R loop - how to use it correctly? - php

Question

I am coming from PHP background and still trying to get my head around R.
For instance, I can easily loop an array in PHP and manipulate the data,
$array = [
"site1" => 9
"site2" => 10
"site3" => 18
"site4" => 28
]
foreach($array as $index => $id) {
echo $id . '<br/>';
}
result,
9
10
18
28
But in R, I have this data frame for instance,
siteKey siteCode
1 site1 9
2 site2 10
3 site3 18
4 site4 28
loop,
for(i in 1:length(sites.df$siteKey)) {
print(sites.df$siteCode)
}
result,
[1] "9" "10" "18" "28"
[1] "9" "10" "18" "28"
[1] "9" "10" "18" "28"
[1] "9" "10" "18" "28"
I thought it should be this result below?
"9"
"10"
"18"
"28"
Any idea how I can use the R loop to get the result above that I need?
EDIT:
The query,
# Prepare SQL query1.
dataQuery <- "SELECT
*
FROM speckdata AS s
LEFT JOIN weatherunderground AS w
ON s.wid_timestamp = w.wid_timestamp
LEFT JOIN nodes AS n
ON n.nid = s.nid
AND n.datatype = 'speck'
WHERE n.nid = 'SITE'
"
Maybe better to loop this below?
#
# Data: site 1
#
# Match the pattern and replace it.
data1Query <- sub("SITE", as.character(site1), data1Query)
# Store the result in data1.
data1 = dbGetQuery(DB, data1Query)
#
# Data: site 2
#
data2Query <- sub("SITE", as.character(site2), data2Query)
# Store the result in data.
data2 = dbGetQuery(DB, data2Query)
#
# Data: site 3
#
data3Query <- sub("SITE", as.character(site3), data3Query)
# Store the result in data.
data3 = dbGetQuery(DB, data3Query)
#
# Data: site 4
#
data4Query <- sub("SITE", as.character(site4), data4Query)
# Store the result in data.
data4 = dbGetQuery(DB, data4Query)
Then merge all the data,
# Merge data sets.
set.seed(1)
dataList = list(data1, data2, data3, data4)
allData = Reduce(function(...) merge(..., all=T), dataList)
Then plot the data,
timePlot(
allData,
pollutant = c(species, condition),
avg.time = mean,
lwd = 2,
lty = 1,
type = "site",
group = TRUE,
auto.text = FALSE
)

Answer 1

Add a call to [i] in your loop like so...
for(i in 1:length(sites.df$siteKey)) {
print(sites.df$siteCode[i])
}
This way, each iteration will print the single instance at location [ i ].
Hopefully that makes sense.

Answer 2

The solution of #dsifford is ok in base R. However, I recommend to have a look at the dplyr package early in the switchover from php, because it makes things much more transparent than the hopeless mess of xapply functions (apologies to all radical base-R-ers).
library(dplyr)
site.df = data.frame(siteKey = paste0("site",1:4), siteCode = runif(4,9,28))
site.df %>%
rowwise %>%
do(print(.$siteCode))
In itself this is not really easier, but if you have additional manipulations, it makes for a smoother ride.

Answer 3

Your question as written is "R loop: How to use it correctly?"
You propose this code
for(i in 1:length(sites.df$siteKey)) {
print(sites.df$siteCode)
}
This code would be an example of using an R loop correctly (if you insisted on it, even though it is discouraged).
What this code is saying is for each pass print out the "column" or complete list of values for $siteCode. The loop is repeating that print request for each site, so you are getting the same complete list for each pass.
As suggested above by #dsifford changing it to
for(i in 1:length(sites.df$siteKey)) {
print(sites.df$siteCode[i])
}
will mean that instead of printing the entire list it will print just the ith value from that list for each pass.
The new code you added has different issues, specifically you are repeating the same code over and over for each site. You might want to consider writing functions to replace the duplicate code and then feeding the functions the list of sites you are using. That is a case where a loop may possibly make sense (although it may not). But that is all a side issue from your original question.