R 获取数据帧中特定类别变量的计数

我有以下数据框

cars <- c("Toyota","Nissan","BMW","Nissan","Ford","Toyota","Nissan")
price <- c(21000, 23400, 26800,21000, 23400, 26800,21000)
salesDF <- data.frame(cars, price)

---

cars实现这一点的方法很多，这里有两种：

var = 'Nissan'
#1.
value <- sum(salesDF$cars == var)
value
#[1] 3

#2.
value <- nrow(subset(salesDF, cars == var))

var='Nissan'
#1.
值用于获取所有计数，并获取所需计数：

counts <- as.data.frame(table(salesDF$cars))

print(counts)
##     Var1 Freq
## 1    BMW    1
## 2   Ford    1
## 3 Nissan    3
## 4 Toyota    2

print(subset(counts, Var1 == 'Nissan', select = -Var1))
##   Freq
## 3    3

计数