[R] read.table, NA assignment, and sep

[Juliet Hannah]

I place the following data in a file

id 	 rs835 	 rs169 	 rs174
1001 	 CC 	 GG 	 CC
10032 	 CC 	 GG 	 CC
10066 	 CC 	 NA 	 CC

If I read it in as
tempDat <- read.table("tempDat.txt",na.strings="NA",header=TRUE)

I get the following.

     id rs835 rs169 rs174
1  1001    CC    GG    CC
2 10032    CC    GG    CC
3 10066    CC  <NA>    CC

NA has been assigned a missing value.

If I add the separator

tempDat <- read.table("tempDat.txt",na.strings="NA",sep="\t",header=TRUE)
tempDat
     id rs835 rs169 rs174
1  1001   CC    GG     CC
2 10032   CC    GG     CC
3 10066   CC    NA     CC

NA does not get assigned a missing value.

Could you help me understand the logic of this. Thanks!