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I was going through some earlier post-

Quickest way to read a subset of rows of a CSV

One way to select subset of data is 

write.csv(iris,"iris.csv")

fread("shuf -n 5 iris.csv")

However I was wondering if I can pass some SQL query instead of top 5 rows e.g. only import those rows that have V6 = versicolor

Is there any way to do this using fread function?

Matt Summersgill
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Bogaso
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1 Answers1

3

This worked for me in windows (unix alternative is grep)

write.csv(iris,"iris.csv")

fread(cmd = paste('findstr', 'versicolor', 'iris.csv'))

    V1  V2  V3  V4  V5         V6
 1:  51 7.0 3.2 4.7 1.4 versicolor
 2:  52 6.4 3.2 4.5 1.5 versicolor
 3:  53 6.9 3.1 4.9 1.5 versicolor
 4:  54 5.5 2.3 4.0 1.3 versicolor
 5:  55 6.5 2.8 4.6 1.5 versicolor
 6:  56 5.7 2.8 4.5 1.3 versicolor
 7:  57 6.3 3.3 4.7 1.6 versicolor
 8:  58 4.9 2.4 3.3 1.0 versicolor
 9:  59 6.6 2.9 4.6 1.3 versicolor
10:  60 5.2 2.7 3.9 1.4 versicolor
11:  61 5.0 2.0 3.5 1.0 versicolor

It outputs only those rows that contain "versicolor" in any field.

Humpelstielzchen
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  • In macOS, I am getting error as > fread(grep = paste('findstr', 'versicolor', 'iris.csv')) Error in fread(grep = paste("findstr", "versicolor", "iris.csv")) : unused argument (grep = paste("findstr", "versicolor", "iris.csv")) – Bogaso Jul 02 '19 at 19:35
  • No, you have to replace `findstr` with `grep`. – Humpelstielzchen Jul 03 '19 at 04:42
  • Thanks. It works for my problem. But still, it would be a better idea if I could pass some SQL like query to fetch data with a complex relationship – Bogaso Jul 03 '19 at 05:26
  • Well then the answer is no. You can't pass an SQL query to fread. – Humpelstielzchen Jul 03 '19 at 05:44