Worksheet 6 - Subprograms

Subroutines and Functions

This worksheet is also available in PDF format.

This worksheet makes use of several examples programs that are all available for download from this website.


By the end of this worksheet you will be able to:

  • Understand the use of subroutines and functions to make your code more efficient and easier to read.

Re-using code – the subroutine

Examine the following program

   program output
   implicit none
   real,dimension(3)  :: a,b,c
   character ::  answer*1
   !initialise arrays
   a = 1.5
   b = 2.5
   c = 3.5
   write(*,1) 'a',a
   print *, 'type y to continue or any other key  to finish'
   read *, answer
   if (answer /= 'y') stop
   write(*,1) 'b',b
   print *, 'type y to continue or any other key  to finish'
   read *, answer
   if (answer /= 'y') stop
   write(*,1) 'c',c
   print *, 'type y to continue or any other key  to finish'
   read *, answer
   if (answer /= 'y') stop
   write(*,1)  'a*b*c',a * b * c
   1 format(a,3f8.3)
   end program  output

The program sets up some arrays and then outputs them. At three stages in the program (bolded), it asks whether it should continue; it stops if the answer is not 'y'.  Notice that the three bolded parts of the code are identical.

Simple enough – but look at the amount of code! Most of it is the same – wouldn't it be nice to re-use the code and cut down on the typing? The answer is to use subroutines.

   program output1
   implicit none
   real,dimension(3) :: a,b,c
   !initialise arrays
   a = 1.5
   b = 2.5
   c = 3.5
   write(*,1) 'a',a
  call prompt()
   write(*,1) 'b',b
  call prompt()
   write(*,1) 'c',c
  call prompt()
   write(*,1) 'a*b*c',a * b * c
   1        format(a,3f8.3)
   end program output1
  subroutine prompt()
  !prompts for a keypress
  implicit none
  character answer*1
  print *, 'type y to continue or any other key to finish'
  read *, answer
  if (answer /= 'y') stop
  end subroutine prompt

Examine the code, each time we use type

                 call  prompt()

the program jumps to the line

                 subroutine prompt()

then executes each line of the code it finds in the subroutine until it reaches the line

                 end  subroutine prompt

and then returns to the main program and carries on where it left off.

The program is much easier to understand now. All the code for prompting is in one place. If we ever need to change the code which prompts the user to continue, we will only ever need to change it once. This makes the program more maintainable.

Arguments to subroutines

We have seen that subroutines are very useful where we need to execute the same bit of code repeatedly. 

The subroutine can be thought of as a separate program which we can call on whenever we wish to do a specific task. It is independent of the main program – it knows nothing about the variables used in the main program. Also, the main program knows nothing about the variables used in the subroutine.  This can be useful – we can write a subroutine using any variable names we wish and we know that they will not interfere with anything we have already set up in the main program.

This immediately poses a problem – what if we want the subroutine to do calculations for us that we can use in the main program? The following program uses  arguments to do just that.

Example: a program that calculates the difference in volume between 2 spheres.

   program vols
   !Calculates difference in volume of 2 spheres
   implicit none
   real :: rad1,rad2,vol1,vol2
   character :: response
   print *, 'Please enter the two radii'
   read *, rad1,rad2
   call volume(rad1,vol1)
   call volume(rad2,vol2)
   write(*,10) 'The difference in volumes is, ',abs(vol1-vol2)
   10       format(a,2f10.3)
   print *, 'Any more? - hit Y for yes, otherwise hit any key'
   read *, response
   if (response /= 'Y' .and. response /= 'y') stop
   end do
   end program vols
   subroutine volume(rad,vol)   
   implicit none
   real :: rad,vol,pi
   !calculates the volume of a sphere
   !It's a little quicker in processing to  do r*r*r than r**3!
   end subroutine volume      

When the program reaches the lines

          call volume(rad1,vol1)

It jumps to the line

          subroutine  volume(rad,vol)   

The values, rad1 and vol1 are passed to the subroutine. The subroutine calculates a value for the volume and when the line :

          end subroutine volume      

is reached, the value of the volume is returned to the main program

Points to notice – these are very important – please read carefully

  • You may have several subroutines in your program. Ideally, a subroutine should do a specific task – reflected by its name.
  • All the variables in subroutines, apart from the ones passed as arguments, are 'hidden' from the main program. That means that you can use the same names in your subroutine as in the main program and the values stored in each will be unaffected – unless the variable is passed as an argument to the subroutine.
  • It is very easy to forget to declare variables in subroutines. Always use implicit none in your subroutines to guard against this.
  • All the variables in the subroutine must be declared.
  • The positioning of the arguments (in this case, rad and vol) is important. The subroutine has no knowledge of what the variables are called in the main program. It is vital that the arguments agree both in position and type. So, if an argument to the subroutine is real  in the main program, it must also be real  in the subroutine.
  • If an argument to the subroutine is an array, it must also be declared as an array in the subroutine.
Exercise 6.1

Write a program that calculates the difference in area between two triangles. Your program should prompt the user for the information it needs to do the calculation. Use a subroutine to calculate the actual area. Pass information to the subroutine using arguments.

User Defined Functions

We have already met FORTRAN intrinsic functions like abs, cos, sqrt. We can also define our own functions – they work in a similar way to subroutines.

As an example, let's write a program (func.f95) that does some trigonometry. As you know, the trig routines in FORTRAN use radians, not degrees - so it would be nice to write a function that does all the conversion for us.

   print *,'Enter a  number'
   read *, a
   print *,'the sine of  ',a,' is ',sin(a*pi/180)

In this snippet, we are having to code the conversion from degrees to radians directly into the main part of the program. That's OK for a 'one-off', but what if we needed to do the conversion several times. Now look at this:

   program func                                
   !demonstrates use of user defined functions
   implicit none
   integer, parameter ::  ikind=selected_real_kind(p=15)
   real (kind=ikind)::  deg,rads
   print *, 'Enter an  angle in degrees'
   read  *, deg
   write(*,10) 'sin =  ',sin(rads(deg))
   write(*,10) 'tan =  ',tan(rads(deg))
   write(*,10) 'cos =  ',cos(rads(deg))
   10   format(a,f10.8)
   end program func
   function rads(degrees)
   implicit none
   integer, parameter ::  ikind=selected_real_kind(p=15)
   !    returns radians
   real (kind=ikind) ::  pi,degrees,rads
   end function rads           

What we have done, in effect, is to create our own function rads, which is used in an identical way to the intrinsic ones you have used already like sqrt, cos, and abs.

When the line

     write(*,10) 'sin =  ',sin(rads(deg))

is reached, the program jumps to

      function rads(degrees)

the value, degrees, is passed to the function. The function does some computation, then finally returns the calculated value to the main program with the line

Note carefully that it doesn't return the value in the argument list (as does a subroutine) but actually assigns the value to its own name rads.
  • The function rads converts the value of the argument, degrees, to radians.
  • Notice that we must declare the data type of the function both in the main program, and in the function itself as if it were a variable.
  • Functions return one value. This value, when calculated, is assigned to the name of the function as if it were a variable –
Exercise 6.2

Write a program that includes a function called

real function average(n,list)

where n is integer and is the number of items in the list, and list is a real array.

Write suitable code for reading the numbers from a file (or keyboard), and output the average of the numbers.

Exercise 6.3

Write a  program that allows a user to enter the size of a square matrix. In the program write a subroutine to compute a finite difference matrix. Ensure your output is neatly formatted in rows and columns.

So, for a 10 by 10 matrix, we expect output to look like this

    2  -1    0   0   0   0   0    0   0   0
   -1   2   -1   0   0   0   0    0   0   0
    0  -1    2  -1   0   0   0    0   0   0
    0   0   -1   2  -1   0   0    0   0   0
    0   0    0  -1   2  -1   0    0   0   0
    0   0    0   0  -1   2  -1    0   0   0
    0   0    0   0   0  -1   2   -1   0   0
    0   0    0   0   0   0  -1    2  -1   0
    0   0    0   0   0   0   0   -1   2  -1
    0   0    0   0   0   0   0    0  -1   2  

Check your attempt with finite.diffs.f95 on the website.