[R] help with dealing with integer(0) returns from grep used within a conditional loop

[Mark Kimpel]

I am using grep to locate colnames to automate a report build and have
run into a problem when a colname is not found. The use of integer(0)
in a conditional statement seems to be a no no as it has length 0.
Below is a self-contained trivial example. I would like to get
something like "NA" or -1 for the position when it is not found OR
learn a way to use integer(0) or some "cast" of it in a logical
statement. Example, output, and sessionInfo follow. Thanks, Mark

####################################3
vec.1 <- c("a", "b", "c")
vec.2 <- c("a", "c", "d")
for (i in 1:length(vec.1)){
  for (j in 1:length(vec.2)){
    print(paste("i:", i, " j:", j, sep = ""))
    pos <- grep(vec.1[i], vec.2[j])
    if (pos > 0){
      print("pos identified")
    }
    else{
      print("pos not found")
    }
  }
}
################################

> #################################3
> vec.1 <- c("a", "b", "c")
> vec.2 <- c("a", "c", "d")
> for (i in 1:length(vec.1)){
+   for (j in 1:length(vec.2)){
+     print(paste("i:", i, " j:", j, sep = ""))
+     pos <- grep(vec.1[i], vec.2[j])
+     if (pos > 0){
+       print("pos identified")
+     }
+     else{
+       print("pos not found")
+     }
+   }
+ }
[1] "i:1 j:1"
[1] "pos identified"
[1] "i:1 j:2"
Error in if (pos > 0) { : argument is of length zero
No suitable frames for recover()
> ###############
> sessionInfo()
R version 2.9.1 (2009-06-26)
x86_64-unknown-linux-gnu

locale:
LC_CTYPE=en_US.UTF-8;LC_NUMERIC=C;LC_TIME=en_US.UTF-8;LC_COLLATE=en_US.UTF-8;LC_MONETARY=C;LC_MESSAGES=en_US.UTF-8;LC_PAPER=en_US.UTF-8;LC_NAME=C;LC_ADDRESS=C;LC_TELEPHONE=C;LC_MEASUREMENT=en_US.UTF-8;LC_IDENTIFICATION=C

attached base packages:
[1] tcltk     stats     graphics  grDevices datasets  utils     methods
[8] base

other attached packages:
 [1] sfsmisc_1.0-7       KEGG.db_2.2.11      GO.db_2.2.11
 [4] rat2302.db_2.2.11   GOstats_2.10.0      RSQLite_0.7-1
 [7] DBI_0.2-4           graph_1.22.2        Category_2.10.1
[10] AnnotationDbi_1.6.1 qvalue_1.18.0       limma_2.18.2
[13] affy_1.22.0         Biobase_2.4.1

loaded via a namespace (and not attached):
 [1] affyio_1.12.0        annotate_1.22.0      genefilter_1.24.2
 [4] GSEABase_1.6.1       preprocessCore_1.6.0 RBGL_1.20.0
 [7] splines_2.9.1        survival_2.35-4      tools_2.9.1
[10] XML_2.5-3            xtable_1.5-5
------------------------------------------------------------
Mark W. Kimpel MD  ** Neuroinformatics ** Dept. of Psychiatry
Indiana University School of Medicine

15032 Hunter Court, Westfield, IN  46074

(317) 490-5129 Work, & Mobile & VoiceMail

"The real problem is not whether machines think but whether men do."
-- B. F. Skinner