Читаем C++ Primer Plus полностью

The loop performs initialization just once. Typically, programs use this expression to set a variable to a starting value and then use the variable to count loop cycles.

test-expression determines whether the loop body gets executed. Typically, this expression is a relational expression—that is, one that compares two values. Our example compares the value of i to 5, checking whether i is less than 5. If the comparison is true, the program executes the loop body. Actually, C++ doesn’t limit test-expression to true/false comparisons. You can use any expression, and C++ will type cast it to type bool. Thus, an expression with a value of 0 is converted to the bool value false, and the loop terminates. If the expression evaluates to nonzero, it is type cast to the bool value true, and the loop continues. Listing 5.2 demonstrates this by using the expression i as the test condition. (In the update section, i-- is similar to i++ except that it decreases the value of i by 1 each time it’s used.)

Listing 5.2. num_test.cpp

// num_test.cpp -- use numeric test in for loop

#include

int main()

{

    using namespace std;

    cout << "Enter the starting countdown value: ";

    int limit;

    cin >> limit;

    int i;

    for (i = limit; i; i--)     // quits when i is 0

        cout << "i = " << i << "\n";

    cout << "Done now that i = " << i << "\n";

    return 0;

}

Here is the output from the program in Listing 5.2:

Enter the starting countdown value: 4

i = 4

i = 3

i = 2

i = 1

Done now that i = 0

Note that the loop terminates when i reaches 0.

How do relational expressions, such as i < 5, fit into this framework of terminating a loop with a 0 value? Before the bool type was introduced, relational expressions evaluated to 1 if true and 0 if false. Thus, the value of the expression 3 < 5 was 1, and the value of 5 < 5 was 0. Now that C++ has added the bool type, however, relational expressions evaluate to the bool literals true and false instead of 1 and 0. This change doesn’t lead to incompatibilities, however, because a C++ program converts true and false to 1 and 0 where integer values are expected, and it converts 0 to false and nonzero to true where bool values are expected.

The for loop is an entry-condition loop. This means the test expression is evaluated before each loop cycle. The loop never executes the loop body when the test expression is false. For example, suppose you rerun the program in Listing 5.2 but give 0 as a starting value. Because the test condition fails the very first time it’s evaluated, the loop body never gets executed:

Enter the starting countdown value: 0

Done now that i = 0

This look-before-you-loop attitude can help keep a program out of trouble.

update-expression is evaluated at the end of the loop, after the body has been executed. Typically, it’s used to increase or decrease the value of the variable keeping track of the number of loop cycles. However, it can be any valid C++ expression, as can the other control expressions. This makes the for loop capable of much more than simply counting from 0 to 5, the way the first loop example does. You’ll see some examples of this later.

The for loop body consists of a single statement, but you’ll soon learn how to stretch that rule. Figure 5.1 summarizes the for loop design.

Figure 5.1. The design of for loops.

A for statement looks something like a function call because it uses a name followed by paired parentheses. However, for’s status as a C++ keyword prevents the compiler from thinking for is a function. It also prevents you from naming a function for.

Tip

Common C++ style is to place a space between for and the following parenthesis and to omit space between a function name and the following parenthesis:

for (i = 6; i < 10; i++)

      smart_function(i);

Other control statements, such as if and while, are treated similarly to for. This serves to visually reinforce the distinction between a control statement and a function call. Also common practice is to indent the body of a for statement to make it stand out visually.

Expressions and Statements

A for control section uses three expressions. Within its self-imposed limits of syntax, C++ is a very expressive language. Any value or any valid combination of values and operators constitute an expression. For example, 10 is an expression with the value 10 (no surprise), and 28 * 20 is an expression with the value 560. In C++ every expression has a value. Often the value is obvious. For example, the following expression is formed from two values and the addition operator, and it has the value 49:

22 + 27

Sometimes the value is less obvious. For example, the following is an expression because it’s formed from two values and the assignment operator:

x = 20

Перейти на страницу:

Все книги серии Developer's Library

C++ Primer Plus
C++ Primer Plus

C++ Primer Plus is a carefully crafted, complete tutorial on one of the most significant and widely used programming languages today. An accessible and easy-to-use self-study guide, this book is appropriate for both serious students of programming as well as developers already proficient in other languages.The sixth edition of C++ Primer Plus has been updated and expanded to cover the latest developments in C++, including a detailed look at the new C++11 standard.Author and educator Stephen Prata has created an introduction to C++ that is instructive, clear, and insightful. Fundamental programming concepts are explained along with details of the C++ language. Many short, practical examples illustrate just one or two concepts at a time, encouraging readers to master new topics by immediately putting them to use.Review questions and programming exercises at the end of each chapter help readers zero in on the most critical information and digest the most difficult concepts.In C++ Primer Plus, you'll find depth, breadth, and a variety of teaching techniques and tools to enhance your learning:• A new detailed chapter on the changes and additional capabilities introduced in the C++11 standard• Complete, integrated discussion of both basic C language and additional C++ features• Clear guidance about when and why to use a feature• Hands-on learning with concise and simple examples that develop your understanding a concept or two at a time• Hundreds of practical sample programs• Review questions and programming exercises at the end of each chapter to test your understanding• Coverage of generic C++ gives you the greatest possible flexibility• Teaches the ISO standard, including discussions of templates, the Standard Template Library, the string class, exceptions, RTTI, and namespaces

Стивен Прата

Программирование, программы, базы данных

Похожие книги

1С: Бухгалтерия 8 с нуля
1С: Бухгалтерия 8 с нуля

Книга содержит полное описание приемов и методов работы с программой 1С:Бухгалтерия 8. Рассматривается автоматизация всех основных участков бухгалтерии: учет наличных и безналичных денежных средств, основных средств и НМА, прихода и расхода товарно-материальных ценностей, зарплаты, производства. Описано, как вводить исходные данные, заполнять справочники и каталоги, работать с первичными документами, проводить их по учету, формировать разнообразные отчеты, выводить данные на печать, настраивать программу и использовать ее сервисные функции. Каждый урок содержит подробное описание рассматриваемой темы с детальным разбором и иллюстрированием всех этапов.Для широкого круга пользователей.

Алексей Анатольевич Гладкий

Программирование, программы, базы данных / Программное обеспечение / Бухучет и аудит / Финансы и бизнес / Книги по IT / Словари и Энциклопедии
1С: Управление торговлей 8.2
1С: Управление торговлей 8.2

Современные торговые предприятия предлагают своим клиентам широчайший ассортимент товаров, который исчисляется тысячами и десятками тысяч наименований. Причем многие позиции могут реализовываться на разных условиях: предоплата, отсрочка платежи, скидка, наценка, объем партии, и т.д. Клиенты зачастую делятся на категории – VIP-клиент, обычный клиент, постоянный клиент, мелкооптовый клиент, и т.д. Товарные позиции могут комплектоваться и разукомплектовываться, многие товары подлежат обязательной сертификации и гигиеническим исследованиям, некондиционные позиции необходимо списывать, на складах периодически должна проводиться инвентаризация, каждая компания должна иметь свою маркетинговую политику и т.д., вообщем – современное торговое предприятие представляет живой организм, находящийся в постоянном движении.Очевидно, что вся эта кипучая деятельность требует автоматизации. Для решения этой задачи существуют специальные программные средства, и в этой книге мы познакомим вам с самым популярным продуктом, предназначенным для автоматизации деятельности торгового предприятия – «1С Управление торговлей», которое реализовано на новейшей технологической платформе версии 1С 8.2.

Алексей Анатольевич Гладкий

Финансы / Программирование, программы, базы данных