C++ Arrays

An array holds a fixed number of values of the same type, reached by index from 0. C++ has the built-in arrays inherited from C and the safer std::array from the array header. When the size must change while the program runs, use std::vector (see Vectors).

Built-in arrays

#include <iostream>

int main() {
    int marks[5] = {78, 85, 62, 90, 74};
    int total = 0, highest = marks[0];
    for (int m : marks) {
        total += m;
        if (m > highest) highest = m;
    }
    std::cout << "Total: " << total << ", highest: " << highest << "\n";
    std::cout << "Third mark: " << marks[2] << "\n";
    return 0;
}

Total: 389, highest: 90
Third mark: 62

Indexes run from 0 to size − 1. C++ does not check them: marks[5] reads memory that is not part of the array. Safe mode in the editor stops the program at such an index.

std::array

std::array has the same speed as a built-in array, but it knows its size, can be copied and compared, and offers at(i), which checks the index:

#include <array>
#include <iostream>

int main() {
    std::array<int, 4> a = {3, 1, 4, 1};
    std::cout << "size: " << a.size() << "\n";
    std::array<int, 4> b = a;        // a real copy
    b[0] = 9;
    std::cout << a[0] << " " << b[0] << "\n";
    std::cout << "front " << a.front() << ", back " << a.back() << "\n";
    return 0;
}

size: 4
3 9
front 3, back 1

The size is part of the type (std::array<int, 4>) and must be a constant.

Two dimensions

#include <iostream>

int main() {
    int table[2][3] = {{1, 2, 3}, {4, 5, 6}};
    for (int r = 0; r < 2; r++) {
        for (int c = 0; c < 3; c++) {
            std::cout << table[r][c] << " ";
        }
        std::cout << "\n";
    }
    std::cout << "table[1][2] = " << table[1][2] << "\n";
    return 0;
}

1 2 3 
4 5 6 
table[1][2] = 6

Arrays or vectors?

Built-in arrays turn into a pointer when passed to a function and forget their size, which causes many bugs. Prefer std::array for a fixed size and std::vector when the size is known only while the program runs. Most C++ code uses std::vector by default.

Passing arrays to functions

A std::array is passed like any object: by const& to read it, by & to change it, and the function knows its size. A built-in array passed to a function turns into a pointer to its first element and forgets its size, so the size must be passed separately, which is a common source of bugs.

Practice exercises

1. Read 5 numbers into a std::array and print them in reverse order.
2. Count how many elements are above the average.
3. Find the smallest value and its position in one loop.
4. Store a 3 by 3 table and print the sum of each row and each column.