How to change the order of columns in a data frame?

Dear all,

I have a data frame in which the columns need to be ordered. The first column X is at the right position, but the remaining columns X1-Xn should be ordered like this: X1, X2, X3 etc instead of like below.

colnames(pos1)

?[1] "X" ? "X1" ?"X10" "X11" "X12" "X13" "X14" "X15" "X16" "X17" "X18" "X19" "X2" ?"X20" "X3" ?"X4" ?"X5" ?"X6" ?"X7" ?"X8" ?"X9"

pos1[1:5,1:5]

? ? ?X ? ? ? X1 ? ? ? X10 ? ? ? X11 ? ? ? X12
1 100.5 7949.469 18509.064 ?8484.969 17401.056
2 101.5 3080.058 ?7794.691 ?3211.323 ?8211.058
3 102.5 1854.347 ?4347.571 ?1783.846 ?4827.338
4 103.5 2064.441 ?8421.746 ?2012.536 ?8363.785
5 104.5 9650.402 26637.926 10730.647 27053.421

I am trying to first change the first column name to something without an X and save as a vector. I would then remove the X from each position use the vector for renaming the columns. Then the column 2-n could be ordered, I hope...

colnames(pos)[1] <- "Mass"
columnNames <- colnames(pos)

Does any of you have an idea how to do this, or perhaps there is a smoother solution?
Would it be easier to solve it if the contents of the first column were extracted and used as row names instead?

Best regards,

Joel

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