C++

You access each of the array elements by referring to an offset from the array name. Array elements are counted from zero. Therefore, the first array element is arrayName[0]. In the LongArray example, LongArray[0] is the first array element, LongArray[1] the second, and so forth.

This can be somewhat confusing. The array SomeArray[3] has three elements. They are SomeArray[0], SomeArray[1], and SomeArray[2]. More generally, SomeArray[n] has n elements that are numbered SomeArray[0] through SomeArray[n-1].

Therefore, LongArray[25] is numbered from LongArray[0] through LongArray[24]. Listing 11.1 shows how to declare an array of five integers and fill each with a value.

Listing 11.1. Using an integer array.

1: //Listing 11.1 - Arrays

2: #include <iostream.h>

4: int main()

5: {

6: int myArray[5];

7: int i;

8: for ( i=0; i<5; i++) // 0-4

9: {

10: cout << "Value for myArray[" << i << "]: ";

11: cin >> myArray[i];

12: }

13: for (i = 0; i<5; i++)

14: cout << i << ": " << myArray[i] << "\n";

15: return 0;

16: }

Output:

Value for myArray[0]: 3

Value for myArray[1]: 6

Value for myArray[2]: 9

Value for myArray[3]: 12

Value for myArray[4]: 15

0: 3

1: 6

2: 9

3: 12

4: 15

ANALYSIS: Line 6 declares an array called myArray, which holds five integer variables. Line 8 establishes a loop that counts from 0 through 4, which is the proper set of offsets for a five-element array. The user is prompted for a value, and that value is saved at the correct offset into the array.

The first value is saved at myArray[0], the second at myArray[1], and so forth. The second for loop prints each value to the screen.

NOTE: Arrays count from 0, not from 1. This is the cause of many bugs in programs written by C++ novices. Whenever you use an array, remember that an array with 10 elements counts from ArrayName[0] to ArrayName[9]. There is no ArrayName[10].

Writing Past the End of an Array

When you write a value to an element in an array, the compiler computes where to store the value based on the size of each element and the subscript. Suppose that you ask to write over the value at LongArray[5], which is the sixth element. The compiler multiplies the offset (5) by the size of each element—in this case, 4. It then moves that many bytes (20) from the beginning of the array and writes the new value at that location.

If you ask to write at LongArray[50], the compiler ignores the fact that there is no such element. It computes how far past the first element it should look (200 bytes) and then writes over whatever is at that location. This can be virtually any data, and writing your new value there might have unpredictable results. If you’re lucky, your program will crash immediately. If you’re unlucky, you’ll get strange results much later in your program, and you’ll have a difficult time figuring out what went wrong.

The compiler is like a blind man pacing off the distance from a house. He starts out at the first house, MainStreet[0]. When you ask him to go to the sixth house on Main Street, he says to himself, "I must go five more houses. Each house is four big paces. I must go an additional 20 steps." If you ask him to go to MainStreet[100], and Main Street is only 25 houses long, he will pace off 400 steps. Long before he gets there, he will, no doubt, step in front of a moving bus. So be careful where you send him.

Listing 11.2 shows what happens when you write past the end of an array.

WARNING: Do not run this program; it may crash your system!

Listing 11.2. Writing past the end of an array.

1: //Lis1

22:

23: for (i = 0; i<3; i++)

24: {

25: cout << "sentinelOne[" << i << "]: ";

26: cout << sentinelOne[i] << "\n";

27: cout << "sentinelTwo[" << i << "]: ";

28: cout << sentinelTwo[i]<< "\n";

29: }

30:

31: cout << "\nAssigning...";

32: for (i = 0; i<=25; i++)

33: TargetArray[i] = 20;

34:

35: cout << "\nTest 2: \n";

36: cout << "TargetArray[0]: " << TargetArray[0] << "\n";

37: cout << "TargetArray[24]: " << TargetArray[24] << "\n";

38: cout << "TargetArray[25]: " << TargetArray[25] << "\n\n";

39: for (i = 0; i<3; i++)

40: {

41: cout << "sentinelOne[" << i << "]: ";

42: cout << sentinelOne[i]<< "\n";

43: cout << "sentinelTwo[" << i << "]: ";

44: cout << sentinelTwo[i]<< "\n";

45: }

46:

47: return 0;

48: }

OUTPUT:

Test 1:

TargetArray[0]: 0

TargetArray[24]: 0

SentinelOne[0]: 0

SentinelTwo[0]: 0

SentinelOne[1]: 0

SentinelTwo[1]: 0

SentinelOne[2]: 0

SentinelTwo[2]: 0

Assigning...

Test 2:

TargetArray[0]: 20

TargetArray[24]: 20

TargetArray[25]: 20

SentinelOne[0]: 20

SentinelTwo[0]: 0

SentinelOne[1]: 0

SentinelTwo[1]: 0

SentinelOne[2]: 0

SentinelTwo[2]: 0

ANALYSIS: Lines 9 and 10 declare two arrays of three integers that act as sentinels around TargetArray. These sentinel arrays are initialized with the value 0. If memory is written to beyond the end of TargetArray, the sentinels are likely to be changed. Some compilers count down in memory; others count up. For this reason, the sentinels are placed on both sides of TargetArray.

Lines 19-29 confirm the sentinel values in Test 1. In line 33 TargetArray’s members are all initialized to the value 20, but the counter counts to TargetArray offset 25, which doesn’t exist in TargetArray.

Lines 36-38 print TargetArray’s values in Test 2. Note that TargetArray[25] is perfectly happy to print the value 20. However, when SentinelOne and SentinelTwo are printed, SentinelTwo[0] reveals that its value has changed. This is because the memory that is 25 elements after TargetArray[0] is the same memory that is at SentinelTwo[0]. When the nonexistent TargetArray[0] was accessed, what was actually accessed was SentinelTwo[0].

This nasty bug can be very hard to find, because SentinelTwo[0]’s value was changed in a part of the code that was not writing to SentinelTwo at all.

This code uses "magic numbers" such as 3 for the size of the sentinel arrays and 25 for the size of TargetArray. It is safer to use constants, so that you can change all these values in one place.

Fence Post Errors

It is so common to write to one past the end of an array that this bug has its own name. It is called a fence post error. This refers to the problem in counting how many fence posts you need for a 10-foot fence if you need one post for every foot. Most people answer 10, but of course you need 11. Figure 11.2 makes this clear.

Figure 11.2. Fence post errors.

This sort of "off by one" counting can be the bane of any programmer’s life. Over time, however, you’ll get used to the idea that a 25-element array counts only to element 24, and that everything counts from 0. (Programmers are often confused why office buildings don’t have a floor zero. Indeed, some have been known to push the 4 elevator button when they want to get to the fifth floor.)

NOTE: Some programmers refer to ArrayName[0] as the zeroth element. Getting into this habit is a big mistake. If ArrayName[0] is the zeroth element, what is ArrayName[1]? The oneth? If so, when you see ArrayName[24], will you realize that it is not the 24th element, but rather the 25th? It is far better to say that ArrayName[0] is at offset zero and is the first element.