Combination and Permutation Related Functions in R

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1. expand.grid creates a data frame from all possible combinations of supplied vectors or factors. For example,

   > expand.grid(letters[1:2], 1:3, c("+", "-"))
      Var1 Var2 Var3
   1     a    1    +
   2     b    1    +
   3     a    2    +
   4     b    2    +
   5     a    3    +
   6     b    3    +
   7     a    1    -
   8     b    1    -
   9     a    2    -
   10    b    2    -
   11    a    3    -
   12    b    3    -
   

2. combn generates all combinations of \(n\) elements taking \(m\) at a time.