我正在嘗試用numpy創建一個串列陣列

我需要一個包含記錄陣列或記錄串列的陣列。我需要這樣的東西：

people = [['David','Blue','Dog','Car'],['Sally','Yellow','Cat','Boat']]

要么：

people = (['David','Blue','Dog','Car'],['Sally','Yellow','Cat','Boat'])

但我不斷得到：

people = ['David','Blue','Dog','Car','Sally','Yellow','Cat','Boat']

我試過追加 vs 連接，不同的軸，不同的 np 初始化，但結果總是一樣的。這是我的最新版本。我究竟做錯了什么？

import numpy as np

# Tried
# people = np.empty((0,0), dtype='S')
# people = np.array([[]])
people = np.array([])

records = GetRecordsFromDB()

for record in records:

    # Do some stuff    
    
    # Tried
    # person = [name, color, animal, vehicle]
    person = np.array([name, color, animal, vehicle])

    # Tried this with different axis
    # people = np.append(people, person, axis=0)
    people = np.concatenate((people, person))

謝謝你。

編輯：如果有幫助，這將是 Pandas DataFrame 的輸入。

uj5u.com熱心網友回復：

采用np.c_

people = np.c_[people, person]

uj5u.com熱心網友回復：

這是我解決這個問題的方法：

import numpy as np

people = np.array([])

records = GetRecordsFromDB()

for record in records:

    # Do some stuff

    person = np.array([name, color, animal, vehicle])
                  
    if len(people) == 0:
        people = [person]
    else:
        people = np.append(people, [person], axis=0)

uj5u.com熱心網友回復：

編輯

看著你的回答，我意識到我把你的主題行字面化了。[360] 和以下創建一個串列陣列。但是 [356] 是一個二維陣列，其中每個“行”看起來像一個串列 - 但事實并非如此。

早些時候

In [354]: alist = [['David','Blue','Dog','Car'],['Sally','Yellow','Cat','Boat']]
     ...: 
In [355]: alist
Out[355]: [['David', 'Blue', 'Dog', 'Car'], ['Sally', 'Yellow', 'Cat', 'Boat']]
In [356]: np.array(alist)
Out[356]: 
array([['David', 'Blue', 'Dog', 'Car'],
       ['Sally', 'Yellow', 'Cat', 'Boat']], dtype='<U6')

這會生成一個二維字串陣列。該構造與制作二維數值陣列的教科書示例沒有什么不同：

In [358]: np.array([[1, 2], [3, 4]])
Out[358]: 
array([[1, 2],
       [3, 4]])

使用hstack或concatenate：

In [359]: np.hstack(alist)
Out[359]: 
array(['David', 'Blue', 'Dog', 'Car', 'Sally', 'Yellow', 'Cat', 'Boat'],
      dtype='<U6')

要創建一個只有 2 個串列的陣列，您必須初始化一個：

In [360]: arr = np.empty(2, object)
In [361]: arr
Out[361]: array([None, None], dtype=object)
In [362]: arr[:] = alist
In [363]: arr
Out[363]: 
array([list(['David', 'Blue', 'Dog', 'Car']),
       list(['Sally', 'Yellow', 'Cat', 'Boat'])], dtype=object)

如果串列的長度不同，

In [364]: np.array([["David", "Blue"], ["Sally", "Yellow", "Cat"]])
<ipython-input-364-be12d6dec312>:1: VisibleDeprecationWarning: Creating an ndarray from ragged nested sequences (which is a list-or-tuple of lists-or-tuples-or ndarrays with different lengths or shapes) is deprecated. If you meant to do this, you must specify 'dtype=object' when creating the ndarray.
  np.array([["David", "Blue"], ["Sally", "Yellow", "Cat"]])
Out[364]: 
array([list(['David', 'Blue']), list(['Sally', 'Yellow', 'Cat'])],
      dtype=object)

完整閱讀該警告。

默認情況下np.array，嘗試創建基類的多維陣列，如 int 或 string。只有當它做不到時，它才會退回到創建一個物件 dtype 陣列。這種陣列應該被視為二等陣列，只有在真正需要時才使用。通常，串列串列也一樣好。

Your iterative creation is one such case

people = []

records = GetRecordsFromDB()

for record in records:

    # Do some stuff    
    
    # Tried
    # person = [name, color, animal, vehicle]
    person = np.array([name, color, animal, vehicle])
    people = append(person)

Items, whether lists or array (or anything else) can be added to a list in-place with just the addition of a reference. Trying use concatenate to add items of an array is, not only harder to get right, but slower, since it is making a whole new array each time. That means a lot of copying!

np.append is a badly named way of calling concatenate. It is not a list.append clone.

Using np.concatenate requires a careful handling of dimensions. Don't be sloppy, thinking it will figure out what you want.

Similarly this is not a close of list []:

In [365]: np.array([])
Out[365]: array([], dtype=float64)
In [366]: np.array([]).shape
Out[366]: (0,)

It is a 1d array with a specific shape. You can only concatenate it with another 1d array - one the only axis, 0).

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