all-subsets regressiom
leaps() performs an exhaustive search for the best subsets of the
variables in x for predicting y in linear regression, using an efficient
branch-and-bound algorithm.  It is a compatibility wrapper for
regsubsets does the same thing better.
Since the algorithm returns a best model of each size, the results do not depend on a penalty model for model size: it doesn't make any difference whether you want to use AIC, BIC, CIC, DIC, ...
leaps(x=, y=, wt=rep(1, NROW(x)), int=TRUE, method=c("Cp", "adjr2", "r2"), nbest=10,
 names=NULL, df=NROW(x), strictly.compatible=TRUE)| x | A matrix of predictors | 
| y | A response vector | 
| wt | Optional weight vector | 
| int | Add an intercept to the model | 
| method | Calculate Cp, adjusted R-squared or R-squared | 
| nbest | Number of subsets of each size to report | 
| names | vector of names for columns of  | 
| df | Total degrees of freedom to use instead of  | 
| strictly.compatible | Implement misfeatures of leaps() in S | 
A list with components
| which | logical matrix. Each row can be used to select the columns of  | 
| size | Number of variables, including intercept if any, in the model | 
| cp | or  | 
| label | vector of names for the columns of x | 
With strictly.compatible=T the function will stop with an error if x is not of full rank or if it has more than 31 columns. It will ignore the column names of x even if names==NULL and will replace them with "0" to "9", "A" to "Z".
Alan Miller "Subset Selection in Regression" Chapman \& Hall
x<-matrix(rnorm(100),ncol=4) y<-rnorm(25) leaps(x,y)
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