等价于 df[df$x == 1 & df$y == 3, ]

filter(df, x == 1, y == 3)

输入 general quasure，返回 quasure

UQ(q)

unquote, evaluate

"!!"(q)

df %>% group_by(g1) %>% summarise(a = mean(a))

不会用g1来替换 group_var，只是 quote(group_var)

my_summarise(df, g1)

my_summarise2(df, quo(g1))

my_summarise3(df, g1)

summarise(df, mean = mean(!!expr), sum = sum(!!expr), n = n() ) }

diff_input_summarise(df, g1)

mutate(df, !!mean_name := mean(!!expr), !!sum_name := sum(!!expr) ) }

my_mutate(df, a)

df %>% group_by(!!!group_by) %>% summarise(a = mean(a)) }

multiple_arg_summarise(df, g1, g2)