That means that, for example, we can store 5 values of type int in an array without having to declare 5 different variables, each one with a different identifier. Instead of that, using an array we can store 5 different values of the same type, int for example, with a unique identifier.
For example, an array to contain 5 integer values of type int called billy could be represented like this:
where each blank panel represents an element of the array, that in this case are integer values of type int. These elements are numbered from 0 to 4 since in arrays the first index is always 0, independently of its length.
Like a regular variable, an array must be declared before it is used. A typical declaration for an array in C++ is:
type name [elements];
where type is a valid type (like int, float...), name is a valid identifier and the elements field (which is always
enclosed in square brackets []), specifies how many of these elements the array has to contain.
Therefore, in order to declare an array called billy as the one shown in the above diagram it is as simple as:
int billy [5];
NOTE: The elements field within brackets [] which represents the number of elements the array is going to hold, must be a constant value, since arrays are blocks of non-dynamic memory whose size must be determined before execution. In order to create arrays with a variable length dynamic memory is needed, which is explained later in these tutorials.
example of array
Simple array
#include <iostream>
using namespace std;
int billy [] = {16, 2, 77, 40, 12071};
int n, result=0;
int main ()
{
for ( n=0 ; n<5 ; n++ )
{
result += billy[n];
}
cout << result;
return 0;
}
Multidimensional arrays
Multidimensional arrays can be described as "arrays of arrays". For example, a bidimensional array can be imagined as a bidimensional table made of elements, all of them of a same uniform data type.
jimmy represents a bidimensional array of 3 per 5 elements of type int. The way to declare this array in C++ would be:
int jimmy [3][5];
and, for example, the way to reference the second element vertically and fourth horizontally in an expression would be:
jimmy[1][3]
(remember that array indices always begin by zero).
Multidimensional arrays are not limited to two indices (i.e., two dimensions). They can contain as many indices as needed.
But be careful! The amount of memory needed for an array rapidly increases with each
dimension.
For example:
char century [100][365][24][60][60];
declares an array with a char element for each second in a century, that is more than 3 billion chars. So this declaration would consume more than 3 gigabytes of memory!
Multidimensional arrays are just an abstraction for programmers, since we can obtain the same results with a simple array just by putting a factor between its indices:
1
2
int jimmy [3][5]; // is equivalent to
int jimmy [15]; // (3 * 5 = 15)
With the only difference that with multidimensional arrays the compiler remembers the depth of each imaginary dimension for us. Take as example these two pieces of code, with both exactly the same result. One uses a bi-dimensional array and the other one uses a simple array:
multidimensional array
// Example -1
//pseudo-multidimensional array
#define WIDTH 5
#define HEIGHT 3
int jimmy [HEIGHT][WIDTH];
int n,m;
int main ()
{
for (n=0;n<HEIGHT;n++)
for (m=0;m<WIDTH;m++)
{
jimmy[n][m]=(n+1)*(m+1);
}
return 0;
}
// Example -2
//pseudo-multidimensional array
#define WIDTH 5
#define HEIGHT 3
int jimmy [HEIGHT * WIDTH];
int n,m;
int main ()
{
for (n=0;n<HEIGHT;n++)
for (m=0;m<WIDTH;m++)
{
jimmy[n*WIDTH+m]=(n+1)*(m+1);
}
return 0;
}
None of the two source codes above produce any output on the screen, but both assign values to the memory block called jimmy in the following way:
We have used "defined constants" (#define) to simplify possible future modifications of the program. For example, in case that we decided to enlarge the array to a height of 4 instead of 3 it could be done simply by changing the line:
#define HEIGHT 3
to:
#define HEIGHT 4
with no need to make any other modifications to the program.
Arrays as parameters
At some moment we may need to pass an array to a function as a parameter. In C++ it is not possible to pass a complete block of memory by value as a parameter to a function, but we are allowed to pass its
address.
In practice this has almost the same effect and it is a much faster and more efficient operation.
In order to accept arrays as parameters the only thing that we have to do when declaring the function is to specify in its parameters the element type of the array, an identifier and a pair of void brackets [].
For example, the following function: void procedure (int arg[]) accepts a parameter of type "array of int" called arg. In order to pass to this function an array declared as:
int myarray [40];
it would be enough to write a call like this:
procedure (myarray);
Here you have a complete example:
// arrays as parameters
#include <iostream>
using namespace std;
void printarray (int arg[], int length)
{
for (int n=0; n<length; n++)
cout << arg[n] << " ";
cout << "\n";
}
int main ()
{
int firstarray[] = {5, 10, 15};
int secondarray[] = {2, 4, 6, 8, 10};
printarray (firstarray,3);
printarray (secondarray,5);
return 0;
}
As you can see, the first parameter (int arg[]) accepts any array whose elements are of type int, whatever its length.
For that reason we have included a second parameter that tells the function the length of each array that we pass to it as its first parameter.
This allows the for loop that prints out the array to know the range to iterate in the passed array without going out of range.
In a function declaration it is also possible to include multidimensional arrays.
The format for a tridimensional array parameter is:
base_type[][depth][depth]
for example, a function with a multidimensional array as argument could be:
void procedure (int myarray[][3][4])
Notice that the first brackets [] are left empty while the following ones specify sizes for their respective dimensions.
This is necessary in order for the compiler to be able to determine the depth of each additional dimension.
Arrays, both simple or multidimensional, passed as function parameters are a quite common source of errors for novice programmers.
I recommend the reading of the chapter about Pointers for a better understanding on how arrays operate.
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