Subset columns based on list of column names and bring the column before it

Question

I have a larger dataset following the same order, a unique date column, data, unique date column, date, etc. I am trying to subset not just the data column by name but the unique date column also. The code below selects columns based on a list of names, which is part of what I want but any ideas of how I can grab the column immediately before the subsetted column also?

Looking to end up with a DF containing Date1, Fire, Date3, Earth columns (using just the NameList).

Here is my reproducible code:

Cnames <- c("Date1","Fire","Date2","Water","Date3","Earth")
MAINDF <- data.frame(replicate(6,runif(120,-0.03,0.03)))
colnames(MAINDF) <- Cnames

NameList <- c("Fire","Earth")

NewDF <- MAINDF[,colnames(MAINDF) %in% NameList] 

Answer 1

How about

NameList <- c("Fire","Earth")

idx <- match(NameList, names(MAINDF))
idx <- sort(c(idx-1, idx))

NewDF <- MAINDF[,idx] 

Here we use match() to find the index of the desired column, and then we can use index subtraction to grab the column before it

Answer 2

Use which to get the column numbers from the names, and then it's just simple arithmetic:

col.num <- which(colnames(MAINDF) %in% NameList)
NewDF <- MAINDF[,sort(c(col.num, col.num - 1))]

Produces

         Date1         Fire        Date3        Earth
1 -0.010908003  0.007700453 -0.022778726 -0.016413307
2  0.022300509  0.021341360  0.014204445 -0.004492150
3 -0.021544992  0.014187158 -0.015174048 -0.000495121
4 -0.010600955 -0.006960160 -0.024535954 -0.024210771
5 -0.004694499  0.007198620  0.005543146 -0.021676692
6 -0.010623787  0.015977135 -0.027741109 -0.021102651
...