Chapter 11. More Fortran Elements: Functions and Subroutines 11.1 ...

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STOP END In the main program, the variable denoting the number of points is N but in the function the same variable is defined as NPTS. The variable names do not have to match. The first argument in the argument list of the function will be stored as the integer variable NPTS in the function no matter what it is called in the main program. Similarly, in the main program, the step size is called DELTAX, but in the function the same variable is called H. In both the main program and the function subprogram, the array containing the function to be integrated is called F; however, in general the two array names do not have to match. 11.2.4 Compiling Source Code Containing External Functions To compile a Fortran program that uses an external function, the source code may be saved in a single file or in multiple files. If the source code for the main program and the external function is stored in a single file, the source code for the function must be listed after the main program. The compilation with g77 is then no different than that of a single Fortran program: g77 –o pgm.exe file.f If the source code of the main program is stored in one file and the source code of the external function is stored in a second file, then the compilation statement must include both files: g77 –o pgm.exe file1.f file2.f Here, file1.f and file2.f are the two files containing the main program and external function source code, and pgm.exe is the single executable program created by compilation. Note that any number of external functions may be employed in a main program. 11.3 Subroutines A subroutine in Fortran also is a subprogram and is similar to an external function. The difference is that a function may return only one numerical result and a subroutine may return multiple numerical results. 11.3.1 Subroutine Layout The layout of a subroutine is very similar to that of a function. A subroutine has four main sections as shown in Figure 11.2: the subroutine name with specified arguments, variable declarations, the main body of the subroutine, and the end of the subroutine. 70
subroutine name(arg1, arg2, …) start of subroutine implicit none variable declarations real arg1, arg2, x, y integer i, j, k . . . calculations are carried out here . . . return end main body of subroutine end of subroutine Figure 11.2. Layout of a typical Fortran subroutine. The SUBROUTINE statement is placed at the beginning of the subroutine and identifies the subroutine name. The arguments passed to and from the subroutine are enclosed in parentheses after the subroutine name. Any number of arguments may be passed back and forth and the arguments may correspond to any variable type and also may be arrays. The Variable Declaration section of the subroutine has the same format as it does in a regular Fortran program. Note that the arguments passed between the subroutine and the main program must be declared in the Variable Declaration section. The Main Body of the subroutine has the same format as in a regular Fortran program. The main computational tasks of the subroutine are carried out in this section. The End of the Subroutine is indicated by the statement RETURN, which passes execution back to the main program, and END, which signifies the end of the subroutine. 11.3.2 Using a Subroutine in a Fortran Program A subroutine is employed in a Fortran program in a slightly different way than an external function. Because the subroutine may pass back to the main program multiple numerical results, the subroutine is not included as part of an assignment statement. Rather, a CALL statement is employed. The format of a typical CALL statement is shown below. CALL NAME(ARG1, ARG2, …) In the CALL statement, NAME refers to the name of the subroutine. Note that the subroutine name should not be the same as any other variable name and unlike an external function, it should not be declared as a real or integer variable. This is because a subroutine may pass back to the main program the values of many variables, both real and integer, while a function only returns a single value. 71
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Chapter 11. More Fortran Elements: Functions and Subroutines 11.1 ...

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