Question

I have a data frame of the form:

>df
stationid    station      gear sample     lat    lon       date depth
1     25679          CORBOX150    UE4 53.9015 7.8617 15.07.1987    19
2     25681 UE9 Kern CORCRB050    UE9 54.0167 7.3982 15.07.1987    33
3        NA                           54.0167 7.3982 15.07.1987    33

a logical test on stationid gives me, next to the correct first line, an annoying line full of NAs:

> df[df$stationid=="25679",]
stationid station      gear sample     lat    lon       date depth
1      25679         CORBOX150    UE4 53.9015 7.8617 15.07.1987    19
NA        NA    <NA>      <NA>   <NA>      NA     NA       <NA>    NA

why is that?

Somewhere in line 3 of df, things get messed up, I suppose.

Heres the data:

df<-structure(list(stationid = c(25679L, 25681L, NA), station = structure(c(2L, 
3L, 1L), .Label = c("", " ", "UE9 Kern"), class = "factor"), 
gear = structure(c(2L, 3L, 1L), .Label = c("", "CORBOX150", 
"CORCRB050"), class = "factor"), sample = structure(c(2L, 
3L, 1L), .Label = c("", "UE4", "UE9"), class = "factor"), 
lat = c(53.9015, 54.0167, 54.0167), lon = c(7.8617, 7.3982, 
7.3982), date = structure(c(1L, 1L, 1L), .Label = "15.07.1987", class = "factor"), 
depth = c(19L, 33L, 33L)), .Names = c("stationid", "station", 
"gear", "sample", "lat", "lon", "date", "depth"), class = "data.frame", row.names = c(NA, 
-3L))

Answer 1

Any comparison with NA leads to a result of NA (See http://cran.r-project.org/doc/manuals/R-intro.html#Missing-values ) … you can either use

df[df$stationid==25679 & !is.na(df$stationid),]

or (as suggested in the comment above)

df[which(df$stationid==25679),] 

or

subset(df,stationid==25679)

(subset has the sometimes unwanted side-effect of dropping NA values, but in this case it’s exactly what you do want)