An interactive NeXTSTEP interface to a Fortran code for solving coupled differential equations

This paper describes a user‐friendly frontend to a Fortran program that integrates coupled nonlinear ordinary differential equations. The user interface is built using the NeXTSTEP Interface Builder, together with a public‐domain graphical palette for displaying intermediate and final results. The main obstacle was implementing communication between the Objective‐C environment governing the NeXTSTEP interface and the Fortran code. This was overcome by breaking up the Fortran into separate subroutines (which compile as C function calls), corresponding to the various control buttons in the interface. In running the code for a given set of equation parameters the user sees a plot of the solutions at each stage of the iterative process. In the case of a successful sequence of iterations, the initially discontinuous curves smooth out as the scale parameters of the solutions are adjusted to achieve a solution to the nonlinear equations. If the iterative process goes astray, as it often does for a poor choice of starting scale parameters, the user has the opportunity to stop and start over with a better choice, guided by the result of the previous run. The ease of interaction with the equations also allows the user to develop an intuition regarding their solutions and to explore the parameter space for the equations much more quickly.