6 April, 2000 CS1001 Lecture 15 EXAM1 - results SUBPROGRAM - - revisit FUNCTION

6 April, 2000 Exam 1 Average 69 Medium 72 >90 6 between 89 and 80 5 between 79 and between 69 and 60 9 between 59 and 50 3 below 6

6 April, 2000 Function and Subroutine Subprograms -- each is like a miniature program Function consists of: FUNCTION heading Specification part Execution part END FUNCTION statement Subroutine consists of: SUBROUTINE heading Specification part Execution part END SUBROUTINE statement

6 April, 2000 Function Table 6.1 (p. 324) and Appendix A list Fortran library functions Programmer defined Functions –separate program units that perform some function(s) –included with main program –invoked at in assignment statements in the flow of the program –communicates via passing parameters and returning result –should return a single value –must be assigned a type

6 April, 2000 Where to Put Function Code The Code for programmer defined functions are inserted at the end of the main program between a CONTAINS statement and the END PROGRAM PROGRAM ProgramName. CONTAINS Function(s) go here END PROGRAM ProgramName

6 April, 2000 PROGRAM Temperature_Table IMPLICIT NONE INTEGER :: iRange, Init, Limit,Step REAL :: Range, Fahr, CTOF DO iRange = Init, Limit, Step Range = REAL (iRange) Fahr = CTOF(Range) PRINT *, Range, Fahr END DO CONTAINS REAL FUNCTION CTOF(Cels) : END FUNCTION CTOF END PROGRAM Temperature_Table Good practice to declare function type in any program unit that calls the function

6 April, 2000 Function General Form FUNCTION function_name (formal_argument_list) type_identifier :: function-name (Declaration section) … (Execution section) … function_name = expr END FUNCTION function_name formal_argument_list is a list of identifiers (may be blank) type_identify is used to identify the type of the FUNCTION Alternate Heading: type_identifier FUNCTION function_name (formal_argument_list)

6 April, 2000 Converting With Function DO iRange = Init, Limit, Step Range = iRange Fahr = CTOF(Range) PRINT *, Range, Fahr END DO Where: FUNCTION CTOF(Cels) REAL :: CTOF REAL, INTENT(IN) :: Cels CTOF = 1.8 * Cels END FUNCTION CTOF

6 April, 2000 Flow of Control PROGRAM Temperature_Table... REAL FUNCTION CTOF (Cels)... CTOF = 1.8 * Cels END FUNCTION CTOF Range Fahr = CTOF(Range)... END PROGRAM Temperature_Table Range -- actual parameter Cels -- formal parameter

6 April, 2000 Voltage example PROGRAM ComputeVoltage REAL :: Time, Volts, Voltage... Time = 2.5 Volts = Voltage (Time)... CONTAINS FUNCTION Voltage(Time) REAL :: Voltage REAL, INTENT(IN) :: Time Voltage = (Time + 0.1)* EXP(SQRT(Time)) END FUNCTION Voltage END PROGRAM ComputeVoltage Given program statement. Function Voltage is referenced with an argument of Time = 2.5 Expression is evaluated as: ( ) * EXP(SQRT(2.5)) = Volts is then set equal to the computed value of

6 April, 2000 Function Equivalence Following are the same Time = 2.5 Volts = Voltage(Time) Time = 2.5 Volts = (Time + 0.1) * EXP(SQRT(Time)) If the computation is going to be done multiple times within a program (not within a loop), it pays to write a function. Voltage is a FUNCTION

6 April, 2000 Argument (Parameter) Passing Caller ( program or subprogram) function_name (actual_argument_list) function_name (formal_argument_list) Callee Each actual argument is associated with the corresponding formal argument -- must agree in number and type Used to pass values

6 April, 2000 Argument Passing Arguments are used to pass information between (to/from) caller and callee. INTENT Specifier Tells how the arguments are to transfer information – type, INTENT(IN) :: argument for inputs TO either a function or subroutine – type, INTENT(OUT) :: argument for outputs FROM a function or subroutine (but not good practice to have OUT arguments in function) – type, INTENT(INOUT) :: argument for both TO and FROM a subprogram

6 April, 2000 INTENT (IN) specification Used to ensure –value of the actual argument is passed to the formal parameter –the value of the formal argument cannot be changed while the function is being executed Well designed Fortran function –produce a single output value from one or more input values –never modify its own input arguments -- always declare the arguments with INTENT(IN) attribute.

6 April, 2000 Unintended side effects Changing the value of a variable in one part of the program affects, unintentionally, the value of that variable or other variables in other parts of the program. Often dangerous because –they might cause wrong results –are hard to debug (therefore, wrong results might go unnoticed.)

6 April, 2000 Old FORTRAN IV Example In calling program Sum = 0 Sum = addsum (10)... Sum = Sum + 10 Print *, Sum In Function addsum (IMAX)... IMAX = 5 addsum =... (say 20) END Name value in storage Sum 010 IMAX10, then Name value in storage Sum Name value in storage Sum Name value in storage Sum After 5 At 6 Print Sum will give 25 2

6 April, 2000 Scope The portion of the program in which an entity (variable, constant, subprogram, types) is visible, i.e., where it is accessible and can be used. Fundamental Principle -- The scope of an entity is the program or subprogram in which it is declared Scope Rule 1 -- An item declared within a subprogram is not accessible outside that subprogram Scope Rule 2 -- A global entity is accessible throughout the main program and in any internal subprogram in which no local entity has the same name as the global item.