Using lapply to make a plot with factors from a list of data frames

Question

I generated a data frame with two factors, an x-variable, and a y-variable:

set.seed(1)
abc.df <- data.frame(col1 = rep(c("a", "b", "c"), 1000), col2 = rep(1:4, 750),
                      col3 = rnorm(3000), col4 = rnorm(3000, 2))
names(abc.df) <- c("factor1", "factor2", "q", "value")
abc.df$factor1 <- as.factor(abc.df$factor1)
abc.df$factor2 <- as.factor(abc.df$factor2)
abc_list <- split(abc.df, abc.df$factor1)

I want to generate a three plots for each letter (a, b, and c) with the number factors distinguished by color, but it generated an error message:

par(mfrow = c(1, 3))
lapply(abc_list, function(x) {plot(abc_list[[x]]$q, abc_list[[x]]$value,
                                   col = factor2)})
Error in abc_list[[x]] : invalid subscript type 'list'

How can I make the series of plots with the right syntax?

Answer 1

When you use lapply to loop through a list, the argument passed to the function is the element of the list not the index, so you can not use abc_list[[x]] to subset instead, just use x, so for your case:

par(mfrow = c(1, 3))
lapply(abc_list, function(x) {plot(x$q, x$value, col = x$factor2)})

There is another convenient way to get this kind of plot using ggplot2 package with facet_grid, where you don't need to split your original data frame into lists but specify the factor1 as a variable to layout your plots:

library(ggplot2)
ggplot(abc.df, aes(x = q, y = value, col = factor2)) + geom_point() + facet_grid(.~factor1)