【課程2.6】 Pandas數據結構Dataframe：索引
Dataframe既有行索引也有列索引，可以被看做由Series組成的字典（共用一個索引）
選擇列 / 選擇行 / 切片 / 布爾判斷
1.選擇行與列
df = pd.DataFrame(np.random.rand(12).reshape(3,4)*100,
index = ['one','two','three'],
columns = ['a','b','c','d'])
print(df)
data1 = df['a']
data2 = df[['a','c']]
print(data1,type(data1))
print(data2,type(data2))
print('-----')
# 按照列名選擇列，只選擇一列輸出Series，選擇多列輸出Dataframe
data3 = df.loc['one']
data4 = df.loc[['one','two']]
print(data2,type(data3))
print(data3,type(data4))
# 按照index選擇行，只選擇一行輸出Series，選擇多行輸出Dataframe
-----------------------------------------------------------------------
2.df[] - 選擇列
# 一般用於選擇列，也可以選擇行
df = pd.DataFrame(np.random.rand(12).reshape(3,4)*100,
index = ['one','two','three'],
columns = ['a','b','c','d'])
print(df)
print('-----')
data1 = df['a']
data2 = df[['b','c']] # 嘗試輸入 data2 = df[['b','c','e']]
print(data1)
print(data2)
# df[]默認選擇列，[]中寫列名（所以一般數據colunms都會單獨制定，不會用默認數字列名，以免和index衝突）
# 單選列爲Series，print結果爲Series格式
# 多選列爲Dataframe，print結果爲Dataframe格式
data3 = df[:1]
#data3 = df[0]
#data3 = df['one']
print(data3,type(data3))
# df[]中爲數字時，默認選擇行，且只能進行切片的選擇，不能單獨選擇（df[0]）
# 輸出結果爲Dataframe，即便只選擇一行
# df[]不能通過索引標籤名來選擇行(df['one'])
# 核心筆記：df[col]一般用於選擇列，[]中寫列名
-----------------------------------------------------------------------
a b c d
one 88.490183 93.588825 1.605172 74.610087
two 45.905361 49.257001 87.852426 97.490521
three 95.801001 97.991028 74.451954 64.290587
-----
one 88.490183
two 45.905361
three 95.801001
Name: a, dtype: float64
b c
one 93.588825 1.605172
two 49.257001 87.852426
three 97.991028 74.451954
a b c d
one 88.490183 93.588825 1.605172 74.610087 <class 'pandas.core.frame.DataFrame'>
3.df.loc[] - 按index選擇行
df1 = pd.DataFrame(np.random.rand(16).reshape(4,4)*100,
index = ['one','two','three','four'],
columns = ['a','b','c','d'])
df2 = pd.DataFrame(np.random.rand(16).reshape(4,4)*100,
columns = ['a','b','c','d'])
print(df1)
print(df2)
print('-----')
data1 = df1.loc['one']
data2 = df2.loc[1]
print(data1)
print(data2)
print('單標籤索引\n-----')
# 單個標籤索引，返回Series
data3 = df1.loc[['two','three','five']]
data4 = df2.loc[[3,2,1]]
print(data3)
print(data4)
print('多標籤索引\n-----')
# 多個標籤索引，如果標籤不存在，則返回NaN
# 順序可變
data5 = df1.loc['one':'three']
data6 = df2.loc[1:3]
print(data5)
print(data6)
print('切片索引')
# 可以做切片對象
# 末端包含
# 核心筆記：df.loc[label]主要針對index選擇行，同時支持指定index，及默認數字index
-----------------------------------------------------------------------
one 73.070679 7.169884 80.820532 62.299367
two 34.025462 77.849955 96.160170 55.159017
three 27.897582 39.595687 69.280955 49.477429
four 76.723039 44.995970 22.408450 23.273089
a b c d
0 93.871055 28.031989 57.093181 34.695293
1 22.882809 47.499852 86.466393 86.140909
2 80.840336 98.120735 84.495414 8.413039
3 59.695834 1.478707 15.069485 48.775008
-----
a 73.070679
b 7.169884
c 80.820532
d 62.299367
Name: one, dtype: float64
a 22.882809
b 47.499852
c 86.466393
d 86.140909
Name: 1, dtype: float64
單標籤索引
-----
a b c d
two 34.025462 77.849955 96.160170 55.159017
three 27.897582 39.595687 69.280955 49.477429
five NaN NaN NaN NaN
a b c d
3 59.695834 1.478707 15.069485 48.775008
2 80.840336 98.120735 84.495414 8.413039
1 22.882809 47.499852 86.466393 86.140909
多標籤索引
-----
a b c d
one 73.070679 7.169884 80.820532 62.299367
two 34.025462 77.849955 96.160170 55.159017
three 27.897582 39.595687 69.280955 49.477429
a b c d
1 22.882809 47.499852 86.466393 86.140909
2 80.840336 98.120735 84.495414 8.413039
3 59.695834 1.478707 15.069485 48.775008
切片索引
4.df.iloc[] - 按照整數位置（從軸的0到length-1）選擇行
# 類似list的索引，其順序就是dataframe的整數位置，從0開始計
df = pd.DataFrame(np.random.rand(16).reshape(4,4)*100,
index = ['one','two','three','four'],
columns = ['a','b','c','d'])
print(df)
print('------')
print(df.iloc[0])
print(df.iloc[-1])
#print(df.iloc[4])
print('單位置索引\n-----')
# 單位置索引
# 和loc索引不同，不能索引超出數據行數的整數位置
print(df.iloc[[0,2]])
print(df.iloc[[3,2,1]])
print('多位置索引\n-----')
# 多位置索引
# 順序可變
print(df.iloc[1:3])
print(df.iloc[::2])
print('切片索引')
# 切片索引
# 末端不包含
-----------------------------------------------------------------------
a b c d
one 21.848926 2.482328 17.338355 73.014166
two 99.092794 0.601173 18.598736 61.166478
three 87.183015 85.973426 48.839267 99.930097
four 75.007726 84.208576 69.445779 75.546038
------
a 21.848926
b 2.482328
c 17.338355
d 73.014166
Name: one, dtype: float64
a 75.007726
b 84.208576
c 69.445779
d 75.546038
Name: four, dtype: float64
單位置索引
-----
a b c d
one 21.848926 2.482328 17.338355 73.014166
three 87.183015 85.973426 48.839267 99.930097
a b c d
four 75.007726 84.208576 69.445779 75.546038
three 87.183015 85.973426 48.839267 99.930097
two 99.092794 0.601173 18.598736 61.166478
多位置索引
-----
a b c d
two 99.092794 0.601173 18.598736 61.166478
three 87.183015 85.973426 48.839267 99.930097
a b c d
one 21.848926 2.482328 17.338355 73.014166
three 87.183015 85.973426 48.839267 99.930097
切片索引
5.布爾型索引,和Series原理相同
df = pd.DataFrame(np.random.rand(16).reshape(4,4)*100,
index = ['one','two','three','four'],
columns = ['a','b','c','d'])
print(df)
print('------')
b1 = df < 20
print(b1,type(b1))
print(df[b1]) # 也可以書寫爲 df[df < 20]
print('------')
# 不做索引則會對數據每個值進行判斷
# 索引結果保留 所有數據：True返回原數據，False返回值爲NaN
b2 = df['a'] > 50
print(b2,type(b2))
print(df[b2]) # 也可以書寫爲 df[df['a'] > 50]
print('------')
# 單列做判斷
# 索引結果保留 單列判斷爲True的行數據，包括其他列
b3 = df[['a','b']] > 50
print(b3,type(b3))
print(df[b3]) # 也可以書寫爲 df[df[['a','b']] > 50]
print('------')
# 多列做判斷
# 索引結果保留 所有數據：True返回原數據，False返回值爲NaN
b4 = df.loc[['one','three']] < 50
print(b4,type(b4))
print(df[b4]) # 也可以書寫爲 df[df.loc[['one','three']] < 50]
print('------')
# 多行做判斷
# 索引結果保留 所有數據：True返回原數據，False返回值爲NaN
-----------------------------------------------------------------------
a b c d
one 19.185849 20.303217 21.800384 45.189534
two 50.105112 28.478878 93.669529 90.029489
three 35.496053 19.248457 74.811841 20.711431
four 24.604478 57.731456 49.682717 82.132866
------
a b c d
one True False False False
two False False False False
three False True False False
four False False False False <class 'pandas.core.frame.DataFrame'>
a b c d
one 19.185849 NaN NaN NaN
two NaN NaN NaN NaN
three NaN 19.248457 NaN NaN
four NaN NaN NaN NaN
------
one False
two True
three False
four False
Name: a, dtype: bool <class 'pandas.core.series.Series'>
a b c d
two 50.105112 28.478878 93.669529 90.029489
------
a b
one False False
two True False
three False False
four False True <class 'pandas.core.frame.DataFrame'>
a b c d
one NaN NaN NaN NaN
two 50.105112 NaN NaN NaN
three NaN NaN NaN NaN
four NaN 57.731456 NaN NaN
------
a b c d
one True True True True
three True True False True <class 'pandas.core.frame.DataFrame'>
a b c d
one 19.185849 20.303217 21.800384 45.189534
two NaN NaN NaN NaN
three 35.496053 19.248457 NaN 20.711431
four NaN NaN NaN NaN
------
6.多重索引：比如同時索引行和列
# 先選擇列再選擇行 —— 相當於對於一個數據，先篩選字段，再選擇數據量
df = pd.DataFrame(np.random.rand(16).reshape(4,4)*100,
index = ['one','two','three','four'],
columns = ['a','b','c','d'])
print(df)
print('------')
print(df['a'].loc[['one','three']]) # 選擇a列的one，three行
print(df[['b','c','d']].iloc[::2]) # 選擇b，c，d列的one，three行
print(df[df['a'] < 50].iloc[:2]) # 選擇滿足判斷索引的前兩行數據
-----------------------------------------------------------------------
a b c d
one 50.660904 89.827374 51.096827 3.844736
two 70.699721 78.750014 52.988276 48.833037
three 33.653032 27.225202 24.864712 29.662736
four 21.792339 26.450939 6.122134 52.323963
------
one 50.660904
three 33.653032
Name: a, dtype: float64
b c d
one 89.827374 51.096827 3.844736
three 27.225202 24.864712 29.662736
a b c d
three 33.653032 27.225202 24.864712 29.662736
four 21.792339 26.450939 6.122134 52.323963
作業1：如圖創建Dataframe(4*4，值爲0-100的隨機數)，通過索引得到以下值
① 索引得到b，c列的所有值
② 索引得到第三第四行的數據
③ 按順序索引得到two，one行的值
④ 索引得到大於50的值
df = df1 = pd.DataFrame(np.random.rand(16).reshape(4,4)*100,
index = ['one','two','three','four'],
columns = ['a','b','c','d'])
print('創建Dataframe爲：\n',df,'\n-------')
print('b，c列的所有值爲:\n', df[['b','c']],'\n------')
print('第三第四行的值爲:\n', df[2:4],'\n------')
print('按順序索引得到two，one行的值爲:\n', df.loc[['two','one']],'\n------')
print('大於50的值爲:\n', df[df>50],'\n------')