C++ Primer Plus (P15) pptx

Listing 5.19 incorporates an initialized two-dimensional array and a nested loop into a program.. A loop cycles through the same set of instructions repetitively as long as the loop test

Suppose you want to print all the array contents Then, you can use one for loop to change

rows and a second, nested for loop to change columns:

for (int row = 0; row < 4; row++)

{

for (int col = 0; col < 5; col++)

cout << maxtemps[row][col] << "\ t";

cout << "\n";

}

For each value of row, the inner for loop cycles through all the col values This example

prints a tab character (\t in C++ notation) after each value and a newline character after

each complete row

Initializing a Two-Dimensional Array

When you create a two-dimensional array, you have the option of initializing each element

The technique is based on that for initializing a one-dimensional array That method, you

remember, is to provide a comma-separated list of values enclosed in braces:

// initializing a one-dimensional array

int btus[5] = { 23, 26, 24, 31, 28};

For a two-dimensional array, each element is itself an array, so you can initialize each

element by using a form like that in the previous code example Thus, the initialization

consists of a comma-separated series of one-dimensional initializations all enclosed in a

set of braces:

int maxtemps[4][5] = // 2-D array

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{94, 98, 87, 103, 101} , // values for maxtemps[0]

{98, 99, 91, 107, 105} , // values for maxtemps[1]

{93, 91, 90, 101, 104} , // values for maxtemps[2]

{95, 100, 88, 105, 103} // values for maxtemps[3]

};

The term {94, 98, 87, 103, 101} initializes the first row, represented by maxtemps[0] As

a matter of style, placing each row of data on its own line, if possible, makes the data

easier to read

Listing 5.19 incorporates an initialized two-dimensional array and a nested loop into a

program This time the program reverses the order of the loops, placing the column loop

(city index) on the outside and the row loop (year index) on the inside Also, it uses a

common C++ practice of initializing an array of pointers to a set of string constants That is,

cities is declared as an array of pointers-to-char That makes each element, such as

cities[0], a pointer-to-char that can be initialized to the address of a string The program

initializes cities[0] to the address of the "Gribble City" string, and so on Thus, this array

of pointers behaves like an array of strings

Listing 5.19 nested.cpp

// nested.cpp nested loops and 2-D array

#include <iostream>

using namespace std;

const int Cities = 5;

const int Years = 4;

int main()

{

const char * cities[Cities] = // array of pointers

{ // to 5 strings

"Gribble City",

"Gribbletown",

"New Gribble",

"San Gribble",

"Gribble Vista"

};

int maxtemps[Years][Cities] = // 2-D array

{

{95, 99, 86, 100, 104} , // values for maxtemps[0]

{95, 97, 90, 106, 102} , // values for maxtemps[1]

{96, 100, 940, 107, 105} , // values for maxtemps[2]

{97, 102, 89, 108, 104} // values for maxtemps[3]

};

cout << "Maximum temperatures for 1999 - 2002\n\n";

for (int city = 0; city < Cities; city++)

{

cout << cities[city] << ":\ t";

for (int year = 0; year < Years; year++)

cout << maxtemps[year][city] << "\ t";

cout << "\n";

}

return 0;

}

Here is the program output:

Maximum temperatures for 1999 - 2002

Gribble City: 95 95 96 97

Gribbletown: 99 97 100 102

New Gribble: 86 90 940 89

San Gribble: 100 106 107 108

Gribble Vista: 104 102 105 104

Using tabs in the output spaces the data more regularly than using spaces would

However, different tab settings can cause the output to vary in appearance from one

system to another Chapter 17 presents more precise, but more complex, methods for

formatting output

Summary

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C++ offers three varieties of loops: the for loop, the while loop, and the do while loop A

loop cycles through the same set of instructions repetitively as long as the loop test

condition evaluates to true or nonzero, and the loop terminates execution when the test

condition evaluates to false or zero The for loop and the while loop are entry-condition

loops, meaning they examine the test condition before executing the statements in the

body of the loop The do while loop is an exit-condition loop, meaning it examines the test

condition after executing the statements in the body of the loop

The syntax for each loop calls for the loop body to consist of a single statement However,

that statement can be a compound statement, or block, formed by enclosing several

statements within paired curly braces

Relational expressions, which compare two values, are often used as loop test conditions

Relational expressions are formed by using one of the six relational operators: <, <=, ==,

>=, >, or != Relational expressions evaluate to the type bool values true and false

Many programs read text input or text files character-by-character The istream class

provides several ways to do this If ch is a type char variable, the statement

cin >> ch;

reads the next input character into ch However, it skips over spaces, newlines, and tabs

The member function call

cin.get(ch);

reads the next input character, regardless of its value, and places it in ch The member

function call cin.get() returns the next input character, including spaces, newlines, and

tabs, so it can be used as follows:

ch = cin.get();

The cin.get(char) member function call reports encountering the end-of-file condition by

returning a value with the bool conversion of false, whereas the cin.get() member function

call reports end-of-file by returning the value EOF, which is defined in the iostream file

A nested loop is a loop within a loop Nested loops provide a natural way to process

two-dimensional arrays

Review Questions

.1: What's the difference between an entry-condition loop and an exit-condition loop? Which kind is each of the C++ loops?

.2: What would the following code fragment print if it were part of a valid program?

int i;

for (i = 0; i < 5; i++) cout << i;

cout << "\n";

.3: What would the following code fragment print if it were part of a valid program?

int j;

for (j = 0; j < 11; j += 3) cout << j;

cout << "\n" << j << "\n";

.4: What would the following code fragment print if it were part of a valid program?

int j = 5;

while ( ++j < 9) cout << j++ << "\n";

.5: What would the following code fragment print if it were part of a valid program?

int k = 8;

do cout <<" k = " << k << "\n";

while (k++ < 5);

.6: Write a for loop that prints the values 1 2 4 8 16 32 64 by increasing the value

of a counting variable by a factor of 2 each cycle

.7: How do you make a loop body include more than one statement?

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.8: Is the following statement valid? If not, why not? If so, what does it do?

int x = (1,024);

What about the following?

int y;

y = 1,024;

.9: How does cin>>ch differ from cin.get(ch) and ch=cin.get() in how it views input?

Programming Exercises

1: Write a program that requests the user to enter two integers The program then should calculate and report the sum of all the integers between and including the two integers At this point, assume that the smaller integer is entered first For example, if the user enters 2 and 9, the program reports that the sum of all the integers from 2 through 9 is 44

2: Write a program that asks you to type in numbers After each entry, the number reports the cumulative sum of the entries to date The program terminates when you enter a zero

3: Daphne invests $100 at 10% simple interest That is, every year, the investment earns 10% of the original investment, or $10 each and every year:

interest = 0.10 x original balance

At the same time, Cleo invests $100 at 5% compound interest That is, interest

is 5% of the current balance, including previous additions of interest:

interest = 0.05 x current balance

Cleo earns 5% of $100 the first year, giving her $105 The next year she earns

5% of $105, or $5.25, and so on Write a program that finds how many years it takes for the value of Cleo's investment to exceed the value of Daphne's investment and then displays the value of both investments at that time

4: You sell C++ For Fools Write a program that has you enter a year's worth of

monthly sales (in terms of number of books, not of money) The program should use a loop to prompt you by month, using an array of char * initialized to the month strings and storing the input data in an array of int Then, the program should find the sum of the array contents and report the total sales for the year

5: Do Programming Exercise 4 but use a two-dimensional array to store input for three years of monthly sales Report the total sales for each individual year and for the combined years

6: Design a structure called car that holds the following information about an automobile: its make as a string in a character array and the year it was built as

an integer Write a program that asks the user how many cars to catalog The program then should use new to create a dynamic array of that many car structures Next, it should prompt the user to input the make (which might consist of more than one word) and year information for each structure Note that this requires some care, for it alternates reading strings with numeric data (see Chapter 4) Finally, it should display the contents of each structure A sample run should look something like the following:

How many cars do you wish to catalog? 2

Car #1:

Please enter the make: Hudson Hornet

Please enter the year made: 1952

Car #2:

Please enter the make: Kaiser

Please enter the year made: 1951

Here is your collection:

1952 Hudson Hornet

1951 Kaiser

7: Write a program using nested loops that asks the user to enter a value for the number of rows to display It then displays that many rows of asterisks, with one This document was created by an unregistered ChmMagic, please go to http://www.bisenter.com to register it Thanks

asterisk in the first row, two in the second row, and so on For each row, the asterisks are preceded by the number of periods needed to make all the rows display a total number of characters equal to the number of rows A sample run would look like this:

Enter number of rows: 5

*

**

***

.****

*****

CONTENTS

Chapter 6 BRANCHING STATEMENTS AND

LOGICAL OPERATORS

In this chapter you learn

The if Statement Logical Expressions The cctype Library of Character Functions The ?: Operator

The switch Statement The break and continue Statements Number-Reading Loops

Summary Review Questions Programming Exercises

One of the keys to designing intelligent programs is to give them the ability to make

decisions Chapter 5, "Loops and Relational Expressions," shows you one kind of decision

making—looping—in which a program decides whether or not to continue looping Now

you investigate how C++ lets you use branching statements to decide among alternative

actions Which vampire-protection scheme (garlic or cross) should the program use? What

menu choice has the user selected? Did the user enter a zero? C++ provides the if and

switch statements to implement decisions, and they are this chapter's main topics You

also look at the conditional operator, which provides another way to make a choice, and

the logical operators, which let you combine two tests into one

The if Statement

When a C++ program must choose whether or not to take a particular action, you usually

implement the choice with an if statement The if comes in two forms: if and if else Let's

investigate the simple if first It's modeled after ordinary English, as in "If you have a

Captain Cookie card, you get a free cookie." The if statement directs a program to execute

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a statement or statement block if a test condition is true and to skip that statement or block

if the condition is false Thus, an if statement lets a program decide whether a particular

statement should be executed

The syntax is similar to the while syntax:

if (test-condition)

statement

A true test-condition causes the program to execute statement, which can be a single

statement or a block A false test-condition causes the program to skip statement. (See

Figure 6.1.) As with loop test conditions, an if test condition is typecast to a bool value, so

zero becomes false and nonzero becomes true The entire if construction counts as a

single statement

Figure 6.1 The if statement.

Most often, test-condition is a relational expression such as those used to control loops.

Suppose, for example, you want a program that counts the spaces in the input as well as

the total number of characters You can use cin.get(char) in a while loop to read the

characters and then use an if statement to identify and count the space characters Listing

6.1 does just that, using the period to recognize the end of a sentence

Listing 6.1 if.cpp

// if.cpp using the if statement

#include <iostream>

using namespace std;

int main()

{

char ch;

int spaces = 0;

int total = 0;

cin.get(ch);

while (ch != '.') // quit at end of sentence

{

if (ch == ' ') // check if ch is a space

spaces++;

total++; // done every time

cin.get(ch);

}

cout << spaces << " spaces, " << total;

cout << " characters total in sentence\n";

return 0;

}

Here's some sample output:

The balloonist was an airhead

with lofty goals.

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6 spaces, 46 characters total in sentence

As the comments indicate, the spaces++; statement is executed only when ch is a space

Because it is outside the if statement, the total++; statement is executed every loop cycle

Note that the total count includes the newline character generated by pressing Enter

The if else Statement

While the if statement lets a program decide whether a particular statement or block is

executed, the if else statement lets a program decide which of two statements or blocks is

executed It's an invaluable statement for creating alternative courses of action The C++ if

else is modeled after simple English, as in "If you have a Captain Cookie card, you get a

Cookie Plus Plus, else you just get a Cookie d'Ordinaire." The if else statement has this

general form:

if (test-condition)

statement1

else

statement2

if (answer == 1492)

cout << "That's right!\n";

else

cout << "You'd better review Chapter 1 again.\n";

prints the first message if answer is 1492 and prints the second message otherwise Each

statement can be either a single statement or a statement block delimited by braces (See

Figure 6.2.) The entire if else construct counts syntactically as a single statement

Figure 6.2 The if else statement.