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AME 150 L

More Subroutine & Functions

(& hints on HW 8)

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Writing Functions

• We will define a function "Noisy_Sine" that add some white noise to the sine

• White Noise is a heterogeneous mixture of signals extending over an infinite frequency range (acoustic definition)

• Fortran has a function that will generate a pseudo-random noise: random_number(x)

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FUNCTION noisy_sine (x,amp) IMPLICIT NONE REAL, INTENT(IN):: x,amp REAL :: noisy_sine, noise LOGICAL :: First_Entry=.TRUE. IF( First_Entry ) THEN First_Entry = .FALSE. !Initialize r-n generator CALL Random_Seed () END IF CALL Random_number(noise) !r-n uniform in 0-1 noisy_sine = SIN(x) + amp * ( noise - .5 ) return END FUNCTION noisy_sine

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Compiling a Subroutine or Function

f90 Noisy_sine.f90

Undefined first referenced

symbol in file

main /usr/spac/sparcompilers/5.1/...

ld: fatal: Symbol referencing errors. No output written to a.out

------------

• One problem is: message wraps on screen!• Why do they do the two column display?• Meaning - No MAIN program! (why didn't they say that?)

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"Calling" Program

• Any Subroutine or Function is used only when invoked (Called)

• Test program (driver) is designed to exercise program (check inputs and outputs)

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PROGRAM testdrive IMPLICIT NONE REAL :: x, Noisy_sine

DO write (*,*)"Enter x=" read (*,*)x WRITE (*,*)x, sin(x), Noisy_sine(x,0.1)

END DO

END PROGRAM testdrive

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Types of Errors

1) Missing symbol: (MAIN) or Subroutine

2) Incorrect calling sequence (arguments)

Omit argument. Does Fortran take 0?

No, It actually took some garbage (GIGO again)

3) Try incorrect type of arguments (Integer or character instead of REAL)

Does it look like Fortran used 0?

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Forcing an Interface

INTERFACE

FUNCTION noisy_sine (x,amp)

IMPLICIT NONE

REAL, INTENT(IN):: x, amp

REAL :: noisy_sine

END FUNCTION noisy_sine

END INTERFACE

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Interface Description

• Does not contain any executable statements

• Protects against interface errors at COMPILE TIME (not run time)

• Use of INCLUDE (Not recommended)

• Older extension (not a Fortran statement)

INCLUDE "filename"• (better to use module facility)

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Common Errors

• Compiler Errors - Wrong Fortran Syntax

• Logic Errors - Fortran statements don't do what you intended (they do what you say)

• Linker Errors - Missing Symbol Names

• Interface Errors - Mismatch in order, type, or usage of arguments to subroutines and/or functions

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Arrays• Collections of data referenced by same

symbol (with an index or subscript)

• Must always describe Type, dimension [and shape]– Modern systems allow dynamic allocation

• Older Fortran only referred to elements of an array, Fortran 90/95 can overload operators (+, -, *) to simplify coding

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Arrays - Examples

• Declaration:REAL, Dimension(10):: a, b, c

! indices go from 1 to 10

REAL :: a(10), b(0:20), c(-10:10)

INTEGER, parameter:: Size=20

REAL , dimension(size,size) :: X

INTEGER,allocatable,dimension(:,:)::k

(we will not use allocatable arrays in AME 150)

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Arrays in Subprograms

• Calling Program:

PROGRAM test

IMPLICIT none

REAL, dimension(10,20)::a

. . .

CALL SUB1 ( a , 10 )

. . .

END PROGRAM test

• Subprogram

Subroutine SUB1 (x, nd)

IMPLICIT none

INTEGER,INTENT (IN)::nd

REAL,dimension(nd,*),& &INTENT(IN OUT):: x

. . .

RETURN

. . .

END Subroutine SUB1

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Storage of Arrays

• An array is stored ONCE in memory– It exists in the Highest Order (towards main

program) routine that references it– Subroutines refer to a Pointer, and do not create

a copy of an array

• On the USC computers, arrays are stored linearly in memory

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Storage of Arrays -2

REAL::A(6) REAL::B(0:4) REAL::C(3,2)

A(1) B(0) C(1,1)

A(2) B(1) C(2,1)

A(3) B(2) C(3,1)

A(4) B(3) C(1,2)

A(5) B(4) C(2,2)

A(6) C(3,2)

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Storage of Arrays -3

• A subroutine only needs to know that a one dimensional array is an array

• For 2 and 3 dimensional arrays, the subroutine MUST know size of all but the last dimension

• Don't count on Fortran to check the dimensions for you