R将相关函数应用于列表

Question

I have a data frame like this:

set.seed(1)
category <- c(rep('A',100), rep('B',100), rep('C',100))
var1 = rnorm(1:300)
var2 = rnorm(1:300)
df<-data.frame(category=category, var1 = var1, var2=var2)

I need to calculate the correlations between var1 and var2 by category. I think I can first split the df by category and apply the cor function to the list. But I am really confused about hot to use the lapply function. Could someone kindly help me out?

Answer 1

This should produce the desired result:

lapply(split(df, category), function(dfs) cor(dfs$var1, dfs$var2))

EDIT:

You can also use by (as suggested by @thelatemail):

by(df, df$category, function(x) cor(x$var1,x$var2)) 

Answer 2

您可以使用sapply获取相同的内容，但将其作为向量而不是列表

sapply(split(df, category), function(dfs) cor(dfs$var1, dfs$var2))

Answer 3

And just for comparison, here's how you'd do it with the dplyr package.

library(dplyr)
df %>% group_by(category) %>% summarize(cor=cor(var1,var2))

#   category         cor
# 1        A -0.05043706
# 2        B  0.13519013
# 3        C -0.04186283