I'm new to R and am trying to extract the factors of a dataframe using numeric indices (e.g. df[1]) that are input to a function definition instead of the other types of references (e.g. df$out).  df[1] is a list(?) whose class is "dataframe".  These indexed lists can be printed successfuly but are not agreeable to the plot() and lm() functions shown below as are their df$out references.  Reading the documentation for plot and lm hasn't helped yet.  Thanks in advance - Stan.

> df=data.frame(out=1:4*3,pred1=1:4,pred2=1:4*2)
> regression=function(tble,a,b) 
+ {
+            plot.new()
+            plot(tble[a]~tble[b])
+            lmm=lm(tble[a]~tble[b])
+            abline(lmm)
+            anova(lmm)
+ }
> df[1]
  out
1   3
2   6
3   9
4  12
> df
  out pred1 pred2
1   3     1     2
2   6     2     4
3   9     3     6
4  12     4     8
> regression(df,1,3)
Error in model.frame(formula, rownames, variables, varnames, extras, extranames,  : 
        invalid type (list) for variable 'tble[a]'
> 


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