Erle Robotics C++ GitBook

Arrays basics

Unidimensional arrays

An array is a fixed number of elements of the same type stored sequentially in memory. Therefore, an integer array holds some number of integers, a character array holds some number of characters, and so on. The size of the array is referred to as its dimension. To declare an array in C++, we write the following:

type arrayName[dimension];


example:

int arr[4];


The elements of an array can be accessed by using an index into the array. Arrays in C++ are zero-indexed, so the first element has an index of 0. So, to access the third element in arr, we write arr[2]; The value returned can then be used just like any other integer.

Like normal variables, the elements of an array must be initialized before they can be used; otherwise we will almost certainly get unexpected results in our program. There are several ways to initialize the array. One way is to declare the array and then initialize some or all of the elements:

int arr[4];

arr[0] = 6;

arr[1] = 0;

arr[2] = 9;

arr[3] = 6;


Another way is to initialize some or all of the values at the time of declaration:

int arr[4] = { 6, 0, 9, 6 };


Sometimes it is more convenient to leave out the size of the array and let the compiler determine the array's size for us, based on how many elements we give it:

int arr[] = { 6, 0, 9, 6, 2, 0, 1, 1 };


Here, the compiler will create an integer array of dimension 8.

The array can also be initialized with values that are not known beforehand:

 #include <iostream>
using namespace std;

int main() {

int arr[4];

cout << "Please enter 4 integers:" << endl;

for(int i = 0; i < 4; i++)
cin >> arr[i];\\Fill in the array

cout << "Values in array are now:";

for(int i = 0; i < 4; i++)
cout << " " << arr[i];\\ Show the content of the array

cout << endl;

return 0;
}


Note that when accessing an array the index given must be a positive integer from 0 to n-1, where n is the dimension of the array. The index itself may be directly provided, derived from a variable, or computed from an expression:

arr[5];

arr[i];

arr[i+3];


Arrays can also be passed as arguments to functions. When declaring the function, simply specify the array as a parameter, without a dimension. The array can then be used as normal within the function. For example:

#include <iostream>
using namespace std;

int sum(const int array[], const int length) {
long sum = 0;
for(int i = 0; i < length; sum += array[i++]);
return sum;
}

int main() {

int arr[] = {1, 2, 3, 4, 5, 6, 7};
cout << "Sum: " << sum(arr, 7) << endl;

return 0;
}


The function sum takes a constant integer array and a constant integer length as its arguments and adds up length elements in the array. It then returns the sum, and the program prints out Sum: 28.

It is important to note that arrays are passed by reference and so any changes made to the array within the function will be observed in the calling scope.

Multidimensional arrays

C++ also supports the creation of multidimensional arrays, through the addition of more than one set of brackets. Thus, a two-dimensional array may be created by the following:

type arrayName[dimension1][dimension2];


The array will have dimension1 x dimension2 elements of the same type and can be thought of as an array of arrays. The first index indicates which of dimension1 subarrays to access, and then the second index accesses one of dimension2 elements within that subarray. Initialization and access thus work similarly to the one-dimensional case:

#include <iostream>
using namespace std;

int main() {

int twoDimArray[2][4];

twoDimArray[0][0] = 6;

twoDimArray[0][1] = 0;

twoDimArray[0][2] = 9;

twoDimArray[0][3] = 6;

twoDimArray[1][0] = 2;

twoDimArray[1][1] = 0;

twoDimArray[1][2] = 1;

twoDimArray[1][3] = 1;

for(int i = 0; i < 2; i++)

for(int j = 0; j < 4; j++)

cout << twoDimArray[i][j];
out << endl;

return 0;

}


The array can also be initialized at declaration in the following ways:

int twoDimArray[2][4] = { 6, 0, 9, 6, 2, 0, 1, 1 };

int twoDimArray[2][4] = { { 6, 0, 9, 6 } , { 2, 0, 1, 1 } };


Note that dimensions must always be provided when initializing multidimensional arrays, as it is otherwise impossible for the compiler to determine what the intended element partitioning is. For the same reason, when multidimensional arrays are specified as arguments to functions, all dimensions but the first must be provided (the first dimension is optional), as in the following:

int aFunction(int arr[][4]) { … }


Multidimensional arrays are merely an abstraction for programmers, as all of the elements in the array are sequential in memory. Declaring int arr[2][4]; is the same thing as declaring int arr[8];.