如何从不以特定字符开头和结尾的R数据帧列中提取值？
有时候，我们只是希望提取数据列的值，这些值基于具有字符串的列的初始值和结束值，有时候，具有字符串的列的值被记录了一些额外的字符，我们希望提取这些值。为此，我们可以使用带有单个方括号的grepl的否定。
示例
考虑以下数据帧-
> x2<-c("Alabama", "Alaska", "American Samoa", "Arizona", "Arkansas", "California",
"Colorado", "Connecticut", "Delaware", "District of Columbia", "Florida", "Georgia",
"Guam", "Hawaii", "Idaho", "Illinois", "Indiana", "Iowa", "Kansas", "Kentucky",
"Louisiana", "Maine", "Maryland", "Massachusetts", "Michigan", "Minnesota", "Minor
Outlying Islands", "Mississippi", "Missouri", "Montana", "Nebraska", "Nevada", "New
Hampshire", "New Jersey", "New Mexico", "New York", "North Carolina", "North
Dakota", "Northern Mariana Islands", "Ohio", "Oklahoma", "Oregon", "Pennsylvania",
"Puerto Rico", "Rhode Island", "South Carolina", "South Dakota", "Tennessee", "Texas",
"U.S. Virgin Islands", "Utah", "Vermont", "Virginia", "Washington", "West Virginia",
"Wisconsin", "Wyoming")> df2<-data.frame(x2)
> head(df2,20)
x2
1 Alabama
2 Alaska
3 American Samoa
4 Arizona
5 Arkansas
6 California
7 Colorado
8 Connecticut
9 Delaware
10 District of Columbia
11 Florida
12 Georgia
13 Guam
14 Hawaii
15 Idaho
16 Illinois
17 Indiana
18 Iowa
19 Kansas
20 Kentucky
查找既不以A开头也不以-a结尾的状态
> df2[!grepl("^A|a$",df2$x2),][1] Colorado Connecticut Delaware
[4] Guam Hawaii Idaho
[7] Illinois Kansas Kentucky
[10] Maine Maryland Massachusetts
[13] Michigan Minor Outlying Islands Mississippi
[16] Missouri New Hampshire New Jersey
[19] New Mexico New York Northern Mariana Islands
[22] Ohio Oregon Puerto Rico
[25] Rhode Island Tennessee Texas
[28] U.S. Virgin Islands Utah Vermont
[31] Washington Wisconsin Wyoming
57 Levels: Alabama Alaska American Samoa Arizona Arkansas ... Wyoming
让我们看看另一个例子
> x1<-
c("Indiaaa","Chinaaa","Russiaa","Canadaaa","Indonesiaaa","Croatiaaa","Mauritaniaaa","
Albaniaaa","Angolaaa","Armeniaaa","Malaysiaaa","Maltaaa","Boliviaaa","Burmaaa","Pa
nama","Romaniaa","Saudi-Arabia","Serbiaaa","Syriaaa","Tongaaa","Koreaaa","Libya")> y1<-sample(1:10,22,replace=TRUE)
> df1<-data.frame(x1,y1)
> df1
x1 y1
1 Indiaaa 6
2 Chinaaa 1
3 Russiaa 9
4 Canadaaa 7
5 Indonesiaaa 7
6 Croatiaaa 3
7 Mauritaniaaa 6
8 Albaniaaa 2
9 Angolaaa 10
10 Armeniaaa 10
11 Malaysiaaa 7
12 Maltaaa 3
13 Boliviaaa 2
14 Burmaaa 10
15 Panama 1
16 Romaniaa 10
17 Saudi-Arabia 10
18 Serbiaaa 8
19 Syriaaa 10
20 Tongaaa 5
21 Koreaaa 7
22 Libya 8
> df1[!grepl("^A|aa$",df1$x1),]x1 y1
15 Panama 1
17 Saudi-Arabia 10
22 Libya 8
> df1[!grepl("^S|aa$",df1$x1),]x1 y1
15 Panama 1
22 Libya 8
> df1[!grepl("^B|aa$",df1$x1),]x1 y1
15 Panama 1
17 Saudi-Arabia 10
22 Libya 8
> df1[!grepl("^P|aa$",df1$x1),]x1 y1
17 Saudi-Arabia 10
22 Libya 8
> df1[!grepl("^L|aa$",df1$x1),]x1 y1
15 Panama 1
17 Saudi-Arabia 10