Arrays and Indexes

An array stores an ordered group of values of the same type. Every element is accessed by index, and the array length is fixed when it is created.

Creating arrays

Use an array initializer when the values are known. int counts[4]{} creates four zero-initialized slots. The element type comes before the variable name, and brackets mean “a group of this type.”

int main() {
    int scores[] = {88, 91, 76, 95, 83};
    int counts[4]{};
    counts[0] = 12;
    counts[1] = 9;
    return 0;
}

Using {} to initialize a numeric array sets every element to 0; an uninitialized local array does not receive automatic default values. The slot count is decided when the array is created.

Accessing array elements

Array indexes start at 0, so the first element is scores[0]. An array of length 5 has indexes 0 through 4. Accessing scores[5] causes undefined behavior.

#include <iostream>

using namespace std;

int main() {
    int scores[] = {88, 91, 76, 95, 83};
    cout << scores[0] << endl; // 88
    cout << scores[4] << endl; // 83
    scores[2] = 80;
    cout << scores[2] << endl; // 80
    return 0;
}

Array length and loops

Built-in arrays have no length field. This example first computes score_count = 5; the last valid index is score_count - 1.

#include <iostream>

using namespace std;

int main() {
    int scores[] = {88, 91, 76, 95, 83};
    const int score_count = 5;

    for (int i = 0; i < score_count; i++) {
        cout << "scores[" << i << "] = " << scores[i] << endl;
    }

    for (int score : scores) {
        cout << score << endl;
    }
    return 0;
}

Fixed length and common operations

You can rewrite existing elements and search with a loop. Neither action adds or removes slots.

#include <iostream>

using namespace std;

int main() {
    int values[] = {4, 0, 7, 0, 2, 9};
    values[2] = 5;
    for (int value : values) {
        cout << value << ' ';
    }
    return 0;
}