问题在于使用赋值运算符=而不是比较运算符==
从Operate on a selection of variables，它说 作用域动词（_if、_at、_all）已被现有动词中的pick(...
Similarly, summarise_if() will summarise columns that satisfy the specified logical conditions. Below, we summarise each character column by reporting the most common value (but for some reason there is no mode() function in R, so we need to write our own). # function to calculate the mode...