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@swathci

VBAユーザがPython・Rを使ってみた：条件分岐

Last updated at Posted at 2021-01-17

はじめに

機械学習の勉強を始めたVBAユーザです。
備忘録としてPython・Rの文法をVBAと比較しながらまとめていきたいと思います。

目次

条件分岐

今回は、条件分岐についてです。
コメントをいただいて、追記しました。

if文

条件分岐（主に、if文）についてです。

Python

Python3
x = 0
if x == 0:
    print(0)
# 0

if x == 0:
    print(0)
else:
    print(1)
# 0

if x == 0:
    print(0)
elif x == 1:
    print(1)
elif x == 2:
    print(2)
else:
    print(3)
# 0

if x in (0,1,2):
    print(x)
else:
    print(3)
# 0

Pythonにはswitch文、case文はなさそうです。
なお、Pythonでは、If文のまとまり（ブロック）を（次のRのように{}で囲んだり、VBAのようにIfではじまってEnd Ifで閉じたり、というような方法ではなく）インデント（字下げ）で表現します（If文に限らず、他の制御構文（繰り返しのFor文）でも同様です。）。

R

R
x <- 0
if (x == 0) {
  print(0)
}
# 0

if (x == 0) {
  print(0)
} else {
  print(1)
}
# 0

if (x == 0) {
  print(0)
} else if (x == 1) {
  print(1)
} else if (x == 2) {
  print(2)
} else {
  print(3)
}
# 0

if (x %in% c(0,1,2)) {
  print(x)
} else {
  print(3)
}

Rには、switch関数がありますが、使いづらそうです（参考：R-Tips）。

VBA

VBA
x = 0
If x = 0 Then
    Debug.Print 0
End If
' 0

If x = 0 Then
    Debug.Print 0
Else
    Debug.Print 1
End If
' 0

If x = 0 Then
    Debug.Print 0
ElseIf x = 1 Then
    Debug.Print 1
ElseIf x = 2 Then
    Debug.Print 2
Else
    Debug.Print 3
End If
' 0

Select Case x
    Case 0
        Debug.Print 0
    Case 1
        Debug.Print 1
    Case 2
        Debug.Print 2
    Case Else
        Debug.Print 3
End Select
' 0

Select Case x
    Case 0, 1, 2
        Debug.Print x
    Case Else
        Debug.Print 3
End Select
' 0

Select Case x
    Case 0 To 2
        Debug.Print x
    Case Else
        Debug.Print 3
End Select
'0

Select Case x
    Case Is <= 2
        Debug.Print x
    Case Else
        Debug.Print 3
End Select
' 0

Debug.Print Switch(x = 0, 0)
' 0
Debug.Print Switch(x = 0, 0, 1, 1)
' 0
Debug.Print Switch(x = 0, 0, x = 1, 1, x = 2, 2, True, 3)
' 0
Debug.Print Switch(x <= 2, x, True, 3)
' 0

VBAにはSelect Case文があります。Switch関数もあります。

case文の実現

（コメントをいただいて追記しました。）
Python、Rにはcase文がありませんが、標準機能を使って実現できます。

例として、日本語の曜日（aaa）を英語の曜日（ddd）に変換して出力することを考えます1。

aaa aaaa ddd dddd
日 日曜日 Sun Sunday
月 月曜日 Mon Monday
火 火曜日 Tue Tuesday
水 水曜日 Wed Wednesday
木 木曜日 Thu Thursday
金 金曜日 Fri Friday
土 土曜日 Sat Saturday

VBA
VBAでは、Select Case文やSwitch関数を使って書くことができます。

VBA
s = "金"

Select Case s
    Case "日": Debug.Print "Sun"
    Case "月": Debug.Print "Mon"
    Case "火": Debug.Print "Tue"
    Case "水": Debug.Print "Wed"
    Case "木": Debug.Print "Thu"
    Case "金": Debug.Print "Fri"
    Case "土": Debug.Print "Sat"
    Case Else: Debug.Print "not found"
End Select
' Fri

Debug.Print Switch(s = "日", "Sun", _
                   s = "月", "Mon", _
                   s = "火", "Tue", _
                   s = "水", "Wed", _
                   s = "木", "Thu", _
                   s = "金", "Fri", _
                   s = "土", "Sat", _
                   True, "not found")
' Fri

これと同じことを、Python、Rの標準機能で実現してみます。

Python
Pythonでは、リスト（list）を使って次のように実現できます。

Python3
aaa = ['日', '月', '火', '水', '木', '金', '土']
ddd = ['Sun', 'Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat']

s = '金'
t = '銀'
print(s in aaa)          # True
print(t in aaa)          # False
print(aaa.index(s))      # 5
# print(aaa.index(t))    # エラー
print(ddd[aaa.index(s)]) # Fri

def aaa_to_ddd(value):
    if value in aaa:
        return ddd[aaa.index(value)]
    else:
        return 'not found'
print(aaa_to_ddd(s))   # Fri
print(aaa_to_ddd(t))   # not found

def print_ddd(value):
    if value in aaa:
        print(ddd[aaa.index(value)])
    else:
        print('not found')
print_ddd(s)   # Fri
print_ddd(t)   # not found

Pythonでは、辞書（dictionary）を使っても実現できます。こちらの方が簡単に書けます。

Python3
aaa = ['日', '月', '火', '水', '木', '金', '土']
ddd = ['Sun', 'Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat']
aaa_ddd = dict(zip(aaa, ddd))   # 辞書
print(aaa_ddd)
# {'日': 'Sun', '月': 'Mon', '火': 'Tue', '水': 'Wed', '木': 'Thu', '金': 'Fri', '土': 'Sat'}

s = '金'
t = '銀'
print(aaa_ddd[s])     # Fri
# print(aaa_ddd[t])   # エラー
print(aaa_ddd.get(s)) # Fri
print(aaa_ddd.get(t)) # None

def aaa_to_ddd(value):
    if value in aaa:
        return aaa_ddd[value]
    else:
        return 'not found'
print(aaa_to_ddd(s))   # Fri
print(aaa_to_ddd(t))   # not found

def aaa_to_ddd(value):
    if value in aaa:
        return aaa_ddd.get(value)
    else:
        return 'not found'
print(aaa_to_ddd(s))   # Fri
print(aaa_to_ddd(t))   # not found

def aaa_to_ddd(value):
    return aaa_ddd.get(value, 'not found')
print(aaa_to_ddd(s))   # Fri
print(aaa_to_ddd(t))   # not found

def print_ddd(value):
    print(aaa_ddd.get(value, 'not found'))
print_ddd(s)   # Fri
print_ddd(t)   # not found

R
Rでは文字列ベクトルを使って実現できます。

R
aaa = c("日", "月", "火", "水", "木", "金", "土")
ddd = c("Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat")

s <- "金"
t <- "銀"
s %in% aaa   # TRUE
t %in% aaa   # FALSE
aaa == s     # FALSE FALSE FALSE FALSE FALSE  TRUE FALSE
aaa == t     # FALSE FALSE FALSE FALSE FALSE FALSE FALSE
which(aaa == s)      # 6
which(aaa == t)      # integer(0)
aaa[which(aaa == s)] # "金"
aaa[which(aaa == t)] # character(0)
aaa[aaa == s]        # "金"
aaa[aaa == t]        # character(0)

aaa_to_ddd <- function(value) {
  if (value %in% aaa) {
    return(ddd[aaa == value])   # ddd[aaa == value][1]
  } else {
    return("not found")
  }
}
aaa_to_ddd(s)   # "Fri"
aaa_to_ddd(t)   # "not found"

print_ddd <- function(value) {
  if (value %in% aaa) {
    print(ddd[aaa == value])
  } else {
    print("not found")
  }
}
print_ddd(s)   # "Fri"
print_ddd(t)   # "not found"

VBA

VBA
Function aaa_to_ddd_1(value As String) As String
' Case文を使って
Select Case value
    Case "日": aaa_to_ddd_1 = "Sun"
    Case "月": aaa_to_ddd_1 = "Mon"
    Case "火": aaa_to_ddd_1 = "Tue"
    Case "水": aaa_to_ddd_1 = "Wed"
    Case "木": aaa_to_ddd_1 = "Thu"
    Case "金": aaa_to_ddd_1 = "Fri"
    Case "土": aaa_to_ddd_1 = "Sat"
    Case Else: aaa_to_ddd_1 = "not found"
End Select
End Function

Function aaa_to_ddd_2(value As String) As String
' Switch関数を使って
Dim v As Variant
v = Switch(value = "日", "Sun", _
           value = "月", "Mon", _
           value = "火", "Tue", _
           value = "水", "Wed", _
           value = "木", "Thu", _
           value = "金", "Fri", _
           value = "土", "Sat")
If IsNull(v) Then
    v = "not found"
End If
aaa_to_ddd_2 = v
End Function

Function aaa_to_ddd_3(value As String) As String
' 配列を使って
Dim aaa As Variant
Dim ddd As Variant
Dim i As Integer
Dim s As String
aaa = Array("日", "月", "火", "水", "木", "金", "土")
ddd = Array("Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat")
s = "not found"
For i = LBound(aaa) To UBound(aaa)
    If value = aaa(i) Then
        s = ddd(i)
    End If
Next i
aaa_to_ddd_3 = s
End Function

Sub print_ddd(value As String)
' Case文を使って
Select Case value
    Case "日": Debug.Print "Sun"
    Case "月": Debug.Print "Mon"
    Case "火": Debug.Print "Tue"
    Case "水": Debug.Print "Wed"
    Case "木": Debug.Print "Thu"
    Case "金": Debug.Print "Fri"
    Case "土": Debug.Print "Sat"
    Case Else: Debug.Print "not found"
End Select
End Sub

Sub test_case()

Dim s As String
Dim t As String

s = "金"
Select Case s
    Case "日": Debug.Print "Sun"
    Case "月": Debug.Print "Mon"
    Case "火": Debug.Print "Tue"
    Case "水": Debug.Print "Wed"
    Case "木": Debug.Print "Thu"
    Case "金": Debug.Print "Fri"
    Case "土": Debug.Print "Sat"
    Case Else: Debug.Print "not found"
End Select
' Fri

Debug.Print Switch(s = "日", "Sun", _
                   s = "月", "Mon", _
                   s = "火", "Tue", _
                   s = "水", "Wed", _
                   s = "木", "Thu", _
                   s = "金", "Fri", _
                   s = "土", "Sat", _
                   True, "not found")
' Fri

s = "金"
t = "銀"

Debug.Print aaa_to_ddd_1(s)   ' Fri
Debug.Print aaa_to_ddd_2(s)   ' Fri
Debug.Print aaa_to_ddd_3(s)   ' Fri
Debug.Print aaa_to_ddd_1(t)   ' not found
Debug.Print aaa_to_ddd_2(t)   ' not found
Debug.Print aaa_to_ddd_3(t)   ' not found

Call print_ddd(s)   ' Fri
Call print_ddd(t)   ' not found

End Sub

まとめ

一覧

各言語で使用するステートメント等を一覧にまとめます。比較のために、EXCELでの計算も示しました。

条件分岐

Python R VBA EXCEL
if文 if P:
　...
elif Q:
　...
elif R:
　...
else:
　... 		if (P) {
　...
} else if (Q) {
　...
} else if (R) {
　...
} else {
　...
} 		If P Then
　...
ElseIf Q Then
　...
ElseIf R Then
　...
Else
　...
End If 		=IF(P,...,
IF(Q,...,
IF(R,...,
...)))
switch文 Select Case X
Case p
　...
Case q
　...
Case r
　...
Case Else
　...
End Select 		=SWITCH(X,
p,...,
q,...,
r,...,
...)

プログラム全体

参考までに使ったプログラムの全体を示します。

Python

Python3
# 条件分岐
x = 0
if x == 0:
    print(0)
# 0

if x == 0:
    print(0)
else:
    print(1)
# 0

if x == 0:
    print(0)
elif x == 1:
    print(1)
elif x == 2:
    print(2)
else:
    print(3)
# 0

if x in (0,1,2):
    print(x)
else:
    print(3)
# 0

R

R
# 条件分岐（If）
x <- 0
if (x == 0) {
  print(0)
}
# 0

if (x == 0) {
  print(0)
} else {
  print(1)
}
# 0

if (x == 0) {
  print(0)
} else if (x == 1) {
  print(1)
} else if (x == 2) {
  print(2)
} else {
  print(3)
}
# 0

if (x %in% c(0,1,2)) {
  print(x)
} else {
  print(3)
}
# 0

VBA

VBA
Sub test_conditional_branch()
Dim x As Integer

' 条件分岐
x = 0
If x = 0 Then
    Debug.Print 0
End If
' 0

If x = 0 Then
    Debug.Print 0
Else
    Debug.Print 1
End If
' 0

If x = 0 Then
    Debug.Print 0
ElseIf x = 1 Then
    Debug.Print 1
ElseIf x = 2 Then
    Debug.Print 2
Else
    Debug.Print 3
End If
' 0

Select Case x
    Case 0
        Debug.Print 0
    Case 1
        Debug.Print 1
    Case 2
        Debug.Print 2
    Case Else
        Debug.Print 3
End Select
' 0

Select Case x
    Case 0, 1, 2
        Debug.Print x
    Case Else
        Debug.Print 3
End Select
' 0

Select Case x
    Case 0 To 2
        Debug.Print x
    Case Else
        Debug.Print 3
End Select
'0

Select Case x
    Case Is <= 2
        Debug.Print x
    Case Else
        Debug.Print 3
End Select
' 0

Debug.Print Switch(x = 0, 0)
' 0
Debug.Print Switch(x = 0, 0, 1, 1)
' 0
Debug.Print Switch(x = 0, 0, x = 1, 1, x = 2, 2, True, 3)
' 0
Debug.Print Switch(x <= 2, x, True, 3)
' 0

End Sub

参考

  1. "aaa","aaaa","ddd","dddd" はExcelの書式の表示形式の文字列です。 ↩

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