tbl <- dplyr::tibble(
  ~a, ~b,
  "1", 1,
  "1", 2,
  "2", 1)
tbl

## Source: local data frame [2 x 2]
##
##       a     b
##   (chr) (chr)
## 1     1     1
## 2     2     2

tbl <- dplyr::tibble(
  ~a, ~b,
  "1", 1,
  "1", 2,
  "2", 2,
  "2", 1)
tbl

## Source: local data frame [2 x 2]
##
##       a     b
##   (chr) (dbl)
## 1     1     1
## 2     2     2

tbl <- dplyr::tibble(
  ~a, ~b, ~Freq,
  1, 1, 5,
  1, 2, 6,
  2, 2, 7,
  2, 1, 8)
tbl

## Source: local data frame [3 x 3]
##
##       a     b  Freq
##   (dbl) (dbl) (dbl)
## 1     1     1     5
## 2     2     6     2
## 3     7     2     1