N * M times.

for (int i = 1; i <= 3; i++) {
    for (int j = 1; j <= 4; j++) {
        cout << "* ";
    }
    cout << endl;
}
// 3 rows, 4 stars each

7. Pattern Printing: The Row-Column Approach

Every pattern can be analyzed using the row-column approach:

1. Identify the number of rows (outer loop variable i).
2. For each row, determine: how many spaces, how many stars/numbers/characters.
3. Express counts as functions of i and the total rows n.

Right-Aligned Triangle

int n = 5;
for (int i = 1; i <= n; i++) {
    for (int j = 1; j <= i; j++) {
        cout << "* ";
    }
    cout << endl;
}

Row 1: 1 star. Row 2: 2 stars. Row i: i stars.

Inverted Triangle

for (int i = n; i >= 1; i--) {
    for (int j = 1; j <= i; j++) {
        cout << "* ";
    }
    cout << endl;
}

Pyramid (Centered Triangle)

for (int i = 1; i <= n; i++) {
    for (int j = 1; j <= n - i; j++) cout << " ";   // spaces
    for (int j = 1; j <= 2 * i - 1; j++) cout << "*"; // stars
    cout << endl;
}

Row i has (n - i) spaces followed by (2*i - 1) stars.

Diamond

A diamond is an upper pyramid (rows 1 to n) followed by an inverted pyramid (rows n-1 to 1).

Floyd's Triangle

int num = 1;
for (int i = 1; i <= n; i++) {
    for (int j = 1; j <= i; j++) {
        cout << num++ << " ";
    }
    cout << endl;
}

Prints consecutive numbers: 1 / 2 3 / 4 5 6 / ...

Pascal's Triangle

for (int i = 0; i < n; i++) {
    int val = 1;
    for (int j = 0; j <= i; j++) {
        cout << val << " ";
        val = val * (i - j) / (j + 1);
    }
    cout << endl;
}

Each element is computed using the combinatorial formula: C(i, j) = C(i, j-1) * (i - j + 1) / j.

8. Loop Optimization Tips

• Avoid recomputing in the condition: for (int i = 0; i < strlen(s); i++) calls strlen every iteration (O(n) each call, making the loop O(n^2)). Compute the length once before the loop.
• Use prefix increment: ++i is generally at least as fast as i++ (for iterators, prefix avoids creating a temporary copy).
• Break early: If searching for an element, break as soon as you find it instead of scanning the entire array.
• Minimize work inside the loop: Move invariant computations outside the loop body.

Code Examples

#include <iostream>
using namespace std;

int main() {
    int num;
    cout << "Enter a number: ";
    cin >> num;

    for (int i = 1; i <= 10; i++) {
        cout << num << " x " << i << " = " << num * i << endl;
    }

    return 0;
}

#include <iostream>
using namespace std;

int main() {
    int n;
    cout << "Enter a number: ";
    cin >> n;

    int sum = 0;
    int temp = n;
    while (temp > 0) {
        sum += temp % 10;
        temp /= 10;
    }

    cout << "Sum of digits of " << n << ": " << sum << endl;
    return 0;
}

#include <iostream>
using namespace std;

int main() {
    int choice;

    do {
        cout << "\n--- Menu ---" << endl;
        cout << "1. Say Hello" << endl;
        cout << "2. Say Goodbye" << endl;
        cout << "3. Exit" << endl;
        cout << "Choice: ";
        cin >> choice;

        switch (choice) {
            case 1: cout << "Hello!" << endl; break;
            case 2: cout << "Goodbye!" << endl; break;
            case 3: cout << "Exiting..." << endl; break;
            default: cout << "Invalid choice." << endl;
        }
    } while (choice != 3);

    return 0;
}

#include <iostream>
using namespace std;

int main() {
    int marks[] = {85, 92, 78, 95, 88};

    int total = 0;
    for (auto m : marks) {
        total += m;
    }

    cout << "Total: " << total << endl;
    cout << "Average: " << (double)total / 5 << endl;

    // Modify using reference
    for (auto& m : marks) {
        m += 5;  // Add 5 grace marks
    }

    cout << "After grace marks: ";
    for (auto m : marks) {
        cout << m << " ";
    }
    cout << endl;

    return 0;
}

#include <iostream>
using namespace std;

int main() {
    int n = 5;

    // Right triangle
    cout << "Right Triangle:" << endl;
    for (int i = 1; i <= n; i++) {
        for (int j = 1; j <= i; j++) {
            cout << "* ";
        }
        cout << endl;
    }

    cout << endl;

    // Pyramid
    cout << "Pyramid:" << endl;
    for (int i = 1; i <= n; i++) {
        for (int j = 1; j <= n - i; j++) {
            cout << " ";
        }
        for (int j = 1; j <= 2 * i - 1; j++) {
            cout << "*";
        }
        cout << endl;
    }

    return 0;
}

#include <iostream>
using namespace std;

int main() {
    int n = 5;

    // Floyd's Triangle
    cout << "Floyd's Triangle:" << endl;
    int num = 1;
    for (int i = 1; i <= n; i++) {
        for (int j = 1; j <= i; j++) {
            cout << num++ << " ";
        }
        cout << endl;
    }

    cout << endl;

    // Pascal's Triangle
    cout << "Pascal's Triangle:" << endl;
    for (int i = 0; i < n; i++) {
        int val = 1;
        for (int j = 0; j <= i; j++) {
            cout << val << " ";
            val = val * (i - j) / (j + 1);
        }
        cout << endl;
    }

    return 0;
}

#include <iostream>
using namespace std;

int main() {
    int n = 5;

    // Upper half (rows 1 to n)
    for (int i = 1; i <= n; i++) {
        for (int j = 1; j <= n - i; j++) cout << " ";
        for (int j = 1; j <= 2 * i - 1; j++) cout << "*";
        cout << endl;
    }

    // Lower half (rows n-1 to 1)
    for (int i = n - 1; i >= 1; i--) {
        for (int j = 1; j <= n - i; j++) cout << " ";
        for (int j = 1; j <= 2 * i - 1; j++) cout << "*";
        cout << endl;
    }

    return 0;
}

#include <iostream>
using namespace std;

int main() {
    int n = 5;
    for (int i = 1; i <= n; i++) {
        char ch = 'A';
        for (int j = 1; j <= i; j++) {
            cout << ch++ << " ";
        }
        cout << endl;
    }

    return 0;
}

Common Mistakes

// Intended: print 1 to 5
for (int i = 1; i < 5; i++) {
    cout << i << " ";
}
// Output: 1 2 3 4 (misses 5)

for (int i = 1; i <= 5; i++) {
    cout << i << " ";
}

int i = 1;
while (i <= 5) {
    cout << i << endl;
    // Forgot i++;
}

int i = 1;
while (i <= 5) {
    cout << i << endl;
    i++;
}

for (int i = 0; i < 5; i++); {
    cout << i << " ";
}
// Compilation error: i is not declared in this scope

for (int i = 0; i < 5; i++) {
    cout << i << " ";
}

int arr[] = {1, 2, 3, 4, 5};
for (auto x : arr) {
    x = x * 2;  // Does NOT modify the array!
}
// arr is still {1, 2, 3, 4, 5}

int arr[] = {1, 2, 3, 4, 5};
for (auto& x : arr) {
    x = x * 2;  // Modifies the array
}
// arr is now {2, 4, 6, 8, 10}

for (int i = 1; i <= 5; i++) {
    for (int j = 1; j <= i; i++) {  // Bug: i++ instead of j++
        cout << "* ";
    }
    cout << endl;
}

for (int i = 1; i <= 5; i++) {
    for (int j = 1; j <= i; j++) {
        cout << "* ";
    }
    cout << endl;
}

char s[] = "hello";
for (int i = 0; i < strlen(s); i++) {
    cout << s[i];
}
// Works but is O(n^2) because strlen is O(n) and called every iteration

char s[] = "hello";
int len = strlen(s);
for (int i = 0; i < len; i++) {
    cout << s[i];
}

Summary