Eval parse for a JSON - json

Question

I am trying to automate a JSON parsing in R (I had to remove the "https:// from the URLs because I don't have enough reputation points):
library(Quandl)
library(jsonlite)
tmp <-
fromJSON("www.quandl.com/api/v3/datasets.json?database_code=WIKI&page=2",flatten = TRUE)
for various numbers in page=X. The above code snippet executes properly. For that I am trying to use eval(parse()) but I am doing something wrong. So I have the following:
text1 <- 'fromJSON("www.quandl.com/api/v3/datasets.json?database_code=WIKI&page='
text2 <- '",flatten = TRUE)'
and to verify that I create the string properly:
> text1
[1] "fromJSON(\www.quandl.com/api/v3/datasets.json?database_code=WIKI&page="
> text2
[1] "\",flatten = TRUE)"
> cat(text1,n,text2,sep="")
fromJSON("www.quandl.com/api/v3/datasets.json?database_code=WIKI&page=2",flatten = TRUE)
BUT when I try to execute:
koko <- eval(parse(text = cat(text1,n,text2,sep="")))
where n<-2 or any other integer then the console freezes with the following error messsage:
?
Error in parse(text = cat(text1, n, text2, sep = "")) :
<stdin>:1:4: unexpected '{'
1: D_{
^
What am I doing wrong here?

Answer 1

Have a read of the difference between paste and cat
cat will just print to the screen, it won't return anything. To create a string you should use paste or paste0.
For example, consider
concat <- cat(text1, n, text2)
p <- paste0(text1, n, text2)
Even when running concat <- cat(text1, n, text2), it prints the output to the console, and concat is empty/NULL
The solution is to use paste0 to create the string expression
text1 <- 'fromJSON("http://www.quandl.com/api/v3/datasets.json?database_code=WIKI&page='
text2 <- '",flatten = TRUE)'
n <- 2
koko <- eval(parse(text = (paste0(text1, n, text2))))
Also, you don't need to use eval, you can use paste0 directly
text1 <- 'http://www.quandl.com/api/v3/datasets.json?database_code=WIKI&page='
n <- 2
koko <- fromJSON(paste0(text1, n), flatten=TRUE)