- 20151210 [R基礎與應用] Ch09：流程控制
- 整理自 R 軟體資料分析基礎與應用 -- Ch09 : 流程控制
- [ 筆記內容 ]
- （一）if 和 else
- [ 背景知識 ] 條件檢測指令 "if"
- 1. "if 敘述"
- 2. 在 "if 敘述" 內加入 "else"
- 3. "else-if 敘述"
- （二）switch
- （三）ifelse
- （四）複合型條件檢測
- [ if、ifelse 和 switch 的複合使用 ]
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整理自 R 軟體資料分析基礎與應用 -- Ch09 : 流程控制
（一）if 和 else
# 在數值上"TRUE"等同於"1"；而"FALSE"等同於"0"> as.numeric(TRUE)[1] 1>> as.numeric(FALSE)[1] 0># 簡單的條件判斷> 1 == 1 # TRUE[1] TRUE>> 1 < 1 # FALSE[1] FALSE>> 1 <= 1 # TRUE[1] TRUE>> 1 > 1 # FALSE[1] FALSE>> 1 >= 1 # TRUE[1] TRUE>> 1 != 1 # FALSE[1] FALSE>> # 建立一個含有 "1" 的變數> tocheck <- 1>> # 若 tocheck 等於 "1"，顯示 "hello"> if (tocheck == 1)+ {+ print("hello")+ }[1] "hello">>> # 現在如果 tocheck 等於 "0"，則顯示 "hello"> if (tocheck == 0)+ {+ print("hello")+ }> # 因為 tocheck 的值為 "1"，所以不會顯示任何東西>> # 首先建立函數> check.bool <- function(x)+ {+ if (x == 1)+ {+ # 若輸入值為"1"則顯示"hello"+ print("hello")+ }+ else+ {+ # 否則顯示"goodbye"+ print("goodbye")+ }+ }>>> # 確認函數能否正確運行> check.bool(1)[1] "hello">> check.bool(0)[1] "goodbye">> check.bool("k")[1] "goodbye">> check.bool(TRUE) # 因為"TRUE"在數值上等同於"1"，所以結果會顯示"hello"[1] "hello">> check.bool <- function(x)+ {+ if (x == 1)+ {+ # 若輸入值等於"1"，顯示"hello"+ print("hello")+ }+ else if (x == 0)+ {+ # 若輸入值等於"0"，顯示"goodbye"+ print("goodbye")+ }+ else+ {+ # 否則顯示"confused"+ print("confused")+ }+ }>> check.bool(1)[1] "hello">> check.bool(0)[1] "goodbye">> check.bool(2)[1] "confused">> check.bool("k")[1] "confused">（二）switch
# 使用範例switch(x, "a" = "first", "b" = "second", "z" = "last", "c" = "third", "other")# 使用 "switch" 建立函數> use.switch <- function(x)+ {+ switch(x, "a" = "first", "b" = "second", "z" = "last", "c" = "third", "other")+ }>> use.switch("a")[1] "first">> use.switch("b")[1] "second">> use.switch("c")[1] "third">> use.switch("d")[1] "other">> use.switch("e")[1] "other">> use.switch("z")[1] "last"># 賦予引數數值> use.switch <- function(x)+ {+ switch(x, "a" = "first", "b" = "second", "z" = "last", "c" = "third", "other")+ }>> use.switch(1)[1] "first">> use.switch(2)[1] "second">> use.switch(3)[1] "last">> use.switch(4)[1] "third">> use.switch(5)[1] "other">> use.switch(6) # 因為"給予的數值"大於"引數的數量"，所以不會回傳任何東西> is.null(use.switch(6)) # 檢測物件是否為"NULL"[1] TRUE>（三）ifelse
# 使用範例ifelse("1 == 1", "Yes", "No")> # 檢測是否 "1 == 1" ?> ifelse(1 == 1, "Yes", "No")[1] "Yes">> # 檢測是否 "1 == 0" ?> ifelse(1 == 0, "Yes", "No")[1] "No">> toTest <- c(1, 1, 0, 1, 0, 1)> ifelse(toTest == 1, "Yes", "No")[1] "Yes" "Yes" "No" "Yes" "No" "Yes">>> # 將檢測的引數用在第二個和第三個引數作為顯示結果> ifelse(toTest == 1, toTest * 3, toTest)[1] 3 3 0 3 0 3>> # FALSE 引數中的值會在每次的檢測結果為"FALSE"時顯示> ifelse(toTest == 1, toTest * 3, "Zero")[1] "3" "3" "Zero" "3" "Zero" "3">> # 讓 toTest 包含"NA"元素，其對應的結果為"NA"> toTest[2] <- NA> ifelse(toTest == 1, "Yes", "No")[1] "Yes" NA "No" "Yes" "No" "Yes"> ifelse(toTest == 1, toTest * 3, toTest)[1] 3 NA 0 3 0 3> ifelse(toTest == 1, toTest * 3, "Zero")[1] "3" NA "Zero" "3" "Zero" "3">（四）複合型條件檢測
> a <- c(1, 1, 0, 1)> b <- c(2, 1, 0, 1)>> # 使用"單符號"檢測 "a" 和 "b" 的每一個元素> ifelse(a == 1 & b == 1, "Yes", "No")[1] "No" "Yes" "No" "Yes">> # 使用"雙符號"檢測 "a" 和 "b" 的第一個元素，因此只回傳一個結果> ifelse(a == 1 && b == 1, "Yes", "No")[1] "No">