"TCPL/1.02_Variables_and_Arithmetic_Expressions"版本比较

1.2 Variables and Arithmetic Expressions 变量与算术表达式

The next program uses the formula ℃=(5/9)(℉-32) to print the following table of Fahrenheit temperatures and their centigrade or Celsius equivalents:

   1    -17
   20   -6
   40   4
   60   15
   80   26
   100  37
   120  48
   140  60
   160  71
   180  82
   200  93
   220  104
   240  115
   260  126
   280  137
   300  148

The program itself still consists of the definition of a single function named main. It is longer than the one that printed "hello, world", but not complicated. It introduces several new ideas, including comments, declarations, variables, arithmetic expressions, loops , and formatted output.

   1    #include <stdio.h>
   2 
   3    /* print Fahrenheit-Celsius table
   4        for fahr = 0, 20, ..., 300 */
   5    main()
   6    {
   7      int fahr, celsius;
   8      int lower, upper, step;
   9 
  10      lower = 0;      /* lower limit of temperature scale */
  11      upper = 300;    /* upper limit */
  12      step = 20;      /* step size */
  13 
  14      fahr = lower;
  15      while (fahr <= upper) {
  16          celsius = 5 * (fahr-32) / 9;
  17          printf("%d\t%d\n", fahr, celsius);
  18          fahr = fahr + step;
  19      }
  20    }

The two lines

  /* print Fahrenheit-Celsius table
      for fahr = 0, 20, ..., 300 */

are a comment, which in this case explains briefly what the program does. Any characters between and are ignored by the compiler; they may be used freely to make a program easier to understand. Comments may appear anywhere where a blank, tab or newline can.

我们来看下一个程序，使用公式℃=(5/9)(℉-32) 打印下列华氏温度与摄氏温度对照表

   1    -17
   20   -6
   40   4
   60   15
   80   26
   100  37
   120  48
   140  60
   160  71
   180  82
   200  93
   220  104
   240  115
   260  126
   280  137
   300  148

此程序中仍然只包括一个名为main的函数定义。它比前面打印"hello, world"的程序长一些，但并不复杂。这个程序中引入了一些新的概念，包括注释、声明、变量、算术表达式、循环以及格式化输出。该程序如下所示：

   1 #include <stdio.h>
   2 
   3 /* print Fahrenheit-Celsius table
   4    for fahr = 0, 20, ..., 300 */
   5 main()
   6 {
   7     int fahr, celsius;
   8     int lower, upper, step;
   9 
  10     lower = 0;      /* lower limit of temperature scale */
  11     upper = 300;    /* upper limit */
  12     step = 20;      /* step size */
  13 
  14     fahr = lower;
  15     while (fahr <= upper) {
  16         celsius = 5 * (fahr-32) / 9;
  17         printf("%d\t%d\n", fahr, celsius);
  18         fahr = fahr + step;
  19     }
  20 }

其中的两行：

  /* print Fahrenheit-Celsius table
      for fahr = 0, 20, ..., 300 */

称为注释，此处，它简单地解释了该程序是做什么用的。包含在/*与*/之间的字符序列将被编译器忽略。注释可以自由地运用在程序中，使得程序更易于理解。程序中允许出现空格、制表符或换行符之处，都可以使用注释。

In C, all variables must be declared before they are used, usually at the beginning of the function before any executable statements. A declaration announces the properties of variables; it consists of a name and a list of variables, such as

    int fahr, celsius;
    int lower, upper, step;

The type int means that the variables listed are integers; by contrast with float, which means floating point, i.e., numbers that may have a fractional part. The range of both int and float depends on the machine you are using; 16-bits ints, which lie between -32768 and +32767, are common, as are 32-bit ints. A float number is typically a 32-bit quantity, with at least six significant digits and magnitude generally between about 10^-38 and 10³⁸.

在C语言中，所有变量都必须先声明后使用。声明通常放在函数起始处，在任何可执行语句之前。声明用于说明变量的属性，它由一个类型名和一个变量表组成，例如：

    int fahr, celsius;
    int lower, upper, step;

其中，类型int表示其后所列变员为整数，与之相对应的，float表示所列变量为浮点数(即，可以带有小数部分的数)。int与float类型的取值范围取决于具体的机器。对于int 类型，通常为16位，其取位范围在-32768～+32767之间，也有用32位表示的int类型。【czk注：英文原意是32位的int类型和16位的一样常见。现在的实际情况是32位更常见一些。】 float类型通常是32位，它至少有6位有效数字，取位范围一般在10^-38～10⁺³⁸之间。

C provides several other data types besides int and float, including:

 char      character - a single byte
 short     short integer
 long      long integer
 double    double-precision floating point 

The size of these objects is also machine-dependent. There are also arrays, structures and unions of these basic types, pointers to them, and functions that return them, all of which we will meet in due course.

除int与float类型之外，C语言还提供了其他一些基本数据类型，例如：

 char      字符 - 一个字节
 short     短整型
 long      长整型
 double    双精度浮点型

这些数据类型对象的大小也取决于具体的机器。另外．还存在这些基本数据类型的数组、结构、联合，指向这些类型的指针以及返回这些类型值的函数。我们将在后续相应的章节中分别介绍。

Computation in the temperature conversion program begins with the assignment statements

    lower = 0;
    upper = 300;
    step = 20;
    fahr = lower;

which set the variables to their initial values. Individual statements are terminated by semicolons.

在上面的温度转换程序中，最开始执行的计算是下列4个赋值语句：

    lower = 0;
    upper = 300;
    step = 20;
    fahr = lower;

它们为变量设置初始值。各语句均以分号结束。

Each line of the table is computed the same way, so we use a loop that repeats once per output line; this is the purpose of the while loop

    while (fahr <= upper) {
       ...
    }

The while loop operates as follows: The condition in parentheses is tested. If it is true (fahr is less than or equal to upper), the body of the loop (the three statements enclosed in braces) is executed. Then the condition is re-tested, and if true, the body is executed again. When the test becomes false (fahr exceeds upper) the loop ends, and execution continues at the statement that follows the loop. There are no further statements in this program, so it terminates.

温度转换表中的各行计算方式相同，因此可以用循环语句重复输出各行。这是while循环语句的用途：

while (fahr <= upper) {
    ...
}

while循环语句的执行方式是这样的：首先测试圆括号中的条件；如果条件为真(fahr<=upper)，则执行循环体(括在花括导中的3条语句)；然后再重新测试圆括号中的条件，如果为真，则再次执行循环体；当圆括号中的条件测试结果为假(fahr>upper)时，循环结束，并继续执行跟在while循环语句之后的下一条语句。在本程序中，循环语句后没有其他语句，因此整个程序的执行终止。

The body of a while can be one or more statements enclosed in braces, as in the temperature converter, or a single statement without braces, as in

   while (i < j)
       i = 2 * i;

In either case, we will always indent the statements controlled by the while by one tab stop (which we have shown as four spaces) so you can see at a glance which statements are inside the loop. The indentation emphasizes the logical structure of the program. Although C compilers do not care about how a program looks, proper indentation and spacing are critical in making programs easy for people to read. We recommend writing only one statement per line, and using blanks around operators to clarify grouping. The position of braces is less important, although people hold passionate beliefs. We have chosen one of several popular styles. Pick a style that suits you, then use it consistently.

while语句的循环体可以是用花括号括起来的一条或多条语句(如上画的温度转换程序)，也可以是不用花括号包括的单条语句，例如：

while (i < j)
    i = 2 * i;

在这两种情况下，我们总是把由while控制的语句缩进一个制表位，这样就可以很容易地看出循环语句中包含哪些语句。这种缩进方式突出了程序的逻辑结构。尽管C编译器并不关心程序的外观形式，但正确的缩址以及保留适当空格的程序设计风格对程序的易渎性非常重要。我们建议每行只书写一条语句，并在运算符两边各加上一个空格字符，这样可以使得运算的结合关系更清楚明了。相比而言，花括号的位置就不那么重要了。我们从比较流行的一些风格巾选择了一种。读者可以选择适合自己的一种风格．并养成一直使用这种风格的好习惯。

Most of the work gets done in the body of the loop. The Celsius temperature is computed and assigned to the variable celsius by the statement

        celsius = 5 * (fahr-32) / 9;

The reason for multiplying by 5 and dividing by 9 instead of just multiplying by 5/9 is that in C, as in many other languages, integer division truncates: any fractional part is discarded. Since 5 and 9 are integers. 5/9 would be truncated to zero and so all the Celsius temperatures would be reported as zero.

This example also shows a bit more of how printf works. printf is a general-purpose output formatting function, which we will describe in detail in Chapter 7. Its first argument is a string of characters to be printed, with each % indicating where one of the other (second, third, ...) arguments is to be substituted, and in what form it is to be printed. For instance, %d specifies an integer argument, so the statement

        printf("%d\t%d\n", fahr, celsius);

causes the values of the two integers fahr and celsius to be printed, with a tab (\t) between them.

Each % construction in the first argument of printf is paired with the corresponding second argument, third argument, etc.; they must match up properly by number and type, or you will get wrong answers.

By the way, printf is not part of the C language; there is no input or output defined in C itself. printf is just a useful function from the standard library of functions that are normally accessible to C programs. The behaviour of printf is defined in the ANSI standard, however, so its properties should be the same with any compiler and library that conforms to the standard.

In order to concentrate on C itself, we don't talk much about input and output until chapter 7. In particular, we will defer formatted input until then. If you have to input numbers, read the discussion of the function scanf in Section 7.4. scanf is like printf, except that it reads input instead of writing output.

There are a couple of problems with the temperature conversion program. The simpler one is that the output isn't very pretty because the numbers are not right-justified. That's easy to fix; if we augment each %d in the printf statement with a width, the numbers printed will be right-justified in their fields. For instance, we might say

   printf("%3d %6d\n", fahr, celsius);

to print the first number of each line in a field three digits wide, and the second in a field six digits wide, like this:

     0     -17
    20      -6
    40       4
    60      15
    80      26
   100      37
   ...

The more serious problem is that because we have used integer arithmetic, the Celsius temperatures are not very accurate; for instance, 0oF is actually about -17.8oC, not -17. To get more accurate answers, we should use floating-point arithmetic instead of integer. This requires some changes in the program. Here is the second version:

   1    #include <stdio.h>
   2 
   3    /* print Fahrenheit-Celsius table
   4        for fahr = 0, 20, ..., 300; floating-point version */
   5    main()
   6    {
   7      float fahr, celsius;
   8      float lower, upper, step;
   9 
  10      lower = 0;      /* lower limit of temperatuire scale */
  11      upper = 300;    /* upper limit */
  12      step = 20;      /* step size */
  13 
  14      fahr = lower;
  15      while (fahr <= upper) {
  16          celsius = (5.0/9.0) * (fahr-32.0);
  17          printf("%3.0f %6.1f\n", fahr, celsius);
  18          fahr = fahr + step;
  19      }
  20    }

This is much the same as before, except that fahr and celsius are declared to be float and the formula for conversion is written in a more natural way. We were unable to use 5/9 in the previous version because integer division would truncate it to zero. A decimal point in a constant indicates that it is floating point, however, so 5.0/9.0 is not truncated because it is the ratio of two floating-point values.

If an arithmetic operator has integer operands, an integer operation is performed. If an arithmetic operator has one floating-point operand and one integer operand, however, the integer will be converted to floating point before the operation is done. If we had written (fahr-32), the 32 would be automatically converted to floating point. Nevertheless, writing floating-point constants with explicit decimal points even when they have integral values emphasizes their floating-point nature for human readers.

The detailed rules for when integers are converted to floating point are in Chapter 2. For now, notice that the assignment

   fahr = lower;

and the test

   while (fahr <= upper)

also work in the natural way - the int is converted to float before the operation is done.

The printf conversion specification %3.0f says that a floating-point number (here fahr) is to be printed at least three characters wide, with no decimal point and no fraction digits. %6.1f describes another number (celsius) that is to be printed at least six characters wide, with 1 digit after the decimal point. The output looks like this:

     0   -17.8
    20    -6.7
    40     4.4
   ...

Width and precision may be omitted from a specification: %6f says that the number is to be at least six characters wide; %.2f specifies two characters after the decimal point, but the width is not constrained; and %f merely says to print the number as floating point.

 %d      print as decimal integer
 %6d     print as decimal integer, at least 6 characters wide
 %f      print as floating point
 %6f     print as floating point, at least 6 characters wide
 %.2f    print as floating point, 2 characters after decimal point
 %6.2f           print as floating point, at least 6 wide and 2 after decimal point 

Among others, printf also recognizes %o for octal, %x for hexadecimal, %c for character, %s for character string and %% for itself.

Exercise 1-3. Modify the temperature conversion program to print a heading above the table.

练习1-3 修改温度转换程序，使之能在转换表的顶部打印一个标题。

Exercise 1-4. Write a program to print the corresponding Celsius to Fahrenheit table.

练习1-4 编写一个程序打印摄氏温度转换为相应华氏温度的转换表。