Fortran to C Code Conversion: Calculating Cosine of Angle Between Vectors

This example illustrates the process of converting a Fortran subroutine to its equivalent C function. The original Fortran code calculates the cosine of the angle between two 3D vectors.

Fortran Subroutine:

SUBROUTINE AnglecosVector(Cosangle,Vector1,Vector2)
IMPLICIT NONE
REAL(8)  Cosangle,Vector1(3),Vector2(3),abdj,aa,bb

abdj=Vector1(1)*Vector2(1)+Vector1(2)*Vector2(2)+
     +     	Vector1(3)*Vector2(3)
aa=DSQRT(Vector1(1)*Vector1(1)+Vector1(2)*Vector1(2)+
     +		Vector1(3)*Vector1(3))
bb=DSQRT(Vector2(1)*Vector2(1)+Vector2(2)*Vector2(2)+
     +		Vector2(3)*Vector2(3))
Cosangle=abdj/(aa*bb+1.0D-50)

RETURN
END

Equivalent C Function:

void AnglecosVector(double *Cosangle, double Vector1[3], double Vector2[3]) {
    double abdj, aa, bb;
    abdj = Vector1[0]*Vector2[0] + Vector1[1]*Vector2[1] + Vector1[2]*Vector2[2];
    aa = sqrt(Vector1[0]*Vector1[0] + Vector1[1]*Vector1[1] + Vector1[2]*Vector1[2]);
    bb = sqrt(Vector2[0]*Vector2[0] + Vector2[1]*Vector2[1] + Vector2[2]*Vector2[2]);
    *Cosangle = abdj / (aa*bb + 1.0e-50);
}

Explanation:

Function Signature: The SUBROUTINE in Fortran becomes the void function in C, accepting a pointer to the output Cosangle, and arrays for input vectors Vector1 and Vector2.
Data Types: REAL(8) in Fortran translates to double in C for floating-point numbers.
Array Indexing: Fortran uses 1-based indexing while C uses 0-based indexing. We adjust the array indices accordingly.
Dot Product and Magnitudes: The core calculation is the same: dot product of vectors (abdj) divided by the product of their magnitudes (aa and bb).
Small Constant Addition: The 1.0D-50 in Fortran (representing 1.0e-50 in C) is a small value added to avoid division by zero when the magnitudes are close to zero.

This example demonstrates a basic conversion process, highlighting key differences between Fortran and C. For more complex code conversions, consider using tools like automatic translators or studying language-specific idioms and conventions.

Fortran to C Code Conversion: Calculating Cosine of Angle Between Vectors