Subsetting R data frame results in mysterious NA rows

Wrap the condition in which:

df[which(df$number1 < df$number2), ]

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How it works:

It returns the row numbers where the condition matches (where the condition is TRUE) and subsets the data frame on those rows accordingly.

Say that:

which(df$number1 < df$number2)

returns row numbers 1, 2, 3, 4 and 5.

As such, writing:

df[which(df$number1 < df$number2), ]

is the same as writing:

df[c(1, 2, 3, 4, 5), ]

Or an even simpler version is:

df[1:5, ]