[r] Convert dataframe column to 1 or 0 for "true"/"false" values and assign to dataframe

Since you're dealing with values that are just supposed to be boolean anyway, just use == and convert the logical response to as.integer:

df <- data.frame(col = c("true", "true", "false"))
df
#     col
# 1  true
# 2  true
# 3 false
df$col <- as.integer(df$col == "true")
df
#   col
# 1   1
# 2   1
# 3   0