Data manipulations in R - r

Question

As part of a project, I am currently using R to analyze some data. I am currently stuck with the retrieving few values from the existing dataset which i have imported from a csv file.
The file looks like:
For my analysis, I wanted to create another column which is the subtraction of the current value of x and its previous value. But the first value of every unique i, x would be the same value as it is currently. I am new to R and i was trying various ways for sometime now but still not able to figure out a way to do so. Request your suggestions in the approach that I can follow to achieve this task.
Mydata structure
structure(list(t = 1:10, x = c(34450L, 34469L, 34470L, 34483L,
34488L, 34512L, 34530L, 34553L, 34575L, 34589L), y = c(268880.73342868,
268902.322359863, 268938.194698248, 268553.521856105, 269175.38273083,
268901.619719038, 268920.864512966, 269636.604121984, 270191.206593437,
269295.344751692), i = c(1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L, 1L,
1L)), .Names = c("t", "x", "y", "i"), row.names = c(NA, 10L), class = "data.frame")

Answer 1

You can use the package data.table to obtain what you want:
library(data.table)
setDT(MyData)[, x_diff := c(x[1], diff(x)), by=i]
MyData
# t x i x_diff
# 1: 1 34287 1 34287
# 2: 2 34789 1 502
# 3: 3 34409 1 -380
# 4: 4 34883 1 474
# 5: 5 34941 1 58
# 6: 6 34045 2 34045
# 7: 7 34528 2 483
# 8: 8 34893 2 365
# 9: 9 34551 2 -342
# 10: 10 34457 2 -94
Data:
set.seed(123)
MyData <- data.frame(t=1:10, x=sample(34000:35000, 10, replace=T), i=rep(1:2, e=5))

Answer 2

You can use the diff() function. If you want to add a new column to your existing data frame, the diff function will return a vector x-1 length of your current data frame though. so in your case you can try this:
# if your data frame is called MyData
MyData$newX = c(NA,diff(MyData$x))
That should input an NA value as the first entry in your new column and the remaining values will be the difference between sequential values in your "x" column
UPDATE:
You can create a simple loop by subsetting through every unique instance of "i" and then calculating the difference between your x values
# initialize a new dataframe
newdf = NULL
values = unique(MyData$i)
for(i in 1:length(values)){
data1 = MyData[MyData$i = values[i],]
data1$newX = c(NA,diff(data1$x))
newdata = rbind(newdata,data1)
}
# and then if you want to overwrite newdf to your original dataframe
MyData = newdf
# remove some variables
rm(data1,newdf,values)