R中使用带If/Else的For循环为DataFrame新增成绩等级列求助

问题原因 #

你当前循环中的i只是从Result向量中读取的临时副本，对i的赋值不会修改原向量的数值，自然无法将转换后的等级存入目标数据框。

解决方法 #

方法1：修改现有循环逻辑 #

保留你已经写好的判断规则，调整循环遍历方式，新增向量存储转换后的等级：

Result <- c(50,67,89,77,65,66,70,73,69,80)
# 新建等长空向量存储转换后的等级
grade <- character(length(Result))

# 遍历索引而非直接遍历数值
for (i in seq_along(Result)){
  score <- Result[i]
  if (score < 16.67) {
    grade[i] <- "G+"
  } else if (score < 26.67) {
    grade[i] <- "F+"
  } else if (score < 36.67) {
    grade[i] <- "E+"
  } else if (score < 40) {
    grade[i] <- "D+"
  } else if (score < 43.33) {
    grade[i] <- "D"
  } else if (score < 46.67) {
    grade[i] <- "D+"
  } else if (score < 50) {
    grade[i] <- "C-"
  } else if (score < 53.33) {
    grade[i] <- "D"
  } else if (score < 56.67) {
    grade[i] <- "D+"
  } else if (score < 60) {
    grade[i] <- "B-"
  } else if (score < 63.33) {
    grade[i] <- "B"
  } else if (score < 66.67) {
    grade[i] <- "B+"
  } else if (score < 70) {
    grade[i] <- "A-"
  } else if (score < 73.33) {
    grade[i] <- "A"
  } else if (score < 100) {
    grade[i] <- "A+"
  }
}

# 存入数据框
df <- data.frame(score = Result, grade = grade)

方法2：使用R原生cut函数（更适合大数据集） #

R内置的cut函数专门用于数值分箱操作，代码更简洁，运行效率远高于自定义循环，完全适配你的需求：

Result <- c(50,67,89,77,65,66,70,73,69,80)
# 定义分数分界点
breaks <- c(-Inf,16.67,26.67,36.67,40,43.33,46.67,50,53.33,56.67,60,63.33,66.67,70,73.33,100)
# 定义对应等级，顺序与分界点间隔一一对应
labels <- c("G+","F+","E+","D+","D","D+","C-","D","D+","B-","B","B+","A-","A","A+")
# 完成分箱转换
grade <- cut(Result, breaks = breaks, labels = labels, right = FALSE)
# 存入数据框
df <- data.frame(score = Result, grade = grade)

注意事项 #

你现有代码中存在一处逻辑不一致：分数小于40的分支，print输出的是D-，但赋值为D+，可根据你实际的等级映射规则调整修正。

内容的提问来源于stack exchange，提问作者MsElaineous