How to specify subjective columns for a formula within a function

Consider dat1 created here:

dat1 <- data.frame(Region = rep(c("r1","r2"), each = 100),
                   State = rep(c("NY","MA","FL","GA"), each = 10),
                   Loc = rep(c("a","b","c","d","e","f","g","h"),each = 5),
                   ID = rep(c(1:10), each=2),
                   var1 = rnorm(200),
                   var2 = rnorm(200),
                   var3 = rnorm(200),
                   var4 = rnorm(200),
                   var5 = rnorm(200))

I want to write a function that will allow me to specify the data, grouping variable (Region,State, or Loc), and row names of the variables (var1:var5) with which I want it to conduct a manova and return the results in tidy format. Running the function would look something like this: manova.test(dat = dat1, groupvar = "Region", cols = 5:9)

The function would look something like this (which doesn't work):

manova.test <- function(dat, groupvar, cols){
   var.mat <- as.matrix(dat[, cols])
   group.man <- manova(cbind(var.mat) ~ groupvar, data = dat)
   summary(group.man)
 }

How do you do this? I am especially confused about how to specify the formula in this format!

You can use get to return the named object within the function. The drawback is that the variable will be labelled get(groupvar) in the summary, so I renamed it within your function:

set.seed(1)
dat1 <- data.frame(Region = rep(c("r1","r2"), each = 100),
                   State = rep(c("NY","MA","FL","GA"), each = 10),
                   Loc = rep(c("a","b","c","d","e","f","g","h"),each = 5),
                   ID = rep(c(1:10), each=2),
                   var1 = rnorm(200),
                   var2 = rnorm(200),
                   var3 = rnorm(200),
                   var4 = rnorm(200),
                   var5 = rnorm(200))

manova.test <- function(dat, groupvar, cols){
    var.mat <- as.matrix(dat[, cols])
    group.man <- manova(cbind(var.mat) ~ get(groupvar), data = dat)
    s <- summary(group.man)
    dimnames(s$stats)[[1]][1] <- groupvar
    s
}

manova.test(dat = dat1, groupvar = "Region", cols = 5:9)
#>            Df   Pillai approx F num Df den Df Pr(>F)
#> Region      1 0.015933   0.6282      5    194 0.6784
#> Residuals 198

^{Created on 2020-05-27 by the reprex package (v0.3.0)}