Avoiding repeated creation of objects in RMarkdown document through if statement - r

Question

I'm working on a RMarkdown document that uses objects that take a long time to create and transform. The syntax is similar to this:
---
title: "Example"
author: "Test"
date: "October 29, 2015"
output: pdf_document
---
Example
```{r}
test_exc <- "NO"
if(exists("some_dta") == FALSE) {
set.seed(1)
# This data is big and messy to transform and I don't want to do it twice
some_dta <- data.frame(speed=runif(n = 1000),nonsense=runif(1000))
test_exc <- "YES"
}
```
You can also embed plots, for example:
```{r, echo=FALSE}
plot(some_dta)
```
Was the code executed: `r test_exc`
As suggested in the code above I would like to avoid repeated execution of the code if(exists("some_dta") == FALSE) { ... }. As illustrated in the code below the code within the loop executes:
I would like to know if there is a way of forcing RStudio markdown creation mechanism to understand that I those objects exists somewhere and there is no need to create them again.

Answer 1

You might want to use caching, as described in the online knitr documentation, e.g.:
---
title: "Example"
author: "Test"
date: "October 29, 2015"
output: pdf_document
---
Example
```{r chunk1,cache=TRUE}
set.seed(1)
# This data is big and messy to transform and I don't want to do it twice
some_dta <- data.frame(speed=runif(n = 1000),nonsense=runif(1000))
}
```

Answer 2

You could save your data to an .rds object and then run a check to see if that file exists
```{r}
if(!file.exists("some_dta.rds")) {
set.seed(1)
# This data is big and messy to transform and I don't want to do it twice
some_dta <- data.frame(speed=runif(n = 1000),nonsense=runif(1000))
saveRDS(some_dta, file='some_dta.rds')
} else {
some_dta <- readRDS('some_dta.rds')
}
```