Arrays and std::vector

When you take 100 readings from a sensor, declaring 100 separate variables makes no sense. To keep many values of the same type under one name, we use an array. C++ has raw arrays (inherited from C) and their modern counterpart, std::vector. As a beginner, I recommend using std::vector.

Raw arrays

int arr[5] = {10, 20, 30, 40, 50};
std::cout << arr[0] << ' ' << arr[4] << '\n';   // 10 50

The size must be fixed at compile time, and the array does not know its own length. On top of that, there is no bounds checking: if you write arr[7], the program may not crash; it simply reads or corrupts some other region of memory. A large share of undefined-behavior bugs come from here.

std::vector

std::vector is an array that can grow. Its header is #include <vector>. The element type is given inside angle brackets.

#include <iomanip>
#include <iostream>
#include <vector>

int main() {
    std::vector<double> readings = {20.5, 21.0, 19.8, 22.1, 20.6};
    readings.push_back(21.4);          // add an element at the end

    double sum = 0.0;
    double smallest = readings[0];
    double largest = readings[0];
    for (double x : readings) {        // range-based for
        sum += x;
        if (x < smallest) smallest = x;
        if (x > largest) largest = x;
    }
    double average = sum / readings.size();

    std::cout << std::fixed << std::setprecision(1);
    std::cout << "Number of readings: " << readings.size() << '\n';
    std::cout << "Average: " << average << '\n';
    std::cout << "Smallest: " << smallest << '\n';
    std::cout << "Largest: " << largest << '\n';
    return 0;
}

Number of readings: 6
Average: 20.9
Smallest: 19.8
Largest: 22.1

The sum is 125.4 and there are 6 elements, so the average comes out as 20.9.

Common operations

The range-based for

The form for (double x : readings) reads as "for each x in readings". You don't need to keep an index. If you want to change the elements, take a reference with for (double& x : readings); if you are only reading, you can avoid the copy with const double&. We will cover references two lessons from now.

Things to watch out for

• size() returns an unsigned integer. Comparing it with int i may trigger a compiler warning; use std::size_t for indexes.
• Don't try to calculate the average of an empty vector: that means dividing by zero. Check empty() first.
• The elements sit next to each other in memory; this is very fast in terms of the cache.

Try it yourself

1. Add 5 readings of your own to the vector and calculate the average.
2. Count the readings above a threshold (e.g. greater than 21).
3. Print the readings in reverse order (use for and an index).
4. Ask the user for readings with cin; keep adding them to the vector until 0 is entered.