[R] Unique?

[Guenther, Cameron]

Hello,
I have sample data set that looks like:

YEAR	MONTH	DAY	CONTINUE	SPL		TIMEFISH
TIMEUNIT	AREA	COUNTY	DEPTH	DEPUNIT	GEAR		TRIPID
CONVUNIT
1992	1	26	1		SP0073928	8
H		7	25		4	NA		1000000
02163399054	161
1992	1	26	1		SP0073928	8
H		7	25		4	NA		1000000
02163399054	8
1992	1	26	2		SP0004228	8
H		7	25		4	NA		1000000
02163399054	161
1992	1	26	2		SP0004228	8
H		7	25		4	NA		1000000
02163399054	8
1992	1	25	NA		SP0052652	8
H		7	25		4	NA		1000000
02163399057	85
1992	1	26	NA		SP0037940	8
H		7	25		4	NA		1000000
02163399058	70
1992	1	27	NA		SP0072357	8
H		7	25		4	NA		1000000
02163399059	15
1992	1	27	NA		SP0072357	8
H		7	25		4	NA		1000000
02163399059	20
1992	1	27	NA		SP0026324	8
H		7	25		4	NA		1000000
02163399060	8
1992	1	28	1		SP0072357	8
H		7	25		4	NA		1000000
02163399062	200

How can I use unique to extract the rows that have repeated tripid's
only, not a unique value for each variable but only for TRIPID.  I then
want to condense the unique values by summing the CONVUNIT for each
unique value of TRIPID.  I posted a similar question last week and
received a sufficient answer of how to do this without using uniqe.  The
solution below worked just fine on this sample data set but the full
data set has 446,000 rows of data and my computer and R simply cannot
handle this follwing code on data this large.

conds<-by(Step4,Step4$TRIPID,function(x)
replace(x[1,],"CONVUNIT",sum(x$CONVUNIT)))
Step5<-do.call(rbind,conds)

Thank you,

Cameron Guenther, Ph.D. 
Associate Research Scientist
FWC/FWRI, Marine Fisheries Research
100 8th Avenue S.E.
St. Petersburg, FL 33701
(727)896-8626 Ext. 4305
cameron.guenther at myfwc.com