CODEWITHSUNDEEP
rfor-loopevalassign
Vint
Vint

Reputation: 499

Using "eval" within a loop

I'd like to dynamically assign a plot to a variable name, and then call that variable latter on within a loop. While using "eval" outside of a loop seems to work just fine, placing it within a loop stops it from working as expected. 

#Sample data frame
x<-c(1,2,3,4,5)
y<-c(5,4,3,2,1)
y2<-c(1,2,3,4,5)


DF<-data.frame(x,y,y2)


#Using ggplot for p and p2
p<-ggplot(DF, aes(x=x, y=y))+
            geom_point()
p2<-ggplot(DF, aes(x=x, y=y2))+
  geom_point()

#Assign p and p2 to string "Plot1" and "Plot2"
assign(paste0("Plot",1), p )
assign(paste0("Plot",2), p2 )


#Create a list to hold all plot names
plotlist<-c("Plot1", "Plot2")




#Print plots to a pdf
pdf(paste0("Plot", "_Test.pdf"), height =8, width=16)

for(i in seq(1,length(plotlist))){
  plotname<-plotlist[i]
  plotter<-eval(parse(text=plotname))
  plotter
  print(plotname)

}

dev.off()

Note that the above does not work. But if I am to run the same eval statements outside of the loop, AKA:

  i=1
  plotname<-plotlist[i]
  plotter<-eval(parse(text=plotname))
  plotter

The plot is created as expected. Is there a way to call "eval" within a loop? And what about being in a loop causes eval statement to work differently?

Note, by removing the for loop, it saves the (first) pdf as expected:

pdf(paste0("Plot", "_Test.pdf"), height =8, width=16)

#for(i in seq(1,length(plotlist))){
  plotname<-plotlist[i]
  plotter<-eval(parse(text=plotname))
  plotter
  print(plotname)

#}

dev.off()

Upvotes: 1

Views: 1805

Answers (1)

MrFlick
MrFlick

Reputation: 206401

A more R-like way to do this avoiding the assign/eval would be

DF <- data.frame(
  x = c(1,2,3,4,5),
  y = c(5,4,3,2,1),
  y2 = c(1,2,3,4,5))

plotlist <- list(
  Plot1 = ggplot(DF, aes(x=x, y=y)) +
    geom_point(),  
  Plot2 = ggplot(DF, aes(x=x, y=y2)) +
    geom_point()
)

pdf(paste0("Plot", "_Test.pdf"), height =8, width=16)
lapply(plotlist, print)
dev.off()

All your plots here are easily stored in a list that we can just lapply() over when needed.

The main problem is that ggplot objects won't render until they are print()ed. When you work in the console, by default the result of the last expression is print()ed. But when you run a loop, that default print()ing doesn't happen. That's described in this previous question: R: ggplot does not work if it is inside a for loop although it works outside of it

Upvotes: 2

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