Remove everything after a string in a data frame column with missing values

问题

I have a data frame resembling the extract below:

Observation Identifier   Value
Obs001      ABC_2001     54
Obs002      ABC_2002     -2
Obs003                   1
Obs004                   1 
Obs005      Def_2001/05  

I would like to transform this data frame into a data frame where portions of the string after the "_" sign would be removed: as illustrated below:

Observation Identifier_NoTime   Value
Obs001      ABC                 54
Obs002      ABC                 -2
Obs003                          1
Obs004                          1 
Obs005      Def  

I tried experimenting with strsplit, gsub and sub as discussed here but cannot force those commends to work. I have to account for the fact that:

1. Column has missing values and I want to leave them where they are
2. String "_" is located in different places in the variable
3. I also want to leave the rest of the data frame the way it is

回答1:

You could try the below sub command to remove all the non-space characters from _ symbol.

sub("_\\S*", "", string)

Explanation:

• _ Matches a literal _ symbol.
• \S* Matches zero or more non-space characters.

OR

This would remove all the characters from _ symbol,

sub("_.*", "", string)

Explanation:

• _ Matches a literal _ symbol.
• .* Matches any character zero or more times.

来源：https://stackoverflow.com/questions/26611922/remove-everything-after-a-string-in-a-data-frame-column-with-missing-values