ELEMENTAL INTEGER FUNCTION factorial(n) INTEGER, INTENT(IN) :: n INTEGER :: i factorial = PRODUCT( (/ (i, i = 1, n) /) ) RETURN END FUNCTION factorial
ELEMENTAL INTEGER FUNCTION permutation(n, r) INTEGER, INTENT(IN) :: n, r INTEGER :: i permutation = PRODUCT( (/ (i, i = n, n - r + 1, -1) /) ) RETURN END FUNCTION permutation
ELEMENTAL INTEGER FUNCTION combination(n, r) INTEGER, INTENT(IN) :: n, r combination = permutation(n, r) / factorial(r) RETURN END FUNCTION combination
PROGRAM twochannel USE m_2chan IMPLICIT NONE INTEGER :: n, k PRINT *, 'INPUT n and k' READ *, n, k PRINT *, 'nCk = ', combination(n, k) STOP END PROGRAM twochannel
>>908 数学的にチューニングすべきだが、提示された式のとおりに作った。PROGRAMは改行多杉になるので省略 integer function factorial(n) implicit none integer i, n i = n factorial = 1 do while(i .gt. 0) factorial = factorial * i i = i - 1; end do end integer function permutation(n, r) implicit none integer n, r, i i = 1 permutation = 1; do while(i .le. r) permutation = permutation * (n - i + 1) i = i + 1 end do end integer function combination(n, r) integer n, r, factorial, permutation combination = permutation(n, r) / factorial(r) end
