Array & Loop

In the previous lesson, you accessed array elements one by one: arr[0], arr[1], arr[2]… If there are 100 elements, are you going to write 100 lines? Of course not!

The array + loop combination is the most commonly used pattern in programming. Almost every program that deals with lots of data uses this pattern.

Analogy: A Teacher Grading Assignments

Imagine a teacher grading 30 students’ assignments. The teacher wouldn’t say: “Open Andi’s book… now Budi’s book… now Citra’s book…” one by one. The teacher would say: “For each assignment book in this pile, check and give a grade.” That’s array + loop!

Example 1: Printing All Array Elements

#include <iostream>

int main() {
    int scores[] = {85, 90, 78, 92, 88, 76, 95};
    int length = sizeof(scores) / sizeof(scores[0]);

    std::cout << "=== Score List ===" << std::endl;

    for (int i = 0; i < length; i++) {
        std::cout << "Student " << i + 1 << ": " << scores[i] << std::endl;
    }

    return 0;
}

Output:

=== Score List ===
Student 1: 85
Student 2: 90
Student 3: 78
Student 4: 92
Student 5: 88
Student 6: 76
Student 7: 95

Example 2: Input Array from User

#include <iostream>

int main() {
    const int COUNT = 5;
    int scores[COUNT];

    std::cout << "Enter scores for " << COUNT << " students:" << std::endl;

    for (int i = 0; i < COUNT; i++) {
        std::cout << "Student " << i + 1 << ": ";
        std::cin >> scores[i];
    }

    std::cout << std::endl;
    std::cout << "=== Entered Scores ===" << std::endl;

    for (int i = 0; i < COUNT; i++) {
        std::cout << "Student " << i + 1 << ": " << scores[i] << std::endl;
    }

    return 0;
}

Output (example input):

Enter scores for 5 students:
Student 1: 85
Student 2: 92
Student 3: 78
Student 4: 90
Student 5: 88

=== Entered Scores ===
Student 1: 85
Student 2: 92
Student 3: 78
Student 4: 90
Student 5: 88

Example 3: Calculate Total and Average

#include <iostream>

int main() {
    int scores[] = {85, 90, 78, 92, 88};
    int length = sizeof(scores) / sizeof(scores[0]);

    int total = 0;

    for (int i = 0; i < length; i++) {
        total += scores[i];
    }

    double average = static_cast<double>(total) / length;

    std::cout << "Total score   : " << total << std::endl;
    std::cout << "Num. students : " << length << std::endl;
    std::cout << "Average       : " << average << std::endl;

    return 0;
}

Output:

Total score   : 433
Num. students : 5
Average       : 86.6

Example 4: Find Maximum and Minimum Values

#include <iostream>

int main() {
    int temp[] = {30, 35, 28, 32, 27, 33, 31};
    int length = sizeof(temp) / sizeof(temp[0]);

    int maxVal = temp[0];  // Assume the first element is the max
    int minVal = temp[0];  // Assume the first element is the min

    for (int i = 1; i < length; i++) {
        if (temp[i] > maxVal) {
            maxVal = temp[i];
        }
        if (temp[i] < minVal) {
            minVal = temp[i];
        }
    }

    std::cout << "=== Weekly Temperature ===" << std::endl;
    for (int i = 0; i < length; i++) {
        std::cout << "Day " << i + 1 << ": " << temp[i] << " C" << std::endl;
    }
    std::cout << "---------------------" << std::endl;
    std::cout << "Highest temp: " << maxVal << " C" << std::endl;
    std::cout << "Lowest temp : " << minVal << " C" << std::endl;

    return 0;
}

Output:

=== Weekly Temperature ===
Day 1: 30 C
Day 2: 35 C
Day 3: 28 C
Day 4: 32 C
Day 5: 27 C
Day 6: 33 C
Day 7: 31 C
---------------------
Highest temp: 35 C
Lowest temp : 27 C

Example 5: Linear Search — Find a Specific Element

#include <iostream>
#include <string>

int main() {
    std::string students[] = {"Andi", "Budi", "Citra", "Dewi", "Eka"};
    int length = sizeof(students) / sizeof(students[0]);

    std::string target;
    std::cout << "Search for student name: ";
    std::cin >> target;

    bool found = false;
    int position = -1;

    for (int i = 0; i < length; i++) {
        if (students[i] == target) {
            found = true;
            position = i;
            break;  // Found it, no need to continue
        }
    }

    if (found) {
        std::cout << target << " found at position " << position + 1 << std::endl;
    } else {
        std::cout << target << " not found!" << std::endl;
    }

    return 0;
}

Output (example input “Citra”):

Search for student name: Citra
Citra found at position 3

Output (example input “Fajar”):

Search for student name: Fajar
Fajar not found!

Example 6: Range-Based For Loop

A modern and cleaner way to traverse arrays:

#include <iostream>
#include <string>

int main() {
    std::string fruits[] = {"Mango", "Orange", "Apple", "Durian", "Rambutan"};

    std::cout << "=== Favorite Fruits ===" << std::endl;

    // Range-based for loop — simple and elegant!
    for (std::string f : fruits) {
        std::cout << "- " << f << std::endl;
    }

    // Can also use auto
    std::cout << std::endl << "=== With auto ===" << std::endl;
    for (auto f : fruits) {
        std::cout << "- " << f << std::endl;
    }

    return 0;
}

Output:

=== Favorite Fruits ===
- Mango
- Orange
- Apple
- Durian
- Rambutan

=== With auto ===
- Mango
- Orange
- Apple
- Durian
- Rambutan

Example 7: Student Report Card Program

A more complete example combining many techniques:

#include <iostream>
#include <string>

int main() {
    const int NUM_STUDENTS = 5;
    std::string names[NUM_STUDENTS];
    int scores[NUM_STUDENTS];

    // Input data
    std::cout << "=== INPUT STUDENT DATA ===" << std::endl;
    for (int i = 0; i < NUM_STUDENTS; i++) {
        std::cout << "Student " << i + 1 << " name : ";
        std::cin >> names[i];
        std::cout << "Student " << i + 1 << " score: ";
        std::cin >> scores[i];
        std::cout << std::endl;
    }

    // Calculate statistics
    int total = 0;
    int maxVal = scores[0];
    int minVal = scores[0];
    int idxMax = 0;
    int idxMin = 0;

    for (int i = 0; i < NUM_STUDENTS; i++) {
        total += scores[i];
        if (scores[i] > maxVal) {
            maxVal = scores[i];
            idxMax = i;
        }
        if (scores[i] < minVal) {
            minVal = scores[i];
            idxMin = i;
        }
    }

    double average = static_cast<double>(total) / NUM_STUDENTS;

    // Display report card
    std::cout << "=============================" << std::endl;
    std::cout << "      CLASS REPORT CARD" << std::endl;
    std::cout << "=============================" << std::endl;

    for (int i = 0; i < NUM_STUDENTS; i++) {
        std::cout << names[i] << "\t: " << scores[i];
        if (scores[i] >= 90) std::cout << " (A)";
        else if (scores[i] >= 80) std::cout << " (B)";
        else if (scores[i] >= 70) std::cout << " (C)";
        else std::cout << " (D)";
        std::cout << std::endl;
    }

    std::cout << "-----------------------------" << std::endl;
    std::cout << "Average  : " << average << std::endl;
    std::cout << "Highest  : " << names[idxMax] << " (" << maxVal << ")" << std::endl;
    std::cout << "Lowest   : " << names[idxMin] << " (" << minVal << ")" << std::endl;

    return 0;
}

Output (example):

=== INPUT STUDENT DATA ===
Student 1 name : Andi
Student 1 score: 85

Student 2 name : Budi
Student 2 score: 92

Student 3 name : Citra
Student 3 score: 78

Student 4 name : Dewi
Student 4 score: 95

Student 5 name : Eka
Student 5 score: 88

=============================
      CLASS REPORT CARD
=============================
Andi	: 85 (B)
Budi	: 92 (A)
Citra	: 78 (C)
Dewi	: 95 (A)
Eka	: 88 (B)
-----------------------------
Average  : 87.6
Highest  : Dewi (95)
Lowest   : Citra (78)

Example 8: Modifying All Elements

What if the teacher wants to add a 5-point bonus to all scores?

#include <iostream>

int main() {
    int scores[] = {75, 82, 68, 90, 71};
    int length = sizeof(scores) / sizeof(scores[0]);

    std::cout << "Before bonus:" << std::endl;
    for (int i = 0; i < length; i++) {
        std::cout << "Student " << i + 1 << ": " << scores[i] << std::endl;
    }

    // Add 5-point bonus
    for (int i = 0; i < length; i++) {
        scores[i] += 5;

        // Make sure it doesn't exceed 100
        if (scores[i] > 100) {
            scores[i] = 100;
        }
    }

    std::cout << std::endl << "After bonus (+5):" << std::endl;
    for (int i = 0; i < length; i++) {
        std::cout << "Student " << i + 1 << ": " << scores[i] << std::endl;
    }

    return 0;
}

Output:

Before bonus:
Student 1: 75
Student 2: 82
Student 3: 68
Student 4: 90
Student 5: 71

After bonus (+5):
Student 1: 80
Student 2: 87
Student 3: 73
Student 4: 95
Student 5: 76

Common Mistakes

1. Off-by-one in loops

int arr[5] = {10, 20, 30, 40, 50};

// WRONG — i <= 5 will access arr[5] which is out of bounds!
for (int i = 0; i <= 5; i++) {
    std::cout << arr[i];  // Crash when i = 5!
}

// CORRECT — i < 5 stops at arr[4]
for (int i = 0; i < 5; i++) {
    std::cout << arr[i];
}

2. Range-based for cannot modify original elements

int numbers[] = {1, 2, 3, 4, 5};

// WRONG — x is a COPY, not the original element
for (int x : numbers) {
    x *= 2;  // Only modifies the copy, the original array stays unchanged!
}

// CORRECT — use a reference (&) to modify the original elements
for (int& x : numbers) {
    x *= 2;  // Now the original array is also modified
}

3. Forgetting to initialize the accumulator variable

// WRONG — total is not initialized, it contains garbage!
int total;
for (int i = 0; i < 5; i++) {
    total += arr[i];  // Adding to a garbage value
}

// CORRECT
int total = 0;  // Always initialize to 0!
for (int i = 0; i < 5; i++) {
    total += arr[i];
}

Exercises

Exercise 1: Create a program that asks the user to enter 7 daily temperatures. Then display:

• All entered temperatures
• The average temperature
• The highest and lowest temperatures
• How many days had a temperature above average

Exercise 2: Create a program that stores 10 numbers in an array, then calculate and display:

• The count of even and odd numbers
• The total of even numbers and total of odd numbers

Exercise 3: Create a “Price Finder” program for a store. Store 5 product names and their prices in two parallel arrays. Ask the user to enter the product name to search for, then display its price (or a “Product not found” message).

Summary

Array + loop is a combination you’ll use over and over again. Master the patterns above, and you’ll be ready to tackle many programming problems! Next, we’ll level up to 2D arrays — arrays inside arrays!