Self study guide: Fortran 95 - University of Cambridge

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subroutine setval(x, y) ! set y to value of x real, intent(in) :: x real, intent(out) :: y y = x end subroutine setval • Modify the setval example above to use this subroutine. • Deliberately introduce an error and within the subroutine attempt to change the value of the first argument; try compiling the program and see what happens. Local variables which are declared within a routine will in general not retain their values between successive calls to the routine. It is possible to preserve values after one return into the next call of the routine by specifying the save attribute in the declaration of that variable, e.g. real, save :: local This of course cannot be used for the arguments of the routine. 37
6. Using Modules As programs grow larger it becomes less convenient to use a single file for the source code. Also it is very often the case that the functions and subroutines you have written can be used in more than one program. We need a way to organise the source code to make this easy to do. In this section we will look at modules which enable the program to be split between multiple files and make it easy to “re-use” the code you write in different programs. 6.1 Modules Modules are a very useful tool when we split programs between multiple files. Modules give us a number of advantages: 1. The module is useful for defining global data, and may be used as an alternative way of transmitting information to a routine. 2. The variables, etc., declared in a module may be made available within any routines at the choice of the programmer. 3. Modules can be imported for use into another program or subroutine. Functions and variables defined in the module become available for use. The compiler is also able to cross-check the calls to subroutines and functions and use of variables just as if they had been defined as internal routines. The definition of a module is module name [statement declarations] [contains [subroutine and function definitions] ] end module [name] The module is incorporated in the program segment or routine in which it is to be used by the use statement: use name • You can have as many different modules as you like. Each module will be contained in a separate file and may be compiled (and checked) separately. • Modules can be re-used between different programs and may be used multiple times in the same program. • The variables declared in the module before the contains statement are global to the module and become global variables in any routine in which the use statement appears. • The use statement can appear in the main program or any other subroutine or module which needs access to the routines or variables of another module. 38
Page 1 and 2: University of Cambridge Department
Page 3 and 4: 8 USE OF NUMERICAL LIBRARIES: NAG 4
Page 5 and 6: Note that except within quotes: ⇒
Page 7 and 8: Unlike in an algebraic expression i
Page 9 and 10: We will meet more options later. Tr
Page 11 and 12: The character data type is used to
Page 13 and 14: Common Intrinsic Functions Name Act
Page 15 and 16: The if construct may also be used i
Page 17 and 18: 1.8 Repeating ourselves with loops:
Page 19 and 20: program linalg implicit none real :
Page 21 and 22: • Here are some more examples whi
Page 23 and 24: 2.2 Self-checking code We have alre
Page 25 and 26: 3. Input to and output from a F95 p
Page 27 and 28: • Modify the program to use “fr
Page 29 and 30: The file data1.dat should contain t
Page 31 and 32: Suggested Exercise 1 Modify the pro
Page 33 and 34: program newton ! ! Solves f(x) = 0
Page 35 and 36: 5.3 Subroutines Subroutines are ver
Page 37: subroutine stats(y, ybar) real, dim
Page 41 and 42: The swapmod.o file contains the com
Page 43 and 44: 7 Numerical precision and more abou
Page 45 and 46: 8 Use of numerical libraries: NAG T
Page 47 and 48: One would expect a subroutine which
Page 49 and 50: The same outcome could have been ac

Self study guide: Fortran 95 - University of Cambridge

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