skip the script if return value is zero(I mean if the dataframe has empty rows)

Question

library(RPostgreSQL)
library(ggplot2)

I want to collect some data from database using Rpostgresql and want to make bar graph for it. Here is the script

l1 <- dbGetQuery(con, "SELECT * FROM event_log where eventtime > '2015-05-29 06:00:00' and sourceid = '1' order by eventtime ")

my_count_table1 <- table(cut(l1$eventtime, breaks = "hour"))
l1_count <- data.frame(my_count_table1)
l1_count$Var1 <- as.POSIXct(l1_count$Var1, format="%Y-%m-%d %H:%M:%S")
p1 <- ggplot(l1_count, aes(x = Var1, y = Freq)) + geom_bar()

l2 <- dbGetQuery(con, "SELECT * FROM event_log where eventtime > '2015-05-29 06:00:00' and id = '3' order by eventtime ")

my_count_table2 <- table(cut(l2$eventtime, breaks = "hour"))
l2_count <- data.frame(my_count_table2)
l2_count$Var2 <- as.POSIXct(l2_count$Var1, format="%Y-%m-%d %H:%M:%S")
p2 <- ggplot(l2_count, aes(x = Var1, y = Freq)) + geom_bar()

multiplot(p1, p2, cols=2)

Normally I run the complete script using source('filename') command. And when return value of l1 and l2 has rows(I mean dataframe), I will get the required output.

But,in some cases if the return value of l1 is equal to zero(no of rwos is zero) or l2 is equal to zero(no of rows is zero), then i will get an error:

Error in cut.default(l2$eventtime, breaks = "hour") : 
  'x' must be numeric

I know these error is because of empty dataframe l2. so in these kind of situation, how to avoid the above error and plot only one graph(I mean to skip the script if the return value/dataframe is empty).

Wrap your code sections in `if (nrow(l1) > 0) { do this }`? – andybega May 29 '15 at 12:19 — andybega, May 29 '15 at 12:19

score 0 · Answer 1 · edited May 23 '17 at 12:30

Wrap your code in a function and do proper input checks. Here is a simplified example:

myfun <- function(DF) {
  stopifnot(nrow(DF) > 0)
  return("result of calculations")

}

myfun(iris)
#[1] "result of calculations"
myfun(iris[0,])
#Error: nrow(DF) > 0 is not TRUE

You can then do error handling:

tryCatch(myfun(iris[0,]), error = function(e) "alternative result")
#[1] "alternative result"

skip the script if return value is zero(I mean if the dataframe has empty rows)

1 Answers1