r fetch data from mysql db loop - mysql

Question

I successfully fetch data from my mysql db using r:
library(RMySQL)
mydb = dbConnect(MySQL(), user='user', password='pass', dbname='fib', host='myhost')
rs = dbSendQuery(mydb, 'SELECT distinct(DATE(date)) as date, open,close FROM stocksng WHERE symbol = "FIB7F";')
data <- fetch(rs, n=-1)
dbHasCompleted(rs)
so now I've an object a list:
> print (typeof(data))
[1] "list"
each elements is a tuple(?) like date(charts),open(long),close(long)
ok well now my problem: I want to get a vector of percentuale difference betwen close (x) and next day open (x+1) until the end BUT I can't access properly to the item!
Example: ((open)/close*100)-100)
I try:
for (item in data){
print (item[2])
}
and all possible combination like:
for (item in data){
print (item[][2])
}
but cannot access to right element :! anyone could help?

Answer 1

You have a bigger problem than this in your MySQL query, because you did not specify an ORDER BY clause. Consider using the following query:
SELECT DISTINCT
DATE(date) AS date,
open,
close
FROM stocksng
WHERE
symbol = "FIB7F"
ORDER BY
date
Here we order the result set by date, so that it makes sense to speak of the current and next open or close. Now with a proper query in place if you wanted to get the percentile difference between the current close and the next day open you could try:
require(dplyr)
(lead(open, 1) / close*100) - 100
Or using base R:
(open[2:(length(open)+1)] / close*100) - 100

Answer 2

naif version:
for (row in 1:nrow(data)){
date <- unname (data[row,"date"])
open <- unname (data[row+1,"open"])
close <- unname (data[row,"close"])
var <- abs((close/open*100)-100)
print (var)
}