How to get two-way tables with marginal proportions?

Question

Consider this

var1<- c('A','B','C','C','G')
var2<- c('G','B','B','F','A')
df = df=data.frame(var1,var2)

> df
  var1 var2
1    A    G
2    B    B
3    C    B
4    C    F
5    G    A

I want to get a two-way table between var1 and var2.

I can use

> df %>% count(var1,var2) %>% spread(var2, n, fill = 0)
Source: local data frame [4 x 5]
Groups: var1 [4]

    var1     A     B     F     G
* <fctr> <dbl> <dbl> <dbl> <dbl>
1      A     0     0     0     1
2      B     0     1     0     0
3      C     0     1     1     0
4      G     1     0     0     0

or using table

> table( df$var1, df$var2)

    A B F G
  A 0 0 0 1
  B 0 1 0 0
  C 0 1 1 0
  G 1 0 0 0

Both give the same results. But now suppose I am interested in the row or columns proportions. I can use table again here

> prop.table(table( df$var1, df$var2), margin =2) # for column

      A   B   F   G
  A 0.0 0.0 0.0 1.0
  B 0.0 0.5 0.0 0.0
  C 0.0 0.5 1.0 0.0
  G 1.0 0.0 0.0 0.0

> prop.table(table( df$var1, df$var2), margin =1) # for rows

      A   B   F   G
  A 0.0 0.0 0.0 1.0
  B 0.0 1.0 0.0 0.0
  C 0.0 0.5 0.5 0.0
  G 1.0 0.0 0.0 0.0

How can I reproduce these with dplyr (the shortest syntax)?

The solution below seems to work (only in one case), although I don't understand why: what does the prop.table(n) function does here?

> df %>%  count(var1,var2) %>% mutate(prop = prop.table(n)) %>% spread(var2, prop, fill = 0)
Source: local data frame [4 x 6]
Groups: var1 [4]

    var1     n     A     B     F     G
* <fctr> <int> <dbl> <dbl> <dbl> <dbl>
1      A     1     0   0.0   0.0     1
2      B     1     0   1.0   0.0     0
3      C     1     0   0.5   0.5     0
4      G     1     1   0.0   0.0     0

Also, using margin in prop.table fails here.

> df %>%  count(var1,var2) %>% mutate(prop = prop.table(n ,margin =1)) %>% spread(var2, prop, fill = 0)
Error in eval(substitute(expr), envir, enclos) : 'x' is not an array

Thanks!

Answer 1

Pipelines are really a magrittr rather than dplyr thing:

df %>% with(table(var1, var2)) %>% prop.table(margin=2)