note-3实操

# 练习3-1
# 1.读取exercise.csv这个文件，赋值给test。
test <- read.csv("exercise.csv")
# 2.求test第一列数值的中位数
median(test$Petal.Length)

## [1] 4.6

# 3.筛选test中，Species列的值为a或c的行
test[test$Species!="b",]

##    Petal.Length Petal.Width Species
## 1           4.6         1.5       a
## 3           4.5         1.5       a
## 5           4.0         1.3       a
## 6           4.7         1.4       a
## 7           1.3         0.2       c
## 8           1.4         0.2       c
## 11          4.9         1.5       a
## 12          1.4         0.2       c
## 13          1.5         0.2       c
## 15          1.4         0.2       c

test[test$Species=="a"|test$Species=="c",]

##    Petal.Length Petal.Width Species
## 1           4.6         1.5       a
## 3           4.5         1.5       a
## 5           4.0         1.3       a
## 6           4.7         1.4       a
## 7           1.3         0.2       c
## 8           1.4         0.2       c
## 11          4.9         1.5       a
## 12          1.4         0.2       c
## 13          1.5         0.2       c
## 15          1.4         0.2       c

test[test$Species %in% c("a","c"),]

##    Petal.Length Petal.Width Species
## 1           4.6         1.5       a
## 3           4.5         1.5       a
## 5           4.0         1.3       a
## 6           4.7         1.4       a
## 7           1.3         0.2       c
## 8           1.4         0.2       c
## 11          4.9         1.5       a
## 12          1.4         0.2       c
## 13          1.5         0.2       c
## 15          1.4         0.2       c

# 练习3-2
# 1.统计内置数据iris最后一列有哪几个取值，每个取值重复了多少次
iris[,ncol(iris)]

##   [1] setosa     setosa     setosa     setosa     setosa     setosa    
##   [7] setosa     setosa     setosa     setosa     setosa     setosa    
##  [13] setosa     setosa     setosa     setosa     setosa     setosa    
##  [19] setosa     setosa     setosa     setosa     setosa     setosa    
##  [25] setosa     setosa     setosa     setosa     setosa     setosa    
##  [31] setosa     setosa     setosa     setosa     setosa     setosa    
##  [37] setosa     setosa     setosa     setosa     setosa     setosa    
##  [43] setosa     setosa     setosa     setosa     setosa     setosa    
##  [49] setosa     setosa     versicolor versicolor versicolor versicolor
##  [55] versicolor versicolor versicolor versicolor versicolor versicolor
##  [61] versicolor versicolor versicolor versicolor versicolor versicolor
##  [67] versicolor versicolor versicolor versicolor versicolor versicolor
##  [73] versicolor versicolor versicolor versicolor versicolor versicolor
##  [79] versicolor versicolor versicolor versicolor versicolor versicolor
##  [85] versicolor versicolor versicolor versicolor versicolor versicolor
##  [91] versicolor versicolor versicolor versicolor versicolor versicolor
##  [97] versicolor versicolor versicolor versicolor virginica  virginica 
## [103] virginica  virginica  virginica  virginica  virginica  virginica 
## [109] virginica  virginica  virginica  virginica  virginica  virginica 
## [115] virginica  virginica  virginica  virginica  virginica  virginica 
## [121] virginica  virginica  virginica  virginica  virginica  virginica 
## [127] virginica  virginica  virginica  virginica  virginica  virginica 
## [133] virginica  virginica  virginica  virginica  virginica  virginica 
## [139] virginica  virginica  virginica  virginica  virginica  virginica 
## [145] virginica  virginica  virginica  virginica  virginica  virginica 
## Levels: setosa versicolor virginica

table(iris$Species)

## 
##     setosa versicolor  virginica 
##         50         50         50

# 2.提取内置数据iris的前5行，前4列，并转换为矩阵，赋值给a。
class(iris)

## [1] "data.frame"

iris[(1:5),(1:4)]

##   Sepal.Length Sepal.Width Petal.Length Petal.Width
## 1          5.1         3.5          1.4         0.2
## 2          4.9         3.0          1.4         0.2
## 3          4.7         3.2          1.3         0.2
## 4          4.6         3.1          1.5         0.2
## 5          5.0         3.6          1.4         0.2

a <- iris[(1:5),(1:4)]
a <- as.matrix(a)
a

##   Sepal.Length Sepal.Width Petal.Length Petal.Width
## 1          5.1         3.5          1.4         0.2
## 2          4.9         3.0          1.4         0.2
## 3          4.7         3.2          1.3         0.2
## 4          4.6         3.1          1.5         0.2
## 5          5.0         3.6          1.4         0.2

class(a)

## [1] "matrix" "array"

# 3.将a的行名改为flower1，flower2...flower5。
rownames(a) <- c("flower1","flower2","flower3","flower4","flower5")
rownames(a) <- paste0("flower",1:nrow(a))
a

##         Sepal.Length Sepal.Width Petal.Length Petal.Width
## flower1          5.1         3.5          1.4         0.2
## flower2          4.9         3.0          1.4         0.2
## flower3          4.7         3.2          1.3         0.2
## flower4          4.6         3.1          1.5         0.2
## flower5          5.0         3.6          1.4         0.2

# 4.探索列表取子集l[2]和l[[2]]的区别（提示：数据结构）