name <- c("Tom", "Jerry", "Alice", "Bob", "Lucy", "Mike", "Emma", "Jack", "Mia", "Lily") 
gender <- c("M", "M", "F", "M", "F", "M", "F", "M", NA, "F") 
age <- c(18, 19, 21, 20, 19, 19, 20, NA, 18, 19) 
score <- c(85, 92, NA, 77, 89, 98, 96, 83, NA, 93)
A1 <- data.frame(name, gender, age, score)
A1 <- A1[complete.cases(A1),]
A1_F <- A1[A1$gender == "F" & A1$age < 20, c("name", "score")]
A1_F_90 <- A1[A1$gender == "F" & A1$age < 20 & A1$score > 90, "name"] num <- length(A1_F_90)
score_mid <- c(79, 81, 85, 88, 92, 78, 83, 91, 90, 87) A2 <- data.frame(name, score_mid)
A3 <- merge(A1, A2, by = "name")
A3 <- A3[order(A3$score, decreasing = F, na.last = T),] # 按期末考试成绩升序排列 
print(A3) <- A3[order(A3$score_mid, decreasing = T, na.last = T),] # 按期中考试成绩降序排列
print(A1)
print(A3)