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.]]
参考サイト