从 R 编程中的一个因素中删除级别 – droplevels()函数

R 编程中的 droplevels()函数用于从因子中删除未使用的级别。

Syntax:
# For vector object
droplevels(x, exclude = if(anyNA(levels(x))) NULL else NA, …)

# For data frame object
droplevels(x, except, exclude)

Parameter values:
x represents object from which unused level has to be dropped
exclude represents factor levels which should be excluded even if present
except represents indices of columns from which levels should not be dropped

示例 1：

# Defining vector
x <- c(1, 3, 4, 8, 1, 5, 4, 4, 5, 6)
  
# Defining factor object for vector
f <- factor(x)
  
# Print factor object
cat("Factor object before deleting value:\n")
print(f)
cat("\n")
  
# Delete value at index 2
f <- f[-2]
  
# Print factor object
cat("Factor object after deleting value:\n")
print(f)
cat("\n")
  
cat("After dropping unused level:\n")
new_f <- droplevels(f)
print(new_f)

输出：

Factor object before deleting value:
 [1] 1 3 4 8 1 5 4 4 5 6
Levels: 1 3 4 5 6 8

Factor object after deleting value:
[1] 1 4 8 1 5 4 4 5 6
Levels: 1 3 4 5 6 8

After dropping unused level:
[1] 1 4 8 1 5 4 4 5 6
Levels: 1 4 5 6 8

示例 2：

# Defining columns
x <- factor(c(7, 3, 3, 7, 5, 5, 1))
y <- factor(c(1, 1, 1, 4, 4, 4, 2))
z <- c(1, 5, 3, 2, 9, 4, 7)
  
# Defining data frame
df <- data.frame(x, y, z)
df <- df[1:6, ]
  
# Print structure of data frame
str(df)
cat("\n")
  
# Drop levels from data frame
df_drop <- droplevels(df)
  
# Print structure of new data frame
str(df_drop)

输出：

'data.frame':   6 obs. of  3 variables:
 $ x: Factor w/ 4 levels "1", "3", "5", "7": 4 2 2 4 3 3
 $ y: Factor w/ 3 levels "1", "2", "4": 1 1 1 3 3 3
 $ z: num  1 5 3 2 9 4
 
 'data.frame':   6 obs. of  3 variables:
 $ x: Factor w/ 3 levels "3", "5", "7": 3 1 1 3 2 2
 $ y: Factor w/ 2 levels "1", "4": 1 1 1 2 2 2
 $ z: num  1 5 3 2 9 4