renaming ggplot2 graphs in a for loop

I have a question about creating ggplot2 graphs in a for loop, renaming them based on the iteration and then arranging the graphs in a grid.

I want to do something like this dummy example

library(ggplot2)

a = c(1, 2, 3)
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b = c(4, 5, 6)

for ( i in c(1:5)){

    x = i*a

    y = i*b

    p = qplot(x, y)

    ... do something to rename p as plot_i...

}

... do something to arranage plots plot_1 ... plot_6 into a 2 x 3 grid

Any suggestions?

You could save the plots into a list:

library(ggplot2) 
library(gridExtra)

a <- c(1, 2, 3) 
b <- c(4, 5, 6)

out <- NULL 
for (i in 1:10){
    take <- data.frame(a = a * i, b = b * i)
    out[[i]] <- ggplot(take, aes(x = a, y = b)) + geom_point() 
} 

grid.arrange(out[[1]], out[[10]], out[[2]], out[[5]], nrow = 2)

An alternative way of approaching this problem is to use use facets in your plot:

a <- 1:3
b <- 4:6

# Create an empty data.frame
pdata <- data.frame()
for( i in 1:6){
  # Create a temporary data.frame to store data for single plot
  tmp <- data.frame(
      plot = i,
      x = i*a,
      y = i*b
  )
  # Use rbind to add this plot data
  pdata <- rbind(pdata, tmp)
}

# Plot results using ggplot with facets
ggplot(pdata, aes(x=x, y=y)) + geom_point() + facet_wrap(~plot)

The cowplot library has the plot_grid function that will do this nicely:

library(ggplot2)
library(cowplot)

makeplot <- function(i, a=c(1,2,3), b=c(4,5,6)) {
    take <- data.frame(a=a*i, b=b*i)
    ggplot(take, aes(x=a, y=b)) + geom_point()
}

nums = 1:10

plots <- lapply(nums, makeplot)

plot_grid(plotlist = plots)