ループ の

ループについて

Pythonには2つのループ構文があります。 回数が決まっていない（数えない）繰り返しにはwhileループ、回数が決まっている（数える）繰り返しにはforループを使います。 break、continue、elseというキーワードを使うと、ループの動作をカスタマイズできます。 range()とenumerate()は、ループの回数の管理とインデックス付けに役立ちます。

whileループ

whileループは、loop expression（「テスト」）がboolean contextでTrueと評価されるかぎり実行を続け、Falseと評価されると終了します。

# Lists are considered "truthy" in a boolean context if they
# contain one or more values, and "falsy" if they are empty.

>>> placeholders = ["spam", "ham", "eggs", "green_spam", "green_ham", "green_eggs"]

>>> while placeholders:
...     print(placeholders.pop(0))
...
'spam'
'ham'
'eggs'
'green_spam'
'green_ham'
'green_eggs'

forループ

Pythonの基本的なfor loopは、むしろ_for eachと呼ぶほうが適切で、任意のiterable objectの値を順にたどり、next()を呼んでも値が返ってこなくなると（StopIterationが送出されます_）終了します。

>>> word_list = ["bird", "chicken", "barrel", "bongo"]

>>> for word in word_list:
...    if word.startswith("b"):
...        print(f"{word.title()} starts with a B.")
...    else:
...        print(f"{word.title()} doesn't start with a B.")
...
'Bird starts with a B.'
'Chicken doesn\'t start with a B.'
'Barrel starts with a B.'
'Bongo starts with a B.'

シーケンスオブジェクトrange()

特定のiterableが与えられていない場合は、特別なrange()シーケンスがループのカウンターとして使われます。 range()には、シーケンスをstopする位置を示すintが必要で、startとstepを任意で指定できます。 startの数値を指定しない場合、シーケンスは0から始まります。 range()オブジェクトは遅延評価で（値は要求されたときに生成されます）、すべてのcommon sequence operationsをサポートし、指定したシーケンスがどれだけ長くても一定のメモリしか使いません。 おもしろいことに、range()はイテレーターではありません。数値のシーケンスが必要になる、ループ以外の多くの場面でも使えます。

# Here we use range to produce some numbers, rather than creating a list of them in memory.
# The values will start with 1 and stop *before* 7.

>>> for number in range(1, 7):
...    if number % 2 == 0:
...       print(f"{number} is even.")
...    else:
...       print(f"{number} is odd.")
'1 is odd.'
'2 is even.'
'3 is odd.'
'4 is even.'
'5 is odd.'
'6 is even.'

# range() can also take a *step* parameter.
# Here we use range to produce only the "odd" numbers, starting with 3 and stopping *before* 15.

>>> for number in range(3, 15, 2):
...    if number % 2 == 0:
...       print(f"{number} is even.")
...    else:
...       print(f"{number} is odd.")
...
'3 is odd.'
'5 is odd.'
'7 is odd.'
'9 is odd.'
'11 is odd.'
'13 is odd.'

enumerate()で値とインデックスを扱う

値とインデックスの両方が必要な場合は、組み込みのenumerate(<iterable>)が、（index、value）のペアを順に返すiteratorを返します。

>>> word_list = ["bird", "chicken", "barrel", "apple"]

# *index* and *word* are the loop variables.
# Loop variables can be any valid python name.

>>> for index, word in enumerate(word_list):
...    if word.startswith("b"):
...        print(f"{word.title()} (at index {index}) starts with a B.")
...    else:
...        print(f"{word.title()} (at index {index}) doesn't start with a B.")
...
'Bird (at index 0) starts with a B.'
'Chicken (at index 1) doesn\'t start with a B.'
'Barrel (at index 2) starts with a B.'
'Apple (at index 3) doesn\'t start with a B.'


# The same method can be used as a "lookup" for pairing items between two lists.
# Of course, if the lengths or indexes don't line up, this doesn't work.

>>> word_list = ["cat", "chicken", "barrel", "apple", "spinach"]
>>> category_list = ["mammal", "bird", "thing", "fruit", "vegetable"]

>>> for index, word in enumerate(word_list):
...    print(f"{word.title()} is in category: {category_list[index]}.")
...
'Cat is in category: mammal.'
'Chicken is in category: bird.'
'Barrel is in category: thing.'
'Apple is in category: fruit.'
'Spinach is in category: vegetable.'

enumerate(<iterable>)関数は、インデックスのカウントを別の数値からstartさせるように設定することもできます。

# Here, the index count will start at 1.
>>> for position, word in enumerate(word_list, start=1):
...    if word.startswith("b"):
...        print(f"{word.title()} (at position {position}) starts with a B.")
...    else:
...        print(f"{word.title()} (at position {position}) doesn't start with a B.")
...
'Bird (at position 1) starts with a B.'
'Chicken (at position 2) doesn\'t start with a B.'
'Barrel (at position 3) starts with a B.'
'Apple (at position 4) doesn\'t start with a B.'

ループの動作を変える

continueキーワードを使うと、次の繰り返しへスキップできます。

>>> word_list = ["bird", "chicken", "barrel", "bongo", "sliver", "apple", "bear"]
...
... # This will skip *bird*, at index 0
... for index, word in enumerate(word_list):
...     if index == 0:
...         continue
...     if word.startswith("b"):
...         print(f"{word.title()} (at index {index}) starts with a b.")
...
'Barrel (at index 2) starts with a b.'
'Bongo (at index 3) starts with a b.'
'Bear (at index 6) starts with a b.'

breakキーワード（多くのC系言語と同じです）を使うと、繰り返しを止めて、もっとも内側のloopから「抜け出す」ことができます。

>>>  word_list = ["bird", "chicken", "barrel", "bongo", "sliver", "apple"]

>>> for index, word in enumerate(word_list):
...    if word.startswith("b"):
...        print(f"{word.title()} (at index {index}) starts with a B.")
...    elif word == "sliver":
...        break
...    else:
...        print(f"{word.title()} doesn't start with a B.")
... print("loop broken.")
...
'Bird (at index 0) starts with a B.'
'Chicken doesn\'t start with a B.'
'Barrel (at index 2) starts with a B.'
'Bongo (at index 3) starts with a B.'
'loop broken.'

ループのelse節はPythonに特有で、繰り返しを使い切ったときの「締めくくり」や「まとめ」の処理に使えます。 elseキーワードに続く文は、繰り返しがbreakで終了した場合は実行されません。

>>> word_list = ["bird", "chicken", "barrel", "bongo", "sliver", "apple"]

# Using enumerate to get both an index and a value.

>>> for index, word in enumerate(word_list):
...    word = word.title()
...    if word.startswith("B"):
...        print(f"{word} (at index {index}) starts with a B.")
...
... # This executes once *StopIteration* is raised and 
... # There are no more items to iterate through.
... # Note the indentation, which lines up with the for keyword.
... else:
...    print(f"Found the above b-words, out of {len(word_list)} words in the word list.")
...
'Bird (at index 0) starts with a B.'
'Barrel (at index 2) starts with a B.'
'Bongo (at index 3) starts with a B.'
'Found the above b-words, out of 6 words in the word list.'


# Terminating a loop via *break* will bypass the loop *else* clause

>>> for index, word in enumerate(word_list):
...    word = word.title()
...    if word.startswith("B"):
...       print(f"{word} (at index {index}) starts with a B.")
...    if word.startswith("S"):
...        print("Found an S, stopping iteration.")
...        break

... # This statement does not run, because a *break* was triggered.
... else:
...    print(f"Found the above b-words, out of {len(word_list)} words in the word list.")
...
'Bird (at index 0) starts with a B.'
'Barrel (at index 2) starts with a B.'
'Bongo (at index 3) starts with a B.'
'Found an S, stopping iteration.'