C - Program - 2e - Chapter06.pdf - po ang - Januszek66

Arrays

CHAPTER OBJECTIVES

Introduce the concept of an array—which is a list of variables

with the same name and requires an index value.

KEY TERMS AND CONCEPTS

array

array dimension

array index

array pointers

arrays and functions

atoi,atof functions

character string

C-string

element

filling arrays from data files

ifstream

multidimensional array

null character

null-terminated string

ofstream

out of bounds array

single-dimensional array

zero-indexed

Demonstrate how the C++ language automatically creates a

pointer to any array that is declared.

Present single (one-dimensional) and multiply dimensioned

(two-dimensional, three-dimensional, etc.) array concepts.

Illustrate how arrays are passed and used in functions.

Explore the problems of an array that is out of bounds in a C++

program.

Explain how to read data from a data file, a particularly useful

tool when working with arrays.

Present example programs that read data from a file into an

array and then pass the array to functions.

KEYWORDS AND OPERATORS

array index [ ]

Run Faster! Jump Higher!

O ne part of writing software is designing the data variables to represent

accurately the situation the program models. The software may be a

computer game, an accounting program, an engineering data analysis

package or drivers for a hardware device. Whatever the application, it is impor-

tant to spend time during the program design phase thinking about what vari-

ables and their data types are needed.

In the previous chapters, we declared variables so that each one provided

a location for storing one value. We had variables such as age, name, sum, and

count. We asked the user his name and age, calculated PI and weekly pay val-

ues. In each case, we had one variable for each data value. Your work built up

your confidence using different variable types. Now we raise the bar and say,

“Run faster! Jump higher!”

In this chapter, we expand our C++ knowledge and learn how to declare

a single variable that contains a list of data values of the same type. These

new data types should be grouped logically i.e., they should relate naturally to

each other. For example, you may declare a single variable that is actually a

list of phone bills for one year. It’s time to run faster, jump higher, and expand

your C++ skills.

6.1

Using Single Data Variables

Let’s start with a programming problem that shows how the ability to create a

single variable containing a list of values can simplify life. Assume that you need

to write a program that totals and averages the phone bills for a year, starting in

January and ending with December. You need twelve variables—one for each

month in the year—and you need to ask the user to enter these twelve values.

Your program calculates and reports the average value. (Eventually it will be

nice to have the program read the numbers from a data file instead of entering

the data by hand. We will learn how to read data files later in this chapter.)

First, we must name our twelve variables. How about using abbreviations for

the months as variable names to keep track of the twelve months? Then we ask for

the numbers and calculate the average cost.

293

294

❚

Chapter 6

Arrays

//Program 6-Incomplete program for finding the yearly total and

//average monthly phone bill.

#include <iostream>

using namespace std;

int main()

{

// Declare 12 individual variables

float jan, feb, mar, apr, may, jun;

float jul, aug, sept, oct, nov, dec;

float ave, yearSum;

// Obtain monthly billing information

cout << "\n Please enter your bill for January:";

cin >> jan;

cout << "\n Please enter your bill for February:";

cin >> feb;

cout << "\n Please enter your bill for March:";

cin >> mar;

// Program needs to ask for values for Apr to Dec here

// This is left as an exercise for the reader. ;-)

// Now average

sum = jan + feb + mar + apr + may + jun + jul

+ aug + sept + oct + nov + dec;

ave = sum/12.0;

cout << "\n Total yearly phone cost is $" << sum

<< "\n Average monthly phone bill is $"

<< ave << endl;

return 0;

}

array

list of variables of the

same data type ref-

erenced with a single

name

Writing a program in this manner is enough to drive anyone crazy. We will need

twelve input statements. Then the average calculation takes three lines. Can you

imagine what a hassle it would be to write out the twelve individual bills? There

must be a better way to do this. Yes, there is. We will use an array .

6.2

Array Fundamentals

C++ allows the programmer to declare an array variable that groups together

variables of the same data type, and references to this group of values can be made

with a single name. Each array member or element is accessed via the array name

and the array index . An array index is an integer value. The general format for an

array declaration is:

dataType arrayName[ size ];

where dataType is float , int , double , etc., the arrayName is the variable name for

the array, and size is an integer that represents how many variables are in this array.

For the phone bills program, we declare an array of twelve floating point values:

float phone_bills[12];

array variable

a variable in C++

that contains a num-

ber of elements of

the same type, refer-

enced with same

name

element

member of an array

array index

the integer value

that references a

specific element or

member of an array

295

❚

Section 6.2

Array Fundamentals

The size is often referred to as the array dimension. When a programmer

creates a list using one dimension (one size) value, as we did with phone_bills ,

it is referred to as a single-dimensional array and can be thought of as a single

list or as a row of values. These values are stored contiguously in memory.

Figure 6-1 illustrates the phone_bills array. It is useful to visualize an array

as a group of boxes. Each box represents a separate variable and each box has its

own name. The array index is used to access the individual elements in the array.

For example, when we assign the values into each element, we must use the array

name and an integer index. This combination of array name plus index is how we

refer to that specific variable location in memory.

It is possible to make arrays in C++ of any data type or class. For example,

here are four more array declaration statements:

single-dimensional

array

an array that repre-

sents a single list or

column of values

int numbers[1000];

// array of 1000 integers, named numbers

double rays[200];

// array of 200 double variables, named rays

string students[25];

// array of 25 strings, named students

char name[25];

// array of 25 characters

The last declaration ( char name[25]; ) is an array of characters known as a

null-terminated character string .This name array is designed to hold up to 24 in-

dividual letters/characters with the 25 th character being a null (a null is ASCII

zero). These character strings or character arrays are also referred to as C-strings ,

and were used in the C language to handle textual data in programs before the

C++ string class was invented. (Remember that the C language was in use by the

late 1970s and C++ didn’t come into existence until the late 1990s.) The C-string

was widely used, and is commonly found in C and C++ code today. We’ll cover it

later in detail in this chapter.

null-terminated

character string or

C-string

a character array

(C-string) that has a

null character (zero)

at the end of the

pertinent data

int main()

{

phone_bills[0]

float phone_bills[12];

phone_bills[1]

phone_bills[2]

Each box represents one element

of the array.

phone_bills[11]

Array

name

Index

Figure 6-1

A single-dimensional

array.

Think of a one-dimensional array as a column of boxes — each

box is referred to with name

integer index.

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❚

Chapter 6

Arrays

Arrays in C++ are Zero Indexed

C++ arrays are referred to as zero indexed , which means that the array elements

(boxes in Figure 6-1) are numbered starting with zero, not one! The name of the

first element of any array has zero as the first index, and the last element’s name

is the size index value. In the phone bill array, the first element (box) is

phone_bills[0] and the last element is phone_bills[11] . Recall that a C++

string object’s first character and a C++ vector’s first element is indexed at 0, too.

Some programming languages, such as FORTRAN, allow the programmer to

specify the starting array index. C++ does not allow this choice. Some beginning C++

programmers want to add an additional array element to the declaration and then

ignore the first (index of zero) array element to make coding “easier.” This technique

is not recommended. All arrays in C++ have the first index value of zero, and the last

element is one less than the size. Do not create your own indexing scheme!

zero-indexed:

when the first ele-

ment of an array is

referenced using a

zero [0]

-1

for Loops and Arrays and the Phone Bills Program

When writing software with arrays, the for loop is the programmer’s best friend.

The for loop provides an efficient method for going through (traversing) an array.

The index of the loop is used not only as a counter for the loop, it can also be used

as the index value for the array. If you are not comfortable writing for loops, go

back to Chapter 3, reread the for loop section, and look at the practice sample

programs.

The Phone Bills program can be written with a for loop that makes coding

much easier. The loop index is used to access each element of our phone bill array.

Program 6-1 asks the user to enter the bill amounts for months 1 to 12. Figure 6-2 illus-

trates how the for loop index variable is used to access the array elements. Figure 6-3

// Obtain monthly billing information

for(i = 0; i < 12; ++i)

{

cout << "\n Enter bill for month # " << i + 1<< "$";

cin >> phone_bills[i];

}

45.14

phone_bills[0]

45.14

phone_bills[1]

47.72

phone_bills[2]

The first time this loop runs, i = 0.

It asks the user for month number 1 (i + 1 = 0 + 1 = 1).

The user's value (i.e., 45.14) is placed in phone_bills[0].

The second time this loop runs, i = 1.

It asks the user for month number 2.

The value the user enters (i.e., 45.14) is placed in phone_bills[1].

The last time this loop runs, i = 11.

It asks the user for month number 12.

The value the user enters (i.e., 48.99) is placed in phone_bills[11].

48.99

phone_bills[11]

Figure 6-2

The for loop is a convenient tool for accessing array elements.

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