Keeping factors with zero occurrences in "table" output

f1 <- sample(1:7, 10, replace=TRUE)
f2 <- sample(1:6, 10, replace=TRUE)
table(f1, f2)

ff1 <- factor(f1, levels=1:7)
ff2 <- factor(f2, levels=1:7)
table(ff1, ff2)

From: Ravi Varadhan

Dear R group,

 

I have a data frame which contains data on preferences on 7 
items (ranks 1
through 7) listed by each participant.  I would like to 
tabulate this in a
7x7 table where the rows would be the items and the columns 
would be the
number of times that item received a particular rank.  

 

I tried doing this by creating a matrix by "rbind"ing each 
vector obtained
using "table" on each item.  This, however, does not work 
because there are
some items which didn't receive some of the ranks, so "table" 
does not list
that item.  Is there a way to force it to list the item and 
give a "0" to
it? Or, is there a simpler way to solve this problem?

 

Thanks for any help,

Ravi.

 

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Ravi Varadhan, Ph.D.

Assistant Professor,  The Center on Aging and Health

Division of Geriatric Medicine and Gerontology

Johns Hopkins University

Ph: (410) 502-2619

Fax: (410) 614-9625

Email:   <mailto:rvaradhan at jhmi.edu> rvaradhan at jhmi.edu

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