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清單（List）

  • 清單是 Python 最常用、也最好用的資料類別，具順序性
  • 甚麼東西都可以裝，裝的東西也可以不同，也可以用清單包清單
  • 想成一個百寶袋，甚麼都可以塞，再拿出來
    • 32位python的儲存上限是536870912 個元素
    • 64位python的儲存上限是1152921504606846975 個元素
  • 前面提到的字母陣列其實概念跟清單很像
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>>> l = [1, 1.0, 10, "test"]
>>> print(l[0])
1

>>> print(l[2])
10

>>> l[2] = 100
>>> print(l[2])
100

>>> print(l[-1])
"test"

>>> print(len(l))
4

>>> l.append("123")
>>> print(l)
[1, 1.0, 100, "test", "123"]

>>> l.pop()
"123"
>>> print(l)
[1, 1.0, 100, "test"]

>>> print(l + l)
[1, 1.0, 100, "test", 1, 1.0, 100, "test"]

>>> print(l * 3)
[1, 1.0, 100, "test", 1, 1.0, 100, "test", 1, 1.0, 100, "test"]
  • Traverse a list：
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l = [1,2,3,4,5]
for i in l:   # or for i in range(len(l))
    print(i * 2)
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Output:
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10
  • 常見操作（Common Operations，供參考）：
Method Usage
list.append(x) Add element x to end of list.
list.sort() Sort (order) the list. A comparison function may be passed as a parameter.
list.reverse() Reverse the list.
list.index(x) Returns index of first occurrence of x.
list.insert(i, x) Insert x into list at index i.
list.count(x) Returns the number of occurrences of x in list.
list.remove(x) Deletes the first occurrence of x in list.
list.pop(i) Deletes the ith element of the list and returns its value.

List Copying

  • 在複製 List 時，要特別留意以下狀況，並非正確的 List 複製方法：
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# Wrong Copy (Reference Copy Only)
aList = [1, 2, 3]
anotherList = aList
anotherList[0] = 5
print(aList)
# Check their address
print(id(aList), id(anotherList))
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Output:
[5, 2, 3]
1805364504896 1805364504896
  • 當我們修改 anotherList 時，原本的 aList 也一同被修改
  • 主要是因為 List 儲存的是記憶體參照（或是說 List 是可變物件，後面會提到），第三行做的事情僅僅是將參照傳給另一個變數，因此也可以發現他們的記憶體其實是相同的
How to copy a list correctly?

有以下幾種方式可以正確地複製 List：

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# Correct Copy
aList = [1, 2, 3]
# Three different ways to copy a list (Shallow)
anotherList = list(aList)
anotherList = aList[:]
anotherList = aList.copy()

anotherList[0] = 5
print(aList)
# Check their address
print(id(aList), id(anotherList))
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Output:
[1, 2, 3]
1805364505024 1805364643392

補充：此處使用的稱為「Shallow Copy」，僅複製容器中元素的地址
若連容器中的元素本身都想完全複製，需要使用「Deep Copy」
延伸閱讀： Python - 淺複製(shallow copy)與深複製(deep copy)

CSV（Comma-separated value）檔案

  • CSV 是常見的儲存資料格式
  • 簡潔、統一、格式化、方便處理
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QuotaAmount,StartDate,OwnerName,Username
150000,2016-01-01,Chris Riley,trailhead9.ub20k5i9t8ou@example.com
150000,2016-02-01,Chris Riley,trailhead9.ub20k5i9t8ou@example.com
150000,2016-03-01,Chris Riley,trailhead9.ub20k5i9t8ou@example.com
150000,2016-01-01,Harold Campbell,trailhead14.jibpbwvuy67t@example.com
150000,2016-02-01,Harold Campbell,trailhead14.jibpbwvuy67t@example.com
150000,2016-03-01,Harold Campbell,trailhead14.jibpbwvuy67t@example.com
150000,2016-01-01,Jessica Nichols,trailhead19.d1fxj2goytkp@example.com
150000,2016-02-01,Jessica Nichols,trailhead19.d1fxj2goytkp@example.com
150000,2016-03-01,Jessica Nichols,trailhead19.d1fxj2goytkp@example.com

結合前面的字串與清單處理方式，我們可以輕鬆的處理 CSV file 中的每一行資料：

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>>> line = 'amount,date,owner,user'
>>> data = line.split(',')
>>> print(data)
['amount', 'date', 'owner', 'user']
>>> print(data[0])
amount

那如何處理整個 CSV file？使用檔案處理相關函數（之後再講）：

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result = []
with open('file.csv') as f:
    data = f.read()
    lines = data.split('\n')
for line in lines:
    result.append(line.split(','))
print(result)
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Output:
[[QuotaAmount,StartDate,OwnerName,Username],
[150000,2016-01-01,Chris Riley,trailhead9.ub20k5i9t8ou@example.com],
[150000,2016-02-01,Chris Riley,trailhead9.ub20k5i9t8ou@example.com],
...]