FUNCTIONS IN FORTRAN

1

• FUNCTIONS IN FORTRAN
• For a complex program difficulties like writing
  it and debugging are encountered. These can be
  minimized by breaking main program into parts
  called sub programs.
• Main program It is a full program in itself
  and can be executed itself.
• Sub program It is a small unit of program and
  cannot be executed itself.It is used by a main
  program or by other sub program.
• It is complete and independent in the sense
• It is complete because it contains necessary type
  declaration ( such as REAL, INTEGER,DIMENSION
  etc. ) and use of library function END
  statement.
• It is independent because all variable names
  (except the function variable name) and all
  statement labels used are local to the subprogram
  and unknown outside. Thus the same variable
  names and statement labels may be used in the
  main(calling) program and diffehent subprograms.
• FUNCTIONS There are three types of
  functions
• (i) Library functions ( Built in functions )
• Statement functions
  
• Function sub program
• (ii) (iii) are user- defined
  functions.

2

• TYPES OF SUB PROGRAM

SUBPROGRAM
SUBROUTINE
FUNCTION
User Defined Fn.
Pre-Defined Built- in Fn./ Intrinsic Fn. /
Library Fn.
(i)
Function Subprogram / External Fn. /
Multiline Fn.
Statement Function

(ii)
(iii
3

• Pre-defined Built in function / Intrinsic
  function / Library function These are
  pre-written programs by the manufacturers of
  compiler for some commonly used mathematical
  functions e.g. SQRT,MOD,SIN,COS etc.
• User- defined functions These are of two types
  
• (a) Statement function It is defined by a
  single arithmetic or logical statement at the
  beginning of the main program or a sub program.
• Syntax
• functionname (list
  of arguments ) expression
• Rules
• List of arguments may be non-subscripted variable
  names called dummy arguments
• The expression on R H S may have some more
  variables in addition to list of arguments
• e.g.
• AVE (A,B,C) (ABC)/3.0 (ii) DIST(X,Y,Z)
  SQRT (XXYYZZ)
• (iii) Use the statement function
  (ii) to evaluate expressiona (a) and (b)
• (iv) Use the statement function DIV (x,y) (xy)
  / (x-y) to evaluate (c),(d)
• (a) P (a5)2 (b7)2 (c9)2
  3/2 , (b) B (u2v2)1/2 / (1( p2 q2
  r2 )1/2
• P DIST (a5.0, b7.0, c9.0) 3 , (b)
  BDIST (u,v,0.0) / (1.0 DIST(p,q,r))
• (c) A (p1/2 5 )/ (p1/2-5) ,
  (d) b 3 tan-1 ((1 r )
  / (1-r ))

4

• Illustration Of Statement Function Program
• F( A,B ) A2 B2
• Write (,) Enter values of X and Y
• Read (,) X,Y
• C To calculate value of X2
• X2 F( X,0.0)
• C To calculate value of Y2
• Y2 F(Y,0.0)
• C To calculate value of ( X2 Y2 )
• X2 Y2 F( X,Y)
• Write (,) X2 , Y2 , X2 Y2
• Stop
• End

5

• (b) Function subprogram / External function /
  Multiline function
• Syntax
• Type Function name (list of
  arguments)
• .
• .
• name expression
• RETURN
• END
• e.g. Integer function square (a, b, c)
• RULES
• Every Function subprogram returns a single value
  using the RETURN statement.
• The first statement in Function subprogram must
  always be the Function statement specifying type,
  name and dummy arguments.If type of Function name
  is declared then the Function name must be
  declared of same type in main or calling program.
• The Function name must appear at least once on
  LHS of an arithmetic expression
  (assignment statement) in its body.
• In MAIN PROGRAM it is called by assignment
  statement.When the name of a FUNCTION subprogram
  is encountered in source program, control is
  transferred to subprogram.The dummy arguments are
  replaced by the actual arguments in the body of
  FUNCTION subprogram and execution of subprogram
  is carried out.After execution is over the RETURN
  statement returns control to particular reference
  point in the calling program.
• The list of dummy arguments are nonsubscriptedl
  variables and must match in number and order with
  calling program.Same variable names and statement
  numbers can be used in MAIN and SUBPROGRAM as
  these are local in function subprogram .
• RETURN is logical end of subprogram and returns
  the control at the calling point.
• Last statement END is physical end of the
  subprogram.

6

• For Example
• Calling Program
• VAR function name (a, b, c)
• END
• Function subprogram
• function name (x, y, z )
• ..
• ..
• name expression
• Return
• End

7

• Determine Factorial of a number using function
  subprogram
• C Main Program
• Integer FACT
• Write(,) Enter the number
• Read(,) N
• IF ( N.LT.0) THEN
• WRITE (,) Factorial of a NEGATIVE
  NUMBER IS NOT DEFINED
• ELSE
• I FACT ( N )
• WRITE (,) N, I
• ENDIF
• STOP
• END
• C Function subprogram
• INTEGER FUNCTION FACT (K)
• IF ( K.EQ.O) THEN
• FACT 1
• RETURN
• ELSE

8

• Compute n C r n! / ( r! (n-r )! ) using
  function subprogram
• C Main Program
• Integer R, FACT
• Write(,) Enter N and R
• Read(,) N, R
• I1 FACT (N)
• I2 FACT (R)
• I3 FACT (N-R )
• NCR I1/ (I2 I3 )
• WRITE (,) N,R,NCR
• STOP
• END
• C Function Subprogram
• INTEGER FUNCTION FACT (K)
• IF ( K.EQ.O) THEN
• FACT 1
• RETURN
• ELSE
• FACT 1

9

• Write Function Subprogram to find the largest of
  three numbers
• Function Big (A,B,C)
• Big A
• If ( Big.LT.B ) Big B
• If ( Big.LT.C ) Big C
• Return
• End

10

• SUBROUTINE
• SYNTAX
• SUBROUTINE name of
  subroutine ( list of arguments )
• .
• .
• Return
• End
• For calling a SUBROUTINE subprogram CALL
  statement is used in Main Program.
• i.e. in Main program
• CALL name of subroutine (
  list of arguments )
• RULES
• It is also a complete and independent program.
• The name of SUBROUTINE subprogram does not have
  a value. So CALL statement is required in Main
  program for using it.
• The communication between Main program and
  SUBROUTINE subprogram is only transmitted through
  parameters and not through its name.
• The execution of CALL transfers the control to
  SUBROUTINE subprogram.The dummy arguments are
  replaced by the actual arguments in the body of
  SUBROUTINE subprogram and execution of subprogram
  is carried out.After execution is over the RETURN
  statement transfers control back to the statement
  immediately following the CALL statement in the
  calling program.

11

• EXAMPLE Multiplication of two numbers using
  SUBROUTINE SUBPROGRAM
• C MAIN PROGRAM
• READ ( , ) IP, IQ
• CALL MUL ( IP , IQ , IT )
• WRITE ( , ) IT
• END
• C SUBROUTINE SUBPROGRAM
• SUBROUTINE MUL ( IA , IB , IC )
• IC IA IB
• RETURN
• END

12

• DIFFERENCE BETWEEN FUNCTION SUBPROGRAM
• FUNCTION SUBPROGRAM
• 1.It can return only one value to calling prog.
• 2.It returns value through its name .
• 3.It has a type associated with its name which
  identifies type of value returned by it.
• 4.There must be at least one argument in the
• dummy list.
• 5. It is called by refering its name.
• 6. After execution of RETURN or END statement ,
  the control is transferred back to the reference
  point in the calling prog. or subprog.
  

• SUBPROGRAM AND SUBROUTINE
• SUBROUTINE SUBPROGRAM

It can return more than one value to calling prog.
It returns value(s) through its arguments .
It has no type associated with its name and
values returned can be of diff. types identified
by type of arguments returning values to the
calling prog.
There is no restriction i. e. it can be without
any arguments.
It is called by a CALL statement.

After execution of RETURN or END statement , the
control is transferred back to the statement
immediately following the CALL statement in the
calling prog. or subprog.
13

• ADVANTAGES AND PURPOSE OF BREAKING A PROGRAM INTO
  SUBPROGRAMS
• Repetition A subprogram may be used to avoid
  rewriting the same set of statements within the
  same program.
• Universal Use A subprogram written once can be
  used in more than one program and by users other
  than its developer.
• Modularity A complex program can be solved by
  modular approaches using the subprogram.
• Team Work A large program can be divided into
  different subprograms which can be written by the
  group of persons.
• Debugging A subprogram makes it easy to debug
  ( i.e. find errors ) in the program.