💡 Python 沒有 do...while 語法。

While Loops

while [condition1]:
    [statement 1]
else:
    [statement 2]

# 巢狀 while 迴圈
while [condition 1]:
    [statement1]
    while [condition 2]:
        [statement2]
    else:
        [statement3]
else:
    [statement4]

語法

• 透過條件成立與否，來控制執行次數。
• statement 中特殊的語法
  • continue - 結束當前迴圈狀態進到下一個迴圈
    • With the continue statement we can stop the current iteration, and continue with the next.
  • break - 跳出/強制離開 當前迴圈
    • With the break statement we can stop the loop even if the while condition is true.
  • pass - 佔位語句，不做任何事，是為了保持程式碼結構的完整性

      💡 只要是在程式碼裡的執行區塊內，pass 都可以使用！
    

程式碼解釋

• 只要判斷為 True，便能一直執行 statement1 的程式碼（不限次數）。
• 如果為 False 的話，就跳到 else 的部分執行 statement2 程式碼後才結束。

範例

• while

    i = 1
    while i < 3:
        print(i)
        i += 1
  
    # 1
    # 2
  

• while..else

    count = 0
    while count < 5:
       print(count, " is  less than 5")
       count = count + 1
    else:
       print(count, " is not less than 5")
  
    # Output:
    # 0  is  less than 5
    # 1  is  less than 5
    # 2  is  less than 5
    # 3  is  less than 5
    # 4  is  less than 5
    # 5  is not less than 5
  

• while + continue

    i = 0
    while i < 6:
        i += 1
        if i == 3:
            continue
        print(i)
  
    # 1
    # 2
    # 4
    # 5
    # 6
  

• while + break

    i = 0
    while i < 6:
        i += 1
        if i == 3:
            break
        print(i)
  
    # 1
    # 2
  

    i = 0
    while i < 6:
        print(i)
        if i == 3:
            break
        i += 1
  
    # 0
    # 1
    # 2
    # 3
  

• while + pass

    number = eval(input())
    while number > 10:
        pass
        # 還不確定要做甚麼動作時，先用 pass 佔位，保持程式碼結構的完整性
    else:
        print("Less than 10.")
  

    count = 0
    while count < 5:
        count += 1
        if count % 3 == 0:
            pass
        else:
            print(count)
  
    # Output:
    # 1
    # 2
    # 4
    # 5
  
    # 若「pass 該行直接為空」，則出現「 IndentationError: expected an indented block 」。
  

• 連續且重複的情況下，可使用 while 來減少程式碼撰寫

    # 從 1 到 100 的總和
  
    counter = 0
    total = 0
  
    while counter < 100:
        counter += 1
        total += counter
  
    print('The Summary from 1 to 100 is', total)
  

• 通常也會使用 while 來處理連續輸入

    while True:
        string = input('Please enter any word：')
        print(string)
  
    # 當輸入 Ctrl + z 或者 Ctrl + c 時，便停止輸入。
  

For Loops

for [var] in [iterable]:
    [statement1]
else:
    [statement2]

# for 巢狀迴圈，唯一要注意的點在於 var1, var2 不能重複

for [var1] in [iterable]:
    for [var2] in [iterable]:
        [statement]

語法

• 使用可迭代物件 (iterable)，來控制執行次數。
  • To loop through a set of code a specified number of times.
  • 可迭代物件 (iterable)
    • 如：list, tuple, dictionary, set, string。
    • 可使用 slicing（切片）、索引（index） 來取出可迭代物件內的元素。

程式碼解釋

• for 迴圈有兩個部分需要設置
  • var - 自行設定的變數，表示可迭代物件中被遍覽的元素。
  • iterable - 可迭代物件，可以被 for loop 遍歷/逐一取出內部資料的東西，用以被遍歷其中的所有元素。
• Python for 迴圈的概念為「遍歷/逐一取出內部資料」，相當於 C 語言的 foreach。

💡 for 迴圈在 python 的寫法設計 與 C / C++ 不相同。

* Python   把所有東西看作 object 來處理。
    # foreach concept
    for i in ['apple', 'banana', 'peach']:
        print(i)

    # counter concept => range(start=0, stop [, step=1])
    for i in range(0, 7, 2):       # but not including 7
        print(i, end="\t")

* C / C++  設定狀態 (Initialization, Condition, Update) 來處理迴圈。
    * Initialization - 用以初始設定迴圈的 Counter
    * Condition - 用以判斷迴圈結束的條件
    * Update - 用以設定迴圈 Counter 的變化

    #include <stdio.h>
    int main() {
        int i;
        for (i = 0; i <= 6; i = i + 2) {
            printf("%d\t", i);
        }
        return 0;
    }

範例

• 字串

    sent = "我的肝呢？"
    for w in sent:
        print(w)
  

• List

    fruits = ['apple', 'banana', 'grape']
    for f in fruits:
        print(f)
  

• Tuple

    fruits = ('apple', 'banana', 'grape')
    for f in fruits:
        print(f)
  

• for..else

    for x in range(6):
        print(x)
    else:
        print("Finally finished!")
  

    for x in range(6):
        if x == 3: 
            break
        print(x)
    else:
        print("Finally finished!")
  
    # If the loop breaks, the else block is not executed.
  

• for + break

    fruits = ["apple", "banana", "cherry"]
    for x in fruits:
        print(x)
        if x == "banana":
            break
  

    fruits = ["apple", "banana", "cherry"]
    for x in fruits:
        if x == "banana":
            break
        print(x)
  

• for + continue

    fruits = ["apple", "banana", "cherry"]
    for x in fruits:
        if x == "banana":
            continue
        print(x)
  

• for + pass

    for x in [0, 1, 2]:
        pass
  

• for 巢狀迴圈

    for i in range(2, 10):
        for j in range(1, 10):
            print('{} * {} = {}'.format(i, j, i*j), end='\t')
        print()
  

while vs. for

i = 0
while i < 5:
    print(i)
    i = i + 1

#相當於
for j in range(5):
    print(j)