Factorial, combinations, & permutations

C

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Calculate factorials, combinations, & permutations in C

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C choose combination combinations combinatorics fac factorial iterative nCr nPr permutation permutations 
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#define nCr(n, r) (factorial(n) / factorial((n)-(r)) / factorial(r))
 /* number of combinations of `n` elements taken `r` at a time */

#define nPr(n, r) (factorial(n) / factorial((n)-(r)))
 /* number of permutations of `n` elements taken `r` at a time */

int factorial(int x) {
 int fac = 1;
 for (int i=2; i<=x; i++) fac *= i;
 return fac;
}