[R] merging data frames gives all NAs

[David Winsemius]

On Feb 1, 2010, at 5:16 PM, James Rome wrote:

> Dear kind R helpers,
>
> I have a vector of runway names in rwy  ("31R", "31L",...  the  
> number is user selectable)
> arrgnd is a data frame with data for all flights and all runways,  
> with a Runway column.
> I am trying to subset arrgnd into a dat frame for each selected  
> runway, and then combine them back together using the following code:
>
> for (j in 1:nr) {    # nr = number of user-selected runways

Safer would be:

  for (j in seq_along(rwy) {

>    ar4rw = arrgnd[arrgnd$Runway==rwy[j],]

Clearer would be :

         ar4rw <- subset(arrgnd, Runway=j) # and I think the NA line's  
will also disappear.


>    if (j == 1) {
>        arrw = ar4rw
>    }
>    else {
>        arrw = merge(arrw, ar4rw)
>    }
> }

You really should give us something like:

dput(rwy)
dput( head(arrgnd, 10) )
>
> but, the merge step gives me a data frame with all NAs. In addition,  
> ar4rw always gets a row with NAs at the start, which I do not  
> understand. There are no rows with all NAs in the arrgnd data frame.
> > ar4rw[1:2,]  # first time through for 31R
>        DateTime       Date month hour minute quarter weekday IATA  
> ICAO Flight
> NA <NA> <NA>    NA   NA     NA      NA      NA <NA> <NA> <NA>
> 529 1/1/09 21:46 2009-01-01     1   21     46      87       5   TA   
> TAI TAI570
>    AircraftType   Tail  Arrived   STA Runway     FromTo Delay
> NA <NA> <NA> <NA> <NA> <NA> <NA>    NA
> 529         A320 N496TA 21:46:58 22:30    31R MSLP /KJFK     0
>                       Operator            dq gw
> NA <NA> <NA> NA
> 529 TACA INTERNATIONAL AIRLINES 2009-01-01 87  1
>
> > ar4rw[1:2,]   # second time through for 31L
>        DateTime       Date month hour minute quarter weekday IATA  
> ICAO Flight
> NA <NA> <NA>    NA   NA     NA      NA      NA <NA> <NA> <NA>
> 552 1/1/09 23:03 2009-01-01     1   23      3      92       5   AA   
> AAL  AAL22
>    AircraftType   Tail  Arrived   STA Runway    FromTo  
> Delay          Operator
> NA <NA> <NA> <NA> <NA> <NA> <NA>    NA <NA>
> 552         B762 N329AA 23:03:35 23:10    31L LAX  /JFK     0  
> AMERICAN AIRLINES
>               dq gw
> NA <NA> NA
>
> But after the merge, I get all NAs. What am I doing wrong?

The data layout gets mangled and I cannot tell what rows are being  
matched to what. Use dput to convey an unambiguous, and easily  
replicated example.
>
> Thanks,
> Jim Rome
>
> 552 2009-01-01 92  1
>
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David Winsemius, MD
Heritage Laboratories
West Hartford, CT