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This section will talk about how we can batch jobs inside R
scripts.
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#### <b>Examples:</b><br>
- <font color="#800000"><b>assign, parse and eval</b></font>
```
assign("a", 3:4)
sum(a)
sum(b <- 3:4)
a
b
(cmd <- paste("sum(", "d", "<-", "3:4)", sep = " "))
eval(parse(text = cmd))
d
```
This is a silly example, but it gives a hint to process a command
from a given string. The command <code>assign()</code> is similar to "<-",
<code>parse()</code> can parse a string/text into an expression object which
can be executed by <code>eval()</code>. At the above, "a", "b", and "d" are
all the same as a vector contains 2 elements "3, 4".
- <font color="#800000"><b>call and eval</b></font>
```
a <- function(x, y = 4, detail = FALSE){
ifelse(detail, x+y, 0)
}
b <- list(quote(a), quote(2), quote(3), quote(detail = TRUE))
mode(b) <- "call"
b
(d <- eval(b))
b[[4]] <- FALSE
b
(e <- eval(b))
```
The object "b" is a "call" structure that can be used in eval().
The result for the object "d" should be 5, not 6,
and the object "e" should be 0, as the following.
```
> b
a(2, 3, TRUE)
> (d <- eval(b))
[1] 5
> b[[4]] <- FALSE
> b
a(2, 3, FALSE)
> (e <- eval(b))
[1] 0
```
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