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CS1001 Lecture 15

CS1001 Lecture 15. EXAM1 - results SUBPROGRAM - - revisit FUNCTION. Exam 1. Average 69 Medium 72 >90 6 between 89 and 80 5 between 79 and 70 11 between 69 and 60 9 between 59 and 50 3 below 6. Function and Subroutine.

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CS1001 Lecture 15

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  1. CS1001 Lecture 15 • EXAM1 - results • SUBPROGRAM - - revisit • FUNCTION

  2. Exam 1 • Average 69 • Medium 72 • >90 6 • between 89 and 80 5 • between 79 and 70 11 • between 69 and 60 9 • between 59 and 50 3 • below 6

  3. 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

  4. 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

  5. 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. 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

  7. 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)

  8. 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 + 32.0 END FUNCTION CTOF

  9. Flow of Control PROGRAM Temperature_Table . . . Range -- actual parameter Cels -- formal parameter 1 Fahr = CTOF(Range) . . . 2 7 Range END PROGRAM Temperature_Table 3 REAL FUNCTION CTOF (Cels) . . . CTOF = 1.8 * Cels + 32.0 END FUNCTION CTOF 4 5 6

  10. 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 Volts is then set equal to the computed value of 12.637 Expression is evaluated as: (2.5 + 0.1) * EXP(SQRT(2.5)) = 12.637

  11. 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

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

  13. 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) :: argumentfor inputs TO either a function or subroutine • type, INTENT(OUT) :: argumentfor 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

  14. 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.

  15. 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.)

  16. Old FORTRAN IV Example Name value in storage Sum 0 10 10 IMAX 10, then 5 In calling program Sum = 0 Sum = addsum (10) . . . Sum = Sum + 10 Print *, Sum In Function addsum (IMAX) . . . IMAX = 5 addsum = . . . (say 20) END 1 2 Name value in storage Sum 20 10 5 3 3 5 4 1 6 Name value in storage Sum 20 10 5 4 5 2 After 5 Name value in storage Sum 25 10 5 Print Sum will give 25 At 6

  17. 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.

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