3.2 If-Else
The if-else statement is used to express decisions. Formally the syntax is
if (expression)
statement1
else
statement2where the else part is optional. The expression is evaluated; if it is true (that is, if expression has a non-zero value), statement1 is executed. If it is false (expression is zero) and if there is an else part, statement2 is executed instead.
if-else语句用于条件判定。其语法如下所示：
if (expression)
statement1
else
statement2其中else部分是可选的。该语句执行时，先计算表达式的值，如果其值为真(即表达式的值为非0)，则执行语句1；如果其值为假(即表达式的值为0)，并且该语句包含else部分，则执行语句2。
Since an if tests the numeric value of an expression, certain coding shortcuts are possible. The most obvious is writing
if (expression)
instead of
if (expression != 0)
Sometimes this is natural and clear; at other times it can be cryptic.
由于if语句只是简单测试表达式的数值，因此可以对某些代码的编写进行简化。最明显的例子是用如下写法
if (expression)
来代替
if (expression != 0)
某些情况下这种形式是自然清晰的，但也有些情况下可能会含义不清。
Because the else part of an if-else is optional,there is an ambiguity when an else if omitted from a nested if sequence. This is resolved by associating the else with the closest previous else-less if. For example, in
if (n > 0)
if (a > b)
z = a;
else
z = b;the else goes to the inner if, as we have shown by indentation. If that isn't what you want, braces must be used to force the proper association:
if (n > 0) {
if (a > b)
z = a;
}
else
z = b;The ambiguity is especially pernicious in situations like this:
if (n > 0)
for (i = 0; i < n; i++)
if (s[i] > 0) {
printf("...");
return i;
}
else /* WRONG */
printf("error -- n is negative\n");The indentation shows unequivocally what you want, but the compiler doesn't get the message, and associates the else with the inner if. This kind of bug can be hard to find; it's a good idea to use braces when there are nested ifs.
By the way, notice that there is a semicolon after z = a in
if (a > b)
z = a;
else
z = b;This is because grammatically, a statement follows the if, and an expression statement like "z = a;" is always terminated by a semicolon.