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