calculate the frequency in R

Question

Here is my data

> a
 [1] Male   Male   Female Male   Male   Male   Female Female Male   Male   Female Male   Male   Male  
[15] Female Female Female Male   Female Male   Female Male   Male   Female Male   Male   Female Male  
[29] Male   Male   Female Male   Male   Male   Female Female Male   Male   Male   Male   Male  
Levels:  Female Male

> b
[1] 0 1 0 1 0 0 0 0 1 1 1 1 0 1 0 0 0 1 0 0 1 0 0 0 1 1 1 0 0 0 0 1 1 0 0 1 0 1 1 1 1
Levels: 0 1

> table(a,b)
        b
a         0  1
          0  0
  Female 10  4
  Male   12 15

I don't know why the result of table(a,b) has a row of (0 0), my expected result is as follows:

> table(a,b)
        b
a         0  1
  Female 10  4
  Male   12 15

Could you tell me why this happens and how to correct it, thank you!

> dput(a)
structure(c(3L, 3L, 2L, 3L, 3L, 3L, 2L, 2L, 3L, 3L, 2L, 3L, 3L, 
3L, 2L, 2L, 2L, 3L, 2L, 3L, 2L, 3L, 3L, 2L, 3L, 3L, 2L, 3L, 3L, 
3L, 2L, 3L, 3L, 3L, 2L, 2L, 3L, 3L, 3L, 3L, 3L), .Label = c("", 
"Female", "Male"), class = "factor")

> dput(b)
structure(c(1L, 2L, 1L, 2L, 1L, 1L, 1L, 1L, 2L, 2L, 2L, 2L, 1L, 
2L, 1L, 1L, 1L, 2L, 1L, 1L, 2L, 1L, 1L, 1L, 2L, 2L, 2L, 1L, 1L, 
1L, 1L, 2L, 2L, 1L, 1L, 2L, 1L, 2L, 2L, 2L, 2L), .Label = c("0", 
"1"), class = "factor")

Answer 1

From the comments above:

This is happening because there is an empty factor level in a:

> levels(a)
[1] ""       "Female" "Male" 

You can keep produce a table that disregards empty factor levels (from @lmo's comment):

table(as.character(a), b)

Alternatively, you can easily remove factor levels without observations (from @Dave2e's comment)

a <- droplevels(a)
table(a, b)