Py - 1차원배열, 2차원배열

[문교식]

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 * 배열; 오류의 경우

>>> C=[]

>>> D=[3,4]

>>> C.append(D)

>>> C.append(D)

>>> C  =>  [[3,4],[3,4]]

>>> C[1][1]  ->  4    # OK

>>> C[1][0] = 5       # update C[[i][0] to a new value (=5)

>>> C  =>  [[5,4],[5,4]]   # Wrong; 5 appears at both places

How to filx it?  ------------------

  Why?  >> D=[3,4] 리스트[3,4]가 변수D에 할당되면 

              D가 리스트를 포인팅한다. 

              D가 C에 2회 입력된다. 

              그러나 두 리스트는 동일한 메모리를 차지한다.

              [3,4]의 어느 하나를 변경하면 나머지도 변경된다.

  Fix >> 

    >>> C=[]

    >>> for j in range(0,2):

    ...          C.append([])

    ...

    >>> C  => [[],[]]

    >>> for j in range(0,2):

    ...          C[j].append(3)

    ...          C[j].append(4)

    ... 

    >>> C  =>  [[3,4],[3,4]]

    >>> C[1][0]=5

    >>> C  =>  [[3,4],[5,4]]    # OK!

 [e.g.] 2차원 배열 만들기.

 >>> A=[]

 >>> for i in range(0,4):

 ...          A.append(0)

 ...

 >>> A  =>  [0,0,0,0]  ; 1차원 배열

 >>> B=[]

 >>> for i in range(0,4):

 ...           B.append(A)

 ...

 >>> B  =>  [ [0,0,0,0], [0,0,0,0], [0,0,0,0], [0,0,0,0] ]

looks good. hwr, indexing it makes some problems.

 >>> A[2]=2  =>  [0,0,2,0]  

 >>> B[0][1]=3

 >>> B  =>  [ [0,3,0,0], [0,3,0,0], [0,3,0,0], [0,3,0,0] ]  ; Wrong!

순서; ⒈ A=[]   # init

⒉ A=[0,0,0,0]

⒊ A=[ [0,0,0,0], [0,0,0,0], [0,0,0,0], [0,0,0,0] ]  ; Wrong!

A different try;

순서; ⒈ C=[]   # init

⒉ C=[ [], [], [], [] ] 

⒊ C=[ [0,0,0,0], [0,0,0,0], [0,0,0,0], [0,0,0,0] ] 

 >>> C=[]

 >>> for i in range(0,4):

 ...          C.append([])

 ...

 >>> C  =>  [ [], [], [], [] ] 

 >>> for i in range(0,4):

 ...          for j in range(0,4):

 ...               C[i].append(0)

 >>> C  =>  C=[ [0,0,0,0], [0,0,0,0], [0,0,0,0], [0,0,0,0] ] 

 >>> C[1][2]=5

 >>> C  => C=[ [0,0,0,0], [0,0,5,0], [0,0,0,0], [0,0,0,0] ]   

 # OK!

Using numpy library; 

>>> import numpy as np 

>>> A = np.zeros((5))     # define A as a one-dimensional array of 5 elements

                                  # initialized to zeros

>>> print(A)

       -> [0. 0. 0. 0. 0.]

>>> C = np.zeros((4,4))     # define C as a two-dimensional array of 4 by 4

                                    # initialized to zeros 

>>> print(C)

       -> [[0. 0. 0. 0.]

>>> C = np.zeros((4,4))     # define C as a two-dimensional array of 4 by 4

                                    # initialized to zeros 

>>> print(C)

       -> [[0. 0. 0. 0.]

            [0. 0. 0. 0.]

            [0. 0. 0. 0.]

            [0. 0. 0. 0.]]

>>> C[1][2] = 5

>>> print(C)

       -> [[0. 0. 0. 0.]

            [0. 0. 5. 0.]

            [0. 0. 0. 0.]

            [0. 0. 0. 0.]]

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