Examples in Fortran programming

Determinant of a Matrix
Determinant and inverse of lower matrices

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Determinant of a Matrix (Fortran 90)

!Function to find the determinant of a square matrix
!Author : Louisda16th a.k.a Ashwith J. Rego
!Description: The subroutine is based on two key points:
!1] A determinant is unaltered when row operations are performed: Hence, using this principle,
!row operations (column operations would work as well) are used
!to convert the matrix into upper traingular form
!2]The determinant of a triangular matrix is obtained by finding the product of the diagonal elements
!
REAL FUNCTION FindDet(matrix, n)
    IMPLICIT NONE
    REAL, DIMENSION(n,n) :: matrix
    INTEGER, INTENT(IN) :: n
    REAL :: m, temp
    INTEGER :: i, j, k, l
    LOGICAL :: DetExists = .TRUE.
    l = 1
    !Convert to upper triangular form
    DO k = 1, n-1
        IF (matrix(k,k) == 0) THEN
            DetExists = .FALSE.
            DO i = k+1, n
                IF (matrix(i,k) /= 0) THEN
                    DO j = 1, n
                        temp = matrix(i,j)
                        matrix(i,j)= matrix(k,j)
                        matrix(k,j) = temp
                    END DO
                    DetExists = .TRUE.
                    l=-l
                    EXIT
                ENDIF
            END DO
            IF (DetExists .EQV. .FALSE.) THEN
                FindDet = 0
                return
            END IF
        ENDIF
        DO j = k+1, n
            m = matrix(j,k)/matrix(k,k)
            DO i = k+1, n
                matrix(j,i) = matrix(j,i) - m*matrix(k,i)
            END DO
        END DO
    END DO

    !Calculate determinant by finding product of diagonal elements
    FindDet = l
    DO i = 1, n
        FindDet = FindDet * matrix(i,i)
    END DO

END FUNCTION FindDet

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Determinant and inverse of lower matrices

Determinant of 2X2, 3X3, 4X4, 5X5, and 6X6.
Inverse matrices for 2X2, 3X3, 4X4, 5X5, and 6X6.

For more interesting Fortran 90 codes, please visit David G. Simpson's websites.
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