I forget about stack and unstack

r_stack_gist.R

# something interesting in R

# `stack` function
# Stacking vectors concatenates multiple vectors into a single vector along
# with a factor indicating where each observation originated. Unstacking
# reverses this operation.

a_list <- list(group1 = letters[1:5],
               group2 = letters[6:10],
               group3 = letters[11:15])

stack(a_list)

#    values    ind
# 1       a group1
# 2       b group1
# 3       c group1
# 4       d group1
# 5       e group1
# 6       f group2
# 7       g group2
# 8       h group2
# 9       i group2
# 10      j group2
# 11      k group3
# 12      l group3
# 13      m group3
# 14      n group3
# 15      o group3

# if you table() the result from stack(), now you have a nice matrix
# of the values in each group
table(stack(a_list))

# this is a table. You can convert to a data.frame
as.data.frame.array(table(stack(a_list)))

# you can also convert the binary matrix to logical TRUE/FALSE
table(stack(a_list)) > 0

# imagine a case where you have the table and some values are greater than 1
# you can use that trick to convert to logical and back to numeric 0/1

b_list <- list(group1 = rep(letters[1],5),
               group2 = letters[6:10],
               group3 = letters[11:15])
table(stack(b_list))
+(table(stack(b_list)) > 0)

# ind
# values group1 group2 group3
# a      1      0      0
# f      0      1      0
# g      0      1      0
# h      0      1      0
# i      0      1      0
# j      0      1      0
# k      0      0      1
# l      0      0      1
# m      0      0      1
# n      0      0      1
# o      0      0      1