TCPL/2.06_Relational_and_Logical_Operators

2.6 Relational and Logical Operators 关系运算符与逻辑运算符

The relational operators are

   >   >=   <   <=

They all have the same precedence. Just below them in precedence are the equality operators:

   ==   !=

Relational operators have lower precedence than arithmetic operators, so an expression like i < lim-1 is taken as i < (lim-1), as would be expected.

关系运算符包括下列几个运算符：

   >   >=   <   <=

它们具有相同的优先级。优先级仅次于它们的是相等性运算符：

   ==   !=

关系运算符的优先级比算术运算符低。因此，表达式i < lim-1等价于i < (1im-1)。

More interesting are the logical operators && and ||. Expressions connected by && or || are evaluated left to right, and evaluation stops as soon as the truth or falsehood of the result is known. Most C programs rely on these properties. For example, here is a loop from the input function getline that we wrote in Chapter 1:

   for (i=0; i < lim-1 && (c=getchar()) != '\n' && c != EOF; ++i)
       s[i] = c;

Before reading a new character it is necessary to check that there is room to store it in the array s, so the test i < lim-1 must be made first. Moreover, if this test fails, we must not go on and read another character.

逻辑运算符&&与||有一些较为特殊的属性，由&&与||连接的表达式按从左到右的顺序进行求值，并且，在知道结果值为真或假后立即停止计算。绝大多数C语言程序运用了这些属性。例如，下列在功能上与第1章的输入函数getline中的循环语句等价的循环语句：

   for (i=0; i < lim-1 && (c=getchar()) != '\n' && c != EOF; ++i)
       s[i] = c;

在读入一个新字符之前必须先检查数组s中是否还有空间存放这个字符，因此必须首先测试条件i<lim-1。如果这一测试失败，就没有必要继续读入下一字符。

Similarly, it would be unfortunate if c were tested against EOF before getchar is called; therefore the call and assignment must occur before the character in c is tested.

类似地，如果在调用getchar函数之前就测试c是否为EOF，结果也是不正确的，因此，函数的调用与赋值都必须在对c中的字符进行测试之前进行。

The precedence of && is higher than that of ||, and both are lower than relational and equality operators, so expressions like

   i < lim-1 && (c=getchar()) != '\n' && c != EOF

need no extra parentheses. But since the precedence of != is higher than assignment, parentheses are needed in

   (c=getchar()) != '\n'

to achieve the desired result of assignment to c and then comparison with '\n'.

运算符&&的优先级比||的优先级高，但两者都比关系运算符和相等性运算符的优先级低。因此，表达式

   i < lim-1 && (c=getchar()) != '\n' && c != EOF

就不需要另外加圆括号了。但是，出于运算符!=的优先级高于赋值运算符的优先级，因此，在表达式

   (c=getchar()) != '\n'

中，就需要使用圆括号，这样才能达到预期的目的：失把函数返回值赋值给c，然后再将c与'\n'进行比较。

By definition, the numeric value of a relational or logical expression is 1 if the relation is true, and 0 if the relation is false.

根据定义，在关系表达式或逻辑表达式中，如果关系为真，则表达式的结果值为数值1；如果为假，则结果值为数值0。

The unary negation operator ! converts a non-zero operand into 0, and a zero operand in 1. A common use of ! is in constructions like

   if (!valid)

rather than

   if (valid == 0)

It's hard to generalize about which form is better. Constructions like !valid read nicely ("if not valid"), but more complicated ones can be hard to understand.

逻辑非运算符!的作用是将非0操作数转换为0，将操作数0转换为1。该运算符通常用于下列类似的结构中：

   if (!valid)

一般不采用下列形式：

   if (valid == 0)

当然，很难评判上述两种形式哪种更好。类似于!valid的用法读起来更直观一些(“如果不是有效的”)，但对于一些更复杂的结构可能会难于理解。

Exercise 2-2. Write a loop equivalent to the for loop above without using && or ||.

练习2-2 在不使用运算符&&或||的条件下编写一个与上面的for循环语句等价的循环语句。