R variable string replacement in a data frame

Question

I have a dataframe that looks as follows:

df <- data.frame(one=c("s1_below_10", "s2_below_20"), 
                 two=c("s3_above_10","s4_above_10"))

I want to replace all the strings by the number preceding the first underscore. In other words, the desired output is

1   3
2   4

I would like to know how I can perform this replacement (the dataset is very large). Thanks for your help.

thelatemail · Accepted Answer · 2013-08-13 20:58:51Z

4

The basic gsub call would be something like:

gsub("^.+?(\\d+)_.+","\\1",df$one)
[1] "1" "2"

Which you could lapply to each column:

data.frame(lapply(df, gsub, pattern="^.+(\\d+)_.+",replacement= "\\1"))
  one two
1   1   3
2   2   4

edited Aug 13, 2013 at 20:58

answered Aug 13, 2013 at 0:18

thelatemail

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1 Comment

Greg Snow Over a year ago

The above pattern will only grab the last digit of a multi digit number (which may be what was wanted, or not) because the first '.+' is greedy and will match as much as possible. A simple correction of changing '.+' to '.+?' will make it non-greedy and let the \\d+ match multiple digits.

Marius · Accepted Answer · 2013-08-13 00:20:57Z

2

If the values you want are always the second character of the string (which seems to be true of all your examples), you can do this with substr:

data.frame(lapply(df, substr, 2, 2))

Output:

  one two
1   1   3
2   2   4

answered Aug 13, 2013 at 0:20

Marius

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