How to check if CSV file has a comma or a semicolon as separator?

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Here are a few approaches assuming that the only difference among the format of the files is whether the separator is semicolon and the decimal is a comma or the separator is a comma and the decimal is a point.

1) fread As mentioned in the comments fread in data.table package will automatically detect the separator for common separators and then read the file in using the separator it detected. This can also handle certain other changes in format such as automatically detecting whether the file has a header.

2) grepl Look at the first line and see if it has a comma or semicolon and then re-read the file:

L <- readLines("myfile", n = 1)
if (grepl(";", L)) read.csv2("myfile") else read.csv("myfile")

3) count.fields We can assume semicolon and then count the fields in the first line. If there is one field then it is comma separated and if not then it is semicolon separated.

L <- readLines("myfile", n = 1)
numfields <- count.fields(textConnection(L), sep = ";")
if (numfields == 1) read.csv("myfile") else read.csv2("myfile")

Update Added (3) and made improvements to all three.

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