R语言 If...Else语句

if语句后面可以是一个可选的else语句，当布尔表达式为false时执行。

语法

在R中创建if ... else语句的基本语法是 -

if(boolean_expression) {
   // statement(s) will execute if the boolean expression is true.
} else {
   // statement(s) will execute if the boolean expression is false.
}

流程图

例

x <- c("what","is","truth")

if("Truth" %in% x) {
   print("Truth is found")
} else {
   print("Truth is not found")
}

[1] "Truth is not found"

if ... else if ... else语句

if语句后面可以跟一个可选的else if ... else语句，这对于使用single if ... else if语句测试各种条件非常有用。

当使用if，else if，else语句有几点要记住。

• 如果可以有零或一个else，它必须在任何其他if之后。
  
• 一个if可以有0到许多else if和它们必须在else之前。
  
• 一旦一个else如果成功，没有任何剩余的else if或else将被测试。
  

语法

在R中创建if ... else if ... else语句的基本语法是 -

if(boolean_expression 1) {
   // Executes when the boolean expression 1 is true.
} else if( boolean_expression 2) {
   // Executes when the boolean expression 2 is true.
} else if( boolean_expression 3) {
   // Executes when the boolean expression 3 is true.
} else {
   // executes when none of the above condition is true.
}

例

x <- c("what","is","truth")

if("Truth" %in% x) {
   print("Truth is found the first time")
} else if ("truth" %in% x) {
   print("truth is found the second time")
} else {
   print("No truth found")
}

[1] "truth is found the second time"