Python: NumPyアレイの添え字操作

2016/8/28 Python

Pythonの数値計算モジュールであるNumPyのアレイに対する添え字操作の基本を記載する。

ソースコード

Python3系で記述している。

import numpy as np# 1行21列のアレイをa rangeメソッドで作成arr = np.arange(0,21)print("arr == ", arr)# 添え字による要素へのアクセスprint("arr[10] == ", arr[10])# 0〜10までの要素をもつアレイを出力print("arr[0:11] ==", arr[0:11])# 0〜10迄の要素にスカラー値を代入arr[0:11] = 100print("arr[0:11] = 100\n", arr, "\n")arr = np.arange(0,11)slice_arr = arr[0:5]print("arr == ", arr)print("slice_arr == ", slice_arr)# slice_arrの全要素にスカラー値100を代入slice_arr[:] = 100print("slice_arr == ", slice_arr)# slice_arrはarrの参照の為、参照元まで書き変わっている。print("arr == ", arr, "\n")# アレイデータのコピー(≠参照)arr_cp = arr.copy()arr_cp[:] = 99print("arr_cp == ", arr_cp)# arr_cp側の変更はarr側へは影響ないprint("arr == ", arr, "\n")# 2次元アレイへの添え字アクセスarr_2d = np.array(([1,2,3],[4,5,6],[7,8,9]))print("arr_2d == \n", arr_2d)print("arr_2d[1] == ", arr_2d[1])# 個別の要素へのアクセスは[row][col] or [row,col]の添え字でアクセスprint("arr_2d[0][1] & arr_2d[0,1] == ", arr_2d[0][1], arr_2d[0,1], "\n")# 2次元アレイのスライスprint("arr_2d[:2,1:] == \n", arr_2d[:2,1:])print("arr_2d[:2,:] == \n", arr_2d[:2,:], "\n")# 9x5のアレイを作成arr_2d = np.zeros((9,5))print("arr_2d == \n", arr_2d)# shapeはアレイの長さをタプル(row, col)で返却する。# shape[0]は行数arr_len = arr_2d.shape[0]for i in range(arr_len):  arr_2d[i] = iprint("arr_2d == \n", arr_2d, "\n")# 指定の行だけを取り出すprint("arr_2d[[1,3,5,7]] == \n", arr_2d[[1,3,5,7]], "\n")# 指定の行だけを順番を変えて取り出すprint("arr_2d[[7,5,1,3]] == \n", arr_2d[[7,5,1,3]], "\n")

実行結果

arr ==  [ 0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20]arr[10] ==  10arr[0:11] == [ 0  1  2  3  4  5  6  7  8  9 10]arr[0:11] = 100 [100 100 100 100 100 100 100 100 100 100 100  11  12  13  14  15  16  17  18  19  20]arr ==  [ 0  1  2  3  4  5  6  7  8  9 10]slice_arr ==  [0 1 2 3 4]slice_arr ==  [100 100 100 100 100]arr ==  [100 100 100 100 100   5   6   7   8   9  10]arr_cp ==  [99 99 99 99 99 99 99 99 99 99 99]arr ==  [100 100 100 100 100   5   6   7   8   9  10]arr_2d == [[1 2 3] [4 5 6] [7 8 9]]arr_2d[1] ==  [4 5 6]arr_2d[0][1] & arr_2d[0,1] ==  2 2arr_2d[:2,1:] == [[2 3] [5 6]]arr_2d[:2,:] == [[1 2 3] [4 5 6]]arr_2d == [[ 0.  0.  0.  0.  0.] [ 0.  0.  0.  0.  0.] [ 0.  0.  0.  0.  0.] [ 0.  0.  0.  0.  0.] [ 0.  0.  0.  0.  0.] [ 0.  0.  0.  0.  0.] [ 0.  0.  0.  0.  0.] [ 0.  0.  0.  0.  0.] [ 0.  0.  0.  0.  0.]]arr_2d == [[ 0.  0.  0.  0.  0.] [ 1.  1.  1.  1.  1.] [ 2.  2.  2.  2.  2.] [ 3.  3.  3.  3.  3.] [ 4.  4.  4.  4.  4.] [ 5.  5.  5.  5.  5.] [ 6.  6.  6.  6.  6.] [ 7.  7.  7.  7.  7.] [ 8.  8.  8.  8.  8.]]arr_2d[[1,3,5,7]] == [[ 1.  1.  1.  1.  1.] [ 3.  3.  3.  3.  3.] [ 5.  5.  5.  5.  5.] [ 7.  7.  7.  7.  7.]]arr_2d[[7,5,1,3]] == [[ 7.  7.  7.  7.  7.] [ 5.  5.  5.  5.  5.] [ 1.  1.  1.  1.  1.] [ 3.  3.  3.  3.  3.]]