R：如何将函数应用于数据框以使用唯一因子组合绘制每个子集的图

Question

I would like to understand how the apply functions or anything similar can be used to make plots for several subsets of a data frame. Let's assume I want to use the mtcars data frame and I want to make a plot for each combination of "gear" and "carb".

I read here , that one can just write a function and then use lapply to create subsets of a data frame and plots for each of them. So how could I rewrite this to use lapply or anything similar?

gg_test = function(data) {
    title = paste("gear",data[,"gear"][1],"carb",data[,"carb"][1])

    require(ggplot2)
    gg1 = ggplot(data, aes(x = mpg, y = cyl, color = disp)) + geom_point() +
        ggtitle(title)
    print(gg1)
} 

for (g in unique(mtcars[,"gear"])){
    for(ca in unique(mtcars[,"carb"])){
        df_sub = subset(mtcars, gear == g & carb == ca)
        gg_test(df_sub)
    }
}

Answer 1

You can use split and interaction to divide a data.frame into chunks that you can then apply your plot function to:

gg_test = function(data) {
  title = paste("gear",data[,"gear"][1],"carb",data[,"carb"][1])

  require(ggplot2)
  ggplot(data, aes(x = mpg, y = cyl, color = disp)) + geom_point() +
    ggtitle(title)
} 

I modified your plot function to return the plot instead of print it out.

plots <- lapply(split(mtcars, interaction(mtcars[,c("gear","carb")]), drop = TRUE), gg_test)

Then I store each plot in another object that can be printed on demand.