Contingency tables in R

Question

What are different ways to create a contingency table? (Examples with explanation)

Answer 1

You can build 2 types of contingency tables.

One way table - 

table() returns simple frequency counts

Ex: 

> table(mpg$cyl)

 4  5  6  8 
81  4 79 70 

prop.table() returns the frequemcy in terms on proportion.

Ex: 

> prop.table(table(mpg$cyl))

         4          5          6          8 
0.34615385 0.01709402 0.33760684 0.29914530

Two way tables - 

table(var1,var2) returns count in row and column values.

Ex: 

table(mpg$cyl,mpg$year)
   
    1999 2008
  4   45   36
  5    0    4
  6   45   34
  8   27   43

xtabs() - returns a contingency table like table().

Here var1+var2 are the variables used to perform the table are calculated wrt full data frame.

Ex: 

xtabs(~cyl+year,mpg)
   year
cyl 1999 2008
  4   45   36
  5    0    4
  6   45   34
  8   27   43

You can see both frequency count and proportion using margin.table(table object)