if語句后面可以是一個可選的else語句，當布爾表達式為false時執(zhí)行。

語法

在R中創(chuàng)建if ... else語句的基本語法是 -

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

流程圖

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語句

if語句后面可以跟一個可選的else if ... else語句，這對于使用single if ... else if語句測試各種條件非常有用。

當使用if，else if，else語句有幾點要記住。

• 如果可以有零或一個else，它必須在任何其他if之后。
  
• 一個if可以有0到許多else if和它們必須在else之前。
  
• 一旦一個else如果成功，沒有任何剩余的else if或else將被測試。
  

語法

在R中創(chuàng)建if ... else if ... else語句的基本語法是 -

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"