if语句后面可以跟一个可选的else语句，它在if语句布尔表达式为false时执行。
语法
Swift 4中if...else语句的语法如下 -
if boolean_expression {
/* statement(s) will execute if the boolean expression is true */
} else {
/* statement(s) will execute if the boolean expression is false */
}
如果布尔表达式的计算结果为true，那么将执行if代码块，否则将执行else代码块。
示例代码
var varA:Int = 100;
/* Check the boolean condition using if statement */
if varA < 20 {
/* If condition is true then print the following */
print("varA is less than 20");
} else {
/* If condition is false then print the following */
print("varA is not less than 20");
}
print("Value of variable varA is \\(varA)");
编译并执行上述代码时，会产生以下结果 -
varA is not less than 20
Value of variable varA is 100
