Transforming contingency tables into frequency tables in R

Problem

We want to tranform a contingency table into a frequency table in R.

# Contingency table
tbl <- table (mtcars[, c("am", "gear")])

   gear
am   3  4  5
  0 15  4  0
  1  0  8  5

Frequency tables

We transform the contingency table into a data frame.

df <- as.data.frame(tbl)
df

  am gear Freq
1  0    3   15
2  1    3    0
3  0    4    4
4  1    4    8
5  0    5    0
6  1    5    5

Transforming frequency tables in contingency tables

To transform a frequency table back to a contingency table.

ftable(xtabs(Freq ~ am + gear, data = df)) 

   gear  3  4  5
am              
0       15  4  0
1        0  8  5

It is the equivalent of:

ftable(mtcars[, c("am", "gear")])

References

• Contingency tables in R
• Proportion tables in R