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Accessing Array Elements

Given the declaration above of a 100 element array the compiler reserves space for 100 consecutive floating point values and accesses these values using an index/subscript that takes values from 0 to 99. The first element in an array in C++ always has the index 0, and if the array has n elements the last element will have the index n-1.

An array element is accessed by writing the identifier of the array followed by the subscript in square brackets. Thus to set the 15th element of the array above to 1.5 the following assignment is used:

annual_temp[14] = 1.5;
Note that since the first element is at index 0, then the ith element is at index i-1. Hence in the above the 15th element has index 14.

An array element can be used anywhere an identifier may be used. Here are some examples assuming the following declarations:

const int NE = 100,
          N = 50;
int i, j, count[N];
float annual_temp[NE];
float sum, av1, av2;
A value can be read into an array element directly, using cin
cin >> count[i];
The element can be increased by 5,
count[i] = count[i] + 5;
or, using the shorthand form of the assignment
count[i] += 5;

Array elements can form part of the condition for an if statement, or indeed, for any other logical expression:

if (annual_temp[j] < 10.0)
   cout << "It was cold this year "
        << endl;
for statements are the usual means of accessing every element in an array. Here, the first NE elements of the array annual_temp are given values from the input stream cin.
for (i = 0; i < NE; i++)
  cin >> annual_temp[i];

The following code finds the average temperature recorded in the first ten elements of the array.

sum = 0.0;
for (i = 0; i <10; i++)
    sum += annual_temp[i];
av1 = sum / 10;
Notice that it is good practice to use named constants, rather than literal numbers such as 10. If the program is changed to take the average of the first 20 entries, then it all too easy to forget to change a 10 to 20. If a const is used consistently, then changing its value will be all that is necessary.

For example, the following example finds the average of the last k entries in the array. k could either be a variable, or a declared constant. Observe that a change in the value of k will still calculate the correct average (provided k<=NE).

sum = 0.0;
for (i = NE - k; i < NE; i++)
  sum += annual_temp[i];
av2 = sum / k;
Important - C++ does not check that the subscript that is used to reference an array element actually lies in the subscript range of the array. Thus C++ will allow the assignment of a value to annual_temp[200], however the effect of this assignment is unpredictable. For example it could lead to the program attempting to assign a value to a memory element that is outside the program's allocated memory space. This would lead to the program being terminated by the operating system. Alternatively it might actually access a memory location that is within the allocated memory space of the program and assign a value to that location, changing the value of the variable in your program which is actually associated with that memory location, or overwriting the machine code of your program. Similarly reading a value from annual_temp[200] might access a value that has not been set by the program or might be the value of another variable. It is the programmer's responsibility to ensure that if an array is declared with n elements then no attempt is made to reference any element with a subscript outside the range 0 to n-1. Using an index, or subscript, that is out of range is called Subscript Overflow. Subscript overflow is one of the commonest causes of erroneous results and can frequently cause very strange and hard to spot errors in programs.


Next: Initializations of arrays Up: Arrays in C++ Previous: Declaration of Arrays