Counted Iteration (for)

For Loop — Computing Array Sum — Watch how the for loop visits each array element exactly once, accumulating a running sum. The loop counter i moves left to right through the array.

#include <iostream>
using namespace std;

int main() {
    // === Basic for loop: print 0 to 4 ===
    cout << "Basic loop: ";
    for (int i = 0; i < 5; i++) {
        cout << i << " ";
    }
    cout << endl;  // Output: 0 1 2 3 4
    
    // === Sum of array elements ===
    int arr[] = {3, 7, 2, 8, 5};
    int n = 5;
    int sum = 0;
    for (int i = 0; i < n; i++) {
        sum += arr[i];
    }
    cout << "Sum: " << sum << endl;  // Output: 25
    
    // === Reverse traversal ===
    cout << "Reversed: ";
    for (int i = n - 1; i >= 0; i--) {
        cout << arr[i] << " ";
    }
    cout << endl;  // Output: 5 8 2 7 3
    
    // === Step by 2 (every other element) ===
    cout << "Every other: ";
    for (int i = 0; i < n; i += 2) {
        cout << arr[i] << " ";
    }
    cout << endl;  // Output: 3 2 5
    
    // === Range-based for loop (C++11) ===
    cout << "Range-based: ";
    for (int val : arr) {
        cout << val << " ";
    }
    cout << endl;  // Output: 3 7 2 8 5
    
    return 0;
}

# === Basic for loop: print 0 to 4 ===
print("Basic loop:", end=" ")
for i in range(5):  # range(5) generates 0, 1, 2, 3, 4
    print(i, end=" ")
print()  # Output: 0 1 2 3 4

# === Sum of array elements ===
arr = [3, 7, 2, 8, 5]
total = 0
for i in range(len(arr)):
    total += arr[i]
print(f"Sum: {total}")  # Output: 25

# === Pythonic: iterate over values directly ===
total2 = 0
for val in arr:  # No index needed!
    total2 += val
print(f"Sum (pythonic): {total2}")  # Output: 25

# === Reverse traversal ===
print("Reversed:", end=" ")
for i in range(len(arr) - 1, -1, -1):  # range(4, -1, -1) → 4, 3, 2, 1, 0
    print(arr[i], end=" ")
print()  # Output: 5 8 2 7 3

# === Step by 2 ===
print("Every other:", end=" ")
for i in range(0, len(arr), 2):  # range(start, stop, step)
    print(arr[i], end=" ")
print()  # Output: 3 2 5

# === enumerate: index + value together ===
for i, val in enumerate(arr):
    print(f"arr[{i}] = {val}")

public class ForLoopDemo {
    public static void main(String[] args) {
        // === Basic for loop: print 0 to 4 ===
        System.out.print("Basic loop: ");
        for (int i = 0; i < 5; i++) {
            System.out.print(i + " ");
        }
        System.out.println();  // Output: 0 1 2 3 4
        
        // === Sum of array elements ===
        int[] arr = {3, 7, 2, 8, 5};
        int sum = 0;
        for (int i = 0; i < arr.length; i++) {
            sum += arr[i];
        }
        System.out.println("Sum: " + sum);  // Output: 25
        
        // === Reverse traversal ===
        System.out.print("Reversed: ");
        for (int i = arr.length - 1; i >= 0; i--) {
            System.out.print(arr[i] + " ");
        }
        System.out.println();  // Output: 5 8 2 7 3
        
        // === Step by 2 ===
        System.out.print("Every other: ");
        for (int i = 0; i < arr.length; i += 2) {
            System.out.print(arr[i] + " ");
        }
        System.out.println();  // Output: 3 2 5
        
        // === Enhanced for loop (for-each) ===
        System.out.print("For-each: ");
        for (int val : arr) {
            System.out.print(val + " ");
        }
        System.out.println();  // Output: 3 7 2 8 5
    }
}

Nested For Loop — Finding All Pairs — Watch how the outer loop fixes one element while the inner loop scans through the remaining elements, generating every unique pair. The total iterations are n×(n-1)/2.

#include <iostream>
#include <vector>
using namespace std;

int main() {
    vector<int> arr = {3, 7, 2, 8, 5};
    int n = arr.size();
    
    // === Generate all pairs ===
    cout << "All pairs:" << endl;
    int pairCount = 0;
    for (int i = 0; i < n; i++) {
        for (int j = i + 1; j < n; j++) {
            cout << "(" << arr[i] << ", " << arr[j] << ")" << endl;
            pairCount++;
        }
    }
    cout << "Total pairs: " << pairCount << endl;
    // n*(n-1)/2 = 5*4/2 = 10 pairs
    
    // === Nested loop: Multiplication table ===
    cout << "\nMultiplication Table (3x3):" << endl;
    for (int i = 1; i <= 3; i++) {
        for (int j = 1; j <= 3; j++) {
            cout << i * j << "\t";
        }
        cout << endl;
    }
    
    return 0;
}

arr = [3, 7, 2, 8, 5]
n = len(arr)

# === Generate all pairs ===
print("All pairs:")
pair_count = 0
for i in range(n):
    for j in range(i + 1, n):
        print(f"({arr[i]}, {arr[j]})")
        pair_count += 1
print(f"Total pairs: {pair_count}")
# n*(n-1)/2 = 5*4/2 = 10 pairs

# === Nested loop: Multiplication table ===
print("\nMultiplication Table (3x3):")
for i in range(1, 4):
    for j in range(1, 4):
        print(f"{i*j}\t", end="")
    print()

# === Common pattern: 2D grid traversal ===
grid = [
    [1, 2, 3],
    [4, 5, 6],
    [7, 8, 9]
]
print("\nGrid traversal:")
for row in range(len(grid)):
    for col in range(len(grid[0])):
        print(f"grid[{row}][{col}] = {grid[row][col]}")

public class NestedForLoop {
    public static void main(String[] args) {
        int[] arr = {3, 7, 2, 8, 5};
        int n = arr.length;
        
        // === Generate all pairs ===
        System.out.println("All pairs:");
        int pairCount = 0;
        for (int i = 0; i < n; i++) {
            for (int j = i + 1; j < n; j++) {
                System.out.println("(" + arr[i] + ", " + arr[j] + ")");
                pairCount++;
            }
        }
        System.out.println("Total pairs: " + pairCount);
        // n*(n-1)/2 = 5*4/2 = 10 pairs
        
        // === Nested loop: Multiplication table ===
        System.out.println("\nMultiplication Table (3x3):");
        for (int i = 1; i <= 3; i++) {
            for (int j = 1; j <= 3; j++) {
                System.out.print(i * j + "\t");
            }
            System.out.println();
        }
        
        // === 2D array traversal ===
        int[][] grid = {{1, 2, 3}, {4, 5, 6}, {7, 8, 9}};
        System.out.println("\nGrid traversal:");
        for (int row = 0; row < grid.length; row++) {
            for (int col = 0; col < grid[0].length; col++) {
                System.out.println("grid[" + row + "][" + col + "] = " + grid[row][col]);
            }
        }
    }
}

#include <iostream>
#include <vector>
#include <climits>
using namespace std;

int main() {
    vector<int> arr = {3, 7, 2, 8, 5};
    int n = arr.size();
    
    // Pattern 1: ACCUMULATE — find sum
    int sum = 0;
    for (int i = 0; i < n; i++) {
        sum += arr[i];
    }
    cout << "Sum: " << sum << endl; // 25
    
    // Pattern 2: SEARCH — find maximum
    int maxVal = arr[0];
    for (int i = 1; i < n; i++) {
        if (arr[i] > maxVal) {
            maxVal = arr[i];
        }
    }
    cout << "Max: " << maxVal << endl; // 8
    
    // Pattern 3: FILTER — collect even numbers
    vector<int> evens;
    for (int i = 0; i < n; i++) {
        if (arr[i] % 2 == 0) {
            evens.push_back(arr[i]);
        }
    }
    cout << "Evens: ";
    for (int e : evens) cout << e << " "; // 2 8
    cout << endl;
    
    // Pattern 4: TRANSFORM — square each element
    vector<int> squared;
    for (int i = 0; i < n; i++) {
        squared.push_back(arr[i] * arr[i]);
    }
    cout << "Squared: ";
    for (int s : squared) cout << s << " "; // 9 49 4 64 25
    cout << endl;
    
    // Pattern 5: COUNT — count elements > 4
    int count = 0;
    for (int i = 0; i < n; i++) {
        if (arr[i] > 4) {
            count++;
        }
    }
    cout << "Elements > 4: " << count << endl; // 3
    
    return 0;
}

arr = [3, 7, 2, 8, 5]

# Pattern 1: ACCUMULATE — find sum
total = 0
for val in arr:
    total += val
print(f"Sum: {total}")  # 25
# Pythonic: sum(arr)

# Pattern 2: SEARCH — find maximum
max_val = arr[0]
for val in arr[1:]:
    if val > max_val:
        max_val = val
print(f"Max: {max_val}")  # 8
# Pythonic: max(arr)

# Pattern 3: FILTER — collect even numbers
evens = []
for val in arr:
    if val % 2 == 0:
        evens.append(val)
print(f"Evens: {evens}")  # [2, 8]
# Pythonic: [x for x in arr if x % 2 == 0]

# Pattern 4: TRANSFORM — square each element
squared = []
for val in arr:
    squared.append(val * val)
print(f"Squared: {squared}")  # [9, 49, 4, 64, 25]
# Pythonic: [x*x for x in arr]

# Pattern 5: COUNT — count elements > 4
count = 0
for val in arr:
    if val > 4:
        count += 1
print(f"Elements > 4: {count}")  # 3
# Pythonic: sum(1 for x in arr if x > 4)

import java.util.ArrayList;
import java.util.List;

public class LoopPatterns {
    public static void main(String[] args) {
        int[] arr = {3, 7, 2, 8, 5};
        
        // Pattern 1: ACCUMULATE — find sum
        int sum = 0;
        for (int i = 0; i < arr.length; i++) {
            sum += arr[i];
        }
        System.out.println("Sum: " + sum); // 25
        
        // Pattern 2: SEARCH — find maximum
        int maxVal = arr[0];
        for (int i = 1; i < arr.length; i++) {
            if (arr[i] > maxVal) {
                maxVal = arr[i];
            }
        }
        System.out.println("Max: " + maxVal); // 8
        
        // Pattern 3: FILTER — collect even numbers
        List<Integer> evens = new ArrayList<>();
        for (int val : arr) {
            if (val % 2 == 0) {
                evens.add(val);
            }
        }
        System.out.println("Evens: " + evens); // [2, 8]
        
        // Pattern 4: TRANSFORM — square each element
        int[] squared = new int[arr.length];
        for (int i = 0; i < arr.length; i++) {
            squared[i] = arr[i] * arr[i];
        }
        System.out.print("Squared: ");
        for (int s : squared) System.out.print(s + " "); // 9 49 4 64 25
        System.out.println();
        
        // Pattern 5: COUNT — count elements > 4
        int count = 0;
        for (int val : arr) {
            if (val > 4) {
                count++;
            }
        }
        System.out.println("Elements > 4: " + count); // 3
    }
}