評分

Python 有兩種迴圈結構。 while迴圈用於_不確定_（不計數）的疊代，for迴圈則用於_確定_（計數）的疊代。 關鍵字break、continue和else可以幫助你自訂迴圈的行為。 range()和enumerate()則能協助處理迴圈的計數與索引。

while 迴圈

while迴圈只要loop expression或「test」在 boolean context中評估為True，就會持續執行，直到評估為False時終止：

# 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 迴圈

Python 裡最基本的 for loop，更貼切的說法是 for each：它會逐一走訪任何可疊代物件的值，並在呼叫 next()不再回傳任何值時結束：

>>> 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.'

序列物件 range()

如果沒有指定特定的iterable，就會使用特殊的 range()序列來當作迴圈計數器。 range()需要一個int來指定序列的stop位置，也可以選擇性地接收start和step參數。 如果沒有提供start數字，序列就會從 0 開始。 range()物件是惰性的（值會在需要時才產生），支援所有常見序列操作，而且無論指定的序列有多長，佔用的記憶體都是固定的。

# 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.'

使用 enumerate() 取值與索引

如果需要同時取得值和索引，內建的 enumerate(<iterable>)會回傳（index、value）配對：

>>> 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.'

改變迴圈行為

continue關鍵字可以讓你跳過目前這一輪，直接進入下一個疊代：

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（就像許多與 C 相關的語言一樣）關鍵字可以用來停止疊代，並從最內層的loop中「跳出」：

>>>  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.'