How to order from highest to lowest in R-CodePudding

My goal is to order multiple variables in a list in R:

" 5x^{5}"   "-2x^{3}"  "5x^{7}"        "0"        "1"

I want to get this order:

"5x^{7}"    " 5x^{5}"   "-2x^{3}"       "1"        "0"

So exponents from highest to lowest, then numerical order for the numbers. How can I achieve this? Decreasing for numerical alone is clear. For the exponents it would be necessary to detect if there is a x in the string and then extract the exponent and order it based on this. But I dont know how to do it.

CodePudding user response：

Well... it works

> x=c(" 5x^{5}","-2x^{3}","5x^{7}","0","1")
> x[order(gsub("(.*\\^\\{)(. )(\\}.*)","\\2",x),decreasing=T)]

[1] "5x^{7}"  " 5x^{5}" "-2x^{3}" "1"       "0"

CodePudding user response：

A more verbose option is to extract the exponents and the multipliers and then use arrange. This has the advantage of having these numbers ready if you need to use them.

library(stringr)
library(dplyr)

dat <- data.frame(x = c(" 5x^{5}", "-2x^{3}", "5x^{7}", "0", "1"))

dat |> mutate(m = as.numeric(str_match(x, "([ -]*\\d )x\\^\\{(\\d)\\}")[, 2]),
              exp = as.numeric(str_match(x, "([ -]*\\d )x\\^\\{(\\d)\\}")[, 3]),
              n = as.numeric(x)) |> 
        arrange(desc(exp), desc(n))

Output

#>         x  m exp  n
#> 1  5x^{7}  5   7 NA
#> 2  5x^{5}  5   5 NA
#> 3 -2x^{3} -2   3 NA
#> 4       1 NA  NA  1
#> 5       0 NA  NA  0

^{Created on 2022-06-13 by the reprex package (v2.0.1)}

CodePudding user response：

Base R solution:

x[
  order(
    gsub(
      ".*\\{(\\d )\\}.*",
      "\\1",
      x
    ),
    decreasing = TRUE
  )
]

Input data:

x <- c(
  " 5x^{5}",
   "-2x^{3}",
   "5x^{7}",
   "0",
   "1"
)