R语言中实现方差和标准差

1、

a <- c(1, 3, 4, 7, 9)

sum_a = 0
for (i in 1:length(a)) {
  sum_a = sum_a + a[i] 
  }
mean_a = sum_a / length(a)

va = 0
for (i in 1:length(a)) {
  va = va + (a[i] - mean_a) * (a[i] - mean_a)
}

s2 = va / (length(a) - 1)
sda = sqrt(s2)

s2
var(a)

sda
sd(a)