How to suppress printing of 0 lines of a table? - r

Question

I have a two factor vectors v1 and v2, which appear to be closely related (the entropy of each is very close to their joint entropy). Indeed, when I do table(v1,v2), I see something like this:
v2
v1 a2 b2 c2
a1 0 100 0
b1 0 0 0
c1 0 0 0
v2
v1 d2 e2 f2
a1 0 0 0
b1 0 0 0
c1 0 0 0
and so on - each factor has dozens of levels, so I get plenty of lines with all 0.
How to I print a table omitting lines which have only zeros in them?

Answer 1

Everybody seems to use rowSums(d)==0 or equivalent, but that will also suppress any row with equal numbers of ones and minus ones or any other zero sum combo. Safer would be to use:
d[ rowSums(d==0) != ncol(d) , ]
I suppose in the case where the object is the result of 'table', there would not be the risk of negative entries, but the risk would occur when this strategy is inappropariately applied to other settings.

Answer 2

Using your example:
v1 <- factor(rep("a1", 100), levels = paste0(letters[1:3], 1))
v2 <- factor(rep("b2", 100), levels = paste0(letters[1:6], 2))
R> table(v1, v2)
v2
v1 a2 b2 c2 d2 e2 f2
a1 0 100 0 0 0 0
b1 0 0 0 0 0 0
c1 0 0 0 0 0 0
Then the rowSums() function will compute the row sums for use. This works because a table is a either a vector or a matrix in disguise. Note in the sequence below showing intermediate steps how we convert the row sums into a logical vector by asking if they exceed 0.
R> rowSums(tab)
a1 b1 c1
100 0 0
R> rowSums(tab) > 0
a1 b1 c1
TRUE FALSE FALSE
R> tab[rowSums(tab) > 0, ]
a2 b2 c2 d2 e2 f2
0 100 0 0 0 0
The above drops the empty dimension. If you want to keep the table format, add drop = FALSE to the call, though note the extra , in there as we want all columns hence the empty argument between , ,:
R> tab[rowSums(tab) > 0, , drop = FALSE]
v2
v1 a2 b2 c2 d2 e2 f2
a1 0 100 0 0 0 0

Answer 3

I'd approach this with rowsums to get a logical vector of those greater than 0. And then use that vextor with indexing as in:
#make an example (please do this for yourself in the future)
d <- table(x=1:5, y=1:5)
d[1, 1] <- 0 #make one row have all 0s
d[rowSums(d) > 0, ]

Answer 4

Borrowing example data from #Gavin's answer
v1 <- factor(rep("a1", 100), levels = paste0(letters[1:3], 1))
v2 <- factor(rep("b2", 100), levels = paste0(letters[1:6], 2))
You can use droplevels to eliminate those value that do not appear anywhere (equivalent to rows with all 0's, or columns with all 0's)
> table(droplevels(v1), droplevels(v2))
b2
a1 100
If you only want to drop rows:
> table(droplevels(v1), v2)
v2
a2 b2 c2 d2 e2 f2
a1 0 100 0 0 0 0