Section 1.4. Control Structures

Part of the compiler's job is to look for errors in the program text. A compiler cannot detect whether the meaning of a program is correct, but it can detect errors in the form of the program. The following are the most common kinds of errors a compiler will detect.

编译器的部分工作是寻找程序代码中的错误。编译器不能查出程序的意义是否正确, 但它可以查出程序形式上的错误。下面是编译器能查出的最普遍的一些错误。

Syntax errors. The programmer has made a grammatical error in the C++ language. The following program illustrates common syntax errors; each comment describes the error on the following line:

语法错误。程序员犯了 C++ 语言中的语法错误。下面代码段说明常见的语法错误；每个注释描述下一行的错误。

                     // error: missing ')' in parameter list for main
       int main ( {
                     // error: used colon, not a semicolon after endl
           std::cout << "Read each file." << std::endl:
                     // error: missing quotes around string literal
           std::cout << Update master. << std::endl;
                     // ok: no errors on this line
           std::cout << "Write new master." <<std::endl;
                     // error: missing ';' on return statement
           return 0
       }

Type errors. Each item of data in C++ has an associated type. The value 10, for example, is an integer. The word "hello" surrounded by double quotation marks is a string literal. One example of a type error is passing a string literal to a function that expects an integer argument.

类型错误。C++ 中每个数据项都有其相关联的类型。例如，值 10 是一个整数。用双引号标注起来的单词“hello”是字符串字面值。类型错误的一个实例是传递了字符串字面值给应该得到整型参数的函数。
Declaration errors. Every name used in a C++ program must be declared before it is used. Failure to declare a name usually results in an error message. The two most common declaration errors are to forget to use std:: when accessing a name from the library or to inadvertently misspell the name of an identifier:

声明错误。C++ 程序中使用的每个名字必须在使用之前声明。没有声明名字通常会导致错误信息。最常见的两种声明错误，是从标准库中访问名字时忘记使用“std::”，以及由于疏忽而拼错标识符名:
```
    #include <iostream>
    int main()
    {
        int v1, v2;
        std::cin >> v >> v2; // error: uses " v "not" v1"
        // cout not defined, should be std::cout
        cout << v1 + v2 << std::endl;
        return 0;
     }
```

An error message contains a line number and a brief description of what the compiler believes we have done wrong. It is a good practice to correct errors in the sequence they are reported. Often a single error can have a cascading effect and cause a compiler to report more errors than actually are present. It is also a good idea to recompile the code after each fixor after making at most a small number of obvious fixes. This cycle is known as edit-compile-debug.

错误信息包含行号和编译器对我们所犯错误的简要描述。按错误报告的顺序改正错误是个好习惯，通常一个错误可能会产生一连串的影响，并导致编译器报告比实际多得多的错误。最好在每次修改后或最多改正了一些显而易见的错误后，就重新编译代码。这个循环就是众所周知的编辑—编译—调试。

1.4. Control Structures

1.4. 控制结构

1.4.1. The `while` Statement

1.4.1. `while` 语句

Key Concept: Indentation and Formatting of C++ Programs

关键概念：C++ 程序的缩排和格式

1.4.2. The `for` Statement

1.4.2. `for` 语句

Compilation Revisited

再谈编译

Exercises Section 1.4.2

1.4.3. The `if` Statement

1.4.3. `if` 语句

Exercises Section 1.4.3

1.4.4. Reading an Unknown Number of Inputs

1.4.4. 读入未知数目的输入

Entering an End-of-file from the Keyboard

从键盘输入文件结束符

Exercises Section 1.4.4