C++ Pointer to an Array

It is most likely that you would not understand this chapter until you go through the chapter related C++ Pointers.

So assuming you have bit understanding on pointers in C++, let us start: An array name is a constant pointer to the first element of the array. Therefore, in the declaration:

double balance[50];

balance is a pointer to &balance[0], which is the address of the first element of the array balance. Thus, the following program fragment assigns p the address of the first element of balance:

double *p;
double balance[10];

p = balance;

It is legal to use array names as constant pointers, and vice versa. Therefore, *(balance + 4) is a legitimate way of accessing the data at balance[4].

Once you store the address of first element in p, you can access array elements using *p, *(p+1), *(p+2) and so on. Below is the example to show all the concepts discussed above:

#include <iostream>
using namespace std;
 
int main ()
{
   // an array with 5 elements.
   double balance[5] = {1000.0, 2.0, 3.4, 17.0, 50.0};
   double *p;

   p = balance;
 
   // output each array element's value 
   cout << "Array values using pointer " << endl; 
   for ( int i = 0; i < 5; i++ )
   {
       cout << "*(p + " << i << ") : ";
       cout << *(p + i) << endl;
   }

   cout << "Array values using balance as address " << endl;
   for ( int i = 0; i < 5; i++ )
   {
       cout << "*(balance + " << i << ") : ";
       cout << *(balance + i) << endl;
   }
 
   return 0;
}

When the above code is compiled and executed, it produces the following result:

Array values using pointer
*(p + 0) : 1000
*(p + 1) : 2
*(p + 2) : 3.4
*(p + 3) : 17
*(p + 4) : 50
Array values using balance as address
*(balance + 0) : 1000
*(balance + 1) : 2
*(balance + 2) : 3.4
*(balance + 3) : 17
*(balance + 4) : 50

In the above example, p is a pointer to double which means it can store address of a variable of double type. Once we have address in p, then *p will give us value available at the address stored in p, as we have shown in the above example.

C++ Multi-dimensional Arrays

C++ allows multidimensional arrays. Here is the general form of a multidimensional array declaration:

type name[size1][size2]...[sizeN];

For example, the following declaration creates a three dimensional 5 . 10 . 4 integer array:

int threedim[5][10][4];

Two-Dimensional Arrays:

The simplest form of the multidimensional array is the two-dimensional array. A two-dimensional array is, in essence, a list of one-dimensional arrays. To declare a two-dimensional integer array of size x,y, you would write something as follows:

type arrayName [ x ][ y ];

Where type can be any valid C++ data type and arrayName will be a valid C++ identifier.

A two-dimensional array can be think as a table, which will have x number of rows and y number of columns. A 2-dimensional array a, which contains three rows and four columns can be shown as below:

Thus, every element in array a is identified by an element name of the form a[ i ][ j ], where a is the name of the array, and i and j are the subscripts that uniquely identify each element in a.

Initializing Two-Dimensional Arrays:

Multidimensioned arrays may be initialized by specifying bracketed values for each row. Following is an array with 3 rows and each row have 4 columns.

int a[3][4] = {  
 {0, 1, 2, 3} ,   /*  initializers for row indexed by 0 */
 {4, 5, 6, 7} ,   /*  initializers for row indexed by 1 */
 {8, 9, 10, 11}   /*  initializers for row indexed by 2 */
};

The nested braces, which indicate the intended row, are optional. The following initialization is equivalent to previous example:

int a[3][4] = {0,1,2,3,4,5,6,7,8,9,10,11};

Accessing Two-Dimensional Array Elements:

An element in 2-dimensional array is accessed by using the subscripts, i.e., row index and column index of the array. For example:

int val = a[2][3];

The above statement will take 4th element from the 3rd row of the array. You can verify it in the above digram.

#include <iostream>
using namespace std;
 
int main ()
{
   // an array with 5 rows and 2 columns.
   int a[5][2] = { {0,0}, {1,2}, {2,4}, {3,6},{4,8}};
 
   // output each array element's value                      
   for ( int i = 0; i < 5; i++ )
      for ( int j = 0; j < 2; j++ )
      {
         cout << "a[" << i << "][" << j << "]: ";
         cout << a[i][j]<< endl;
      }
 
   return 0;
}

When the above code is compiled and executed, it produces the following result:

a[0][0]: 0
a[0][1]: 0
a[1][0]: 1
a[1][1]: 2
a[2][0]: 2
a[2][1]: 4
a[3][0]: 3
a[3][1]: 6
a[4][0]: 4
a[4][1]: 8

As explained above, you can have arrays with any number of dimensions, although it is likely that most of the arrays you create will be of one or two dimensions.

Numbers in C++

Normally, when we work with Numbers, we use primitive data types such as int, short, long, float and double etc. The number data types, their possible values and number ranges have been explained while discussing C++ Data Types.

Defining Numbers in C++:

You have already defined numbers in various examples given in previous chapters. Here is another consolidated example to define various types of numbers in C++:

#include <iostream>
using namespace std;
 int main ()
{
   // number definition:
   short  s;
   int    i;
   long   l;
   float  f;
   double d;
   
   // number assignments;
   s = 10;      
   i = 1000;    
   l = 1000000; 
   f = 230.47;  
   d = 30949.374;
   
   // number printing;
   cout << "short  s :" << s << endl;
   cout << "int    i :" << i << endl;
   cout << "long   l :" << l << endl;
   cout << "float  f :" << f << endl;
   cout << "double d :" << d << endl;
 
   return 0;
}

When the above code is compiled and executed, it produces following result:

short  s :10
int    i :1000
long   l :1000000
float  f :230.47
double d :30949.4

Math Operations in C++:

In addition to the various functions you can create, C++ also includes some useful functions you can use. These functions are available in standard C and C++ libraries and called built-in functions. These are functions that can be included in your program and then use.

C++ has a rich set of mathematical operations which can be performed on various numbers. Following table list down some useful built-in mathematical functions available in C++.

To utilize these functions you need to include the math header file <cmath>.

S.N.	Function & Purpose
1	double cos(double); This function takes an angle (as a double) and returns the cosine.
2	double sin(double); This function takes an angle (as a double) and returns the sine.
3	double tan(double); This function takes an angle (as a double) and returns the tangent.
4	double log(double); This function takes a number and returns the natural log of that number.
5	double pow(double, double); The first is a number you wish to raise and the second is the power you wish to raise it to.
6	double hypot(double, double); If you pass this function the length of two sides of a right triangle, it will return you the length of the hypotenuse.
7	double sqrt(double); You pass this function a number and it gives you this square root.
8	int abs(int); This function returns the absolute value of an integer that is passed to it.
9	double fabs(double); This function returns the absolute value of any decimal number passed to it.
10	double floor(double); Finds the integer which is less than or equal to the argument passed to it.

Following a simple example to show few of the mathematical operations:

#include <iostream>
#include <cmath>
using namespace std;
 int main ()
{
   // number definition:
   short  s = 10;
   int    i = -1000;
   long   l = 100000;
   float  f = 230.47;
   double d = 200.374;

   // mathematical operations;
   cout << "sin(d) :" << sin(d) << endl;
   cout << "abs(i)  :" << abs(i) << endl;
   cout << "floor(d) :" << floor(d) << endl;
   cout << "sqrt(f) :" << sqrt(f) << endl;
   cout << "pow( d, 2) :" << pow(d, 2) << endl;
 
   return 0;
}

When the above code is compiled and executed, it produces following result:

sign(d) :-0.634939
abs(i)  :1000
floor(d) :200
sqrt(f) :15.1812
pow( d, 2 ) :40149.7

Random Numbers in C++:

There are many cases where you will wish to generate a random number. There are actually two functions you will need to know about random number generation. The first is rand(), this function will only return a pseudo random number. The way to fix this is to first call the srand() function.

Following a simple example to generate few random numbers. This example makes use of time()function to get the number of seconds on your system time, to randomly seed the rand() function:

#include <iostream>
#include <ctime>
using namespace std;
 int main ()
{
   int i,j;
 
   // set the seed
   srand( (unsigned)time( NULL ) );

   /* generate 10  random numbers. */
   for( i = 0; i < 10; i++ )
   {
      // generate actual random number
      j= rand();
      cout <<" Random Number : " << j << endl;
   }

   return 0;
}

When the above code is compiled and executed, it produces following result:

 Random Number : 1748144778
 Random Number : 630873888
 Random Number : 2134540646
 Random Number : 219404170
 Random Number : 902129458
 Random Number : 920445370
 Random Number : 1319072661
 Random Number : 257938873
 Random Number : 1256201101
 Random Number : 580322989