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.