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1 The Future

2 Introduction to FORTRAN
OBJECTIVES History and purpose of FORTRAN FORTRAN essentials Program structure Data types and specification statements Essential program control FORTRAN I/O subfunctions and subroutines Extensions for Fortran 95 Pitfalls and common coding problems Sample problems

3 FORTRAN History One of the oldest computer languages Version history
created by John Backus and released in 1957 designed for scientific and engineering computations Version history FORTRAN 1957 FORTRAN II FORTRAN IV FORTRAN 66 (released as ANSI standard in 1966) FORTRAN 77 (ANSI standard in 1977) FORTRAN 90 (ANSI standard in 1990) FORTRAN 95 (ANSI standard version) FORTRAN 2003 (ANSI standard version) Many different “dialects” produced by computer vendors (Digital VAX Fortran, now Intel Fortran) Large majority of existing engineering software is coded in FORTRAN (various versions)

4 Any character: continuation line
Statement Format FORTRAN before 90 requires a fixed format Based on the punch card in use when Fortran was created PROGRAM MAIN C COMMENTS ARE ALLOWED IF A “C” IS PLACED IN COLUMN #1 DIMENSION X(10) READ(5,*) (X(I),I=1,10) WRITE(6,1000) X 1000 FORMAT(1X,’THIS IS A VERY LONG LINE OF TEXT TO SHOW HOW TO CONTINUE ’ * ‘THE STATEMENT TO A SECOND LINE’,/,10F12.4) 1-5 Label 6 7-72 Statements 73-80 Optional Line #s Any character: continuation line

5 Statement Format FORTRAN fixed format
“C” in column 1 indicates that line is a comment columns 1-5 are reserved for statement labels statement labels are not required unless the statement is the target of a goto labels are numeric values only column 6 is the continuation flag any character in column 6, other than space or “0”, indicates that this line is a continuation of the previous line there is usually a limit of 19 on the number of continuations columns 7-72 are contain Fortran statements columns is for sequence information only of any use when using punch cards

6 Statement Format IBM punch card

7 Building a FORTRAN Program
FORTRAN is a complied language (like C) so the source code (what you write) must be converted into machine code before it can be executed (e.g. Make command) Executable File FORTRAN Program FORTRAN Compiler Link with Libraries Libraries Executable Code Source Code Object Code Make Changes in Source Code Test & Debug Program Execute Program

8 Structure of a Fortran Program
Fortran is a compiled language all memory is allocated statically at compile time there is no standard method for allocating additional memory in a Fortran program before Fortran 90 memory is allocated in a predictable manner, a fact which can be used by the programmer to his advantage or distress there is no official recursion before Fortran 90 some vendor implementations had recursive capabilities static memory allocation is at odds with the use of a stack which is needed for recursion Fortran does not guarantee value of un-initialized memory

9 Structure of a Fortran Program
Fortran consists of program units program function subroutine block data The program unit contains the main code and the point where execution starts in Fortran 77 a program begins with the program statement earlier versions of Fortran did not have a program statement unless a vendor dialect provided one the end statement terminates the program unit

10 Fortran Program The program unit contains the main code and the point where execution starts in Fortran 77 a program begins with the program statement earlier versions of Fortran did not have a program statement unless a vendor dialect provided one the end statement terminates the program unit marks end of statements that belong to program during execution it will cause the program to halt a program unit may contain internal sub-programs internal functions internal subroutines

11 Fortran Subroutine The subroutine unit contains Fortran code that can be called from other Fortran code a subroutine begins with a subroutine statement contains a name for the subroutine a list of formal arguments subroutines may be internal or external an internal subroutine is included in the code of program unit and is only callable by the program an external subroutine is created outside of a program unit and is callable from everywhere

12 Fortran Subroutine SUBROUTINE MULT(A,B,C) C = A * B RETURN END
CALL MULT(5.0,X,VALUE)

13 Fortran Function The function unit contains Fortran code that can be called from other Fortran code It differs from a subroutine in that it returns a value a subroutine begins with a function statement contains a name for the function a list of formal arguments specifies a return type functions may be internal or external an internal function is included in the code of program unit and is only callable by the program an external function is created outside of a program unit and is callable from everywhere

14 Fortran Function REAL FUNCTION MULT(A,B) MULT = A * B RETURN END
VALUE = MULT(5.0,X)

15 Fortran Block Data The Block Data program unit does not contain any executable code Used to initialize memory in common blocks It is not “called” by any code in the program, it contains instructions for the initializing memory when the program is loaded into memory for running

16 Fortran Block Data BLOCK DATA COMMON/MEM/A,B DATA A,B/10.0,-3.14/ END

17 Fortran Program Organization
Program units have a statement order header (program, subroutine, function, block data) declarations (variables and types) data initialization (data statements) executable statements (and format statements) internal subprogram units end statement

18 Fortran Variable Variables represent the memory of the program
Fortran variables Fortran IV numbers and letters, at least 6 characters Fortran 77 numbers and letters and “_”, at least 16 characters must start with a letter Up through 77, spaces in a Fortran program are ignored IVALUE and I VAL UE are the same using strange spacing, while acceptable, is bad practice Fortran variables are typed Fortran is case insensitive ivar is the same as IVAR or IvAr

19 Fortran Variable Typing
All Fortran variables are typed INTEGER REAL DOUBLE PRECISION COMPLEX LOGICAL CHARACTER (77+)

20 Fortran Variable Typing
A feature of Fortran – implicit typing when a variable appears that has not been declared previously it is created (at compile time) it is assigned a type based on the first character of the name A-H,O-Z is type REAL I-N is type INTEGER a typo can cause the creation of a new variable – not an error Starting with 77 the implicit statement was added allowed changing the first letter assignments most 77 compilers include the implicit none statement that requires that all variables be explicitly typed – prevents the typo problem It is good practice to use implicit none

21 Fortran Variable Typing
Change implicit typing so that A becomes an INTEGER type Disable implicit typing altogether PROGRAM TEST IMPLICIT INTEGER (A) PROGRAM TEST IMPLICIT NONE

22 Fortran Variable Typing
In Fortran any variable can be explicitly typed In the declarations section enter a type identifier followed by a list of variable names The first letter implicit typing is over-ridden when explicit typing is used A common Fortran error when using implicit typing is to use a variable such as INITIAL_VALUE as if it contained a real (floating point) value INTEGER A,VALUE,ISTART REAL INITIAL_VALUE

23 Fortran Variable Typing
The types presented earlier are the default types The range of both INTEGER and REAL are dependent on the computer architecture one computer may have a 32 bit integer while another may use 16 bit as its default An attempt to deal with this lead to types such as REAL*8, INTEGER*4 the number after the * indicates the number of bytes used most computers have 8 bit bytes not every architecture will have every combination not a practical problem in an Intel world but knowledge of the architecture of the system where a legacy Fortran program was developed is needed to convert to Intel

24 Fortran Variable Typing
Fortran 90+ uses a different method to deal with number ranges that is architecture independent

25 Fortran Variable Typing
The CHARACTER type was introduced in 77 The * notation is used to specify the maximum number of characters the variable can hold Before 77 the Hollerith notation was used common in older Fortran code, even in some 77 code normally placed characters into INTEGER arrays required knowledge of byte length of the variable portability problem CHARACTER*20 TITLE INTEGER*4 TITLE(5) DATA TITLE/4Habcd,4Hefgh,…/

26 Fortran Arrays The array is the only data structure supported in 77 and before An array is a linear allocation of memory An array can contain up to 7 dimensions Arrays are indexed starting a 1 INTEGER A DIMENSION A(10 INTEGER B DIMENSION B(10,10) REAL C(10,10,10)

27 Fortran Arrays Fortran 77 introduced the ability to specify a lower bound for an array dimension During execution of a Fortran program there is normally no check made on array index bounds there may be a compiler option to enable these checks some bound checking code only checks the equivalent linear index not each individual index INTEGER B DIMENSION B(0:10,0:10)

28 Fortran Arrays Fortran character (77) CHARACTER*10 TITLE(4)

29 Fortran Variables and Subroutines
All arguments to a Fortran subroutine are passed by reference the subroutine receives the address of the variable any changes made by the subroutine are seen by the caller most other languages pass by value (the subroutine receives a copy) passing an array as an argument with just the name will pass the address of the first element On entry to a subroutine its local variables are not guaranteed to have any known value the save statement introduced in 77 will ensure that a variable will have on entry the value that it had on its last exit from the subroutine

30 Fortran Common Blocks Normally variables in a Fortran program are local to the unit in which they are declared variables may be made known to subroutines using the arguments variables may be created in a common block Common blocks are shared memory each program unit that declares the common block has access to it

31 Fortran Common Blocks Two types of common
blank common – unnamed named common Most systems do not allow blank common to be initialized Blank common can sometimes be used to allocate unused memory (depends on OS) COMMON A,B(10),C COMMON/SET1/A,B(50,5),C

32 Fortran Common Blocks Problems
each program unit that declares access to a common block defines it’s own view type of each variable in the block size of each array in the block when views between units differ there can be problems – some linkers will warn of size differences PROGRAM TEST COMMON/A/A,B(10),C END SUBROUTINE DOIT COMMON/A/I,J(5),K(20)

33 Fortran Equivalence The EQUIVALENCE statement is used to alias a memory location Usually will include a type change Also used to provide aliases for elements of an array Takes advantage of no index checking INTEGER A(100) EQUIVALENCE (INCREMENT,A(4)) COMMON A(10000) INTEGER IA(1) EQUIVALENCE (IA(1),A(1))

34 Fortran Parameter The PARAMETER statement is used to define constants
A parameter can be used wherever a variable is expected – but cannot be overwritten Can be used in declarations PARAMETER (MAX=20) PARAMETER (MAX=1000) INTEGER A(MAX)

35 Fortran Literals Literals are constants that appear in a Fortran program Number integers - 1, -34 real - 1.0, 4.3E10, 5.1D-5 complex – (5.2,.8) Other logical - .true., .false. character – ‘title line’ Obsolete but still lurking in code Hollerith – 4Habcd, 8Habcdef

36 Fortran Literals INTEGER A A = 34 REAL A(20) A(1) = 31.4159E-1
ITERM = -10.3 COMPLEX Z Z = (10,-10.5) REAL_PART = REAL(Z) AIMAG_PART = AIMAG(Z) Z = CMPLX(REAL_PART * 2,AIMAG_PART)

37 Fortran Expressions Expressions are the heart of Fortran (Formula Translator) There are two types of expressions numeric 2 * * RADIUS**2 SIN(PI) logical IBOOL = .TRUE. I .EQ. 10 .AND. ISTOP note: LOGICAL ISTOP

38 Fortran Numerical Operators
The numerical operators ** (exponentiation) * / unary + - binary + - Parentheses are used to alter the order of evaluation For binary operators, if the types do not match an implicit conversion is performed to the most general type integer -> real -> double precision anything -> complex

39 Fortran Numerical Operators
WARNING: division of an integer by an integer will produce a truncated result 5 / 2 => 2 not 2.5 FLOAT(5)/2 => 2.5 The type-conversion intrinsic functions can be used to get the desired results

40 Intrinsic Functions Fortran includes an extensive set of built-in functions Fortran 66 has different names for these functions depending on the return type and argument type Fortran 77 introduced generic names for intrinsic functions

41 Type Conversion The intrinsic functions have two forms
generic available only in 77 and above argument specific Conversion to integer INT(any) the generic version INT(real) IFIX(real) IDINT(double) Conversion to real REAL(any) the generic version FLOAT(integer) REAL(integer) SNGL(double)

42 Type Conversion Conversion to double Conversion to complex
DBLE(any) the generic version Conversion to complex COMPLX(any) the generic version Character to integer (77+ only) ICHAR(character) Integer to character (77+ only) CHAR(integer)

43 Truncation To integer part, return as real or double
AINT(real or double) the generic version AINT(real) DINT(double) To nearest integer, return as real or double ANINT(real or double) the generic version ANINT(real) DNINT(double) To nearest integer, return as integer NINT(real or double) the generic version NINT(real) IDNINT(double)

44 Math Functions sine and cosine (radians) exponential natural logarithm
SIN(real or double) the generic version SIN(real) DSIN(double) CSIN(complex) exponential EXP(real or double) the generic version EXP(real) DEXP(double) CEXP(complex) natural logarithm LOG(real or double) the generic version ALOG(real) DLOG(double) CLOG(complex)

45 Math Functions tangent (radians) square root hyperbolic sine
TAN(real or double) the generic version TAN(real) DSIN(double) square root SQRT(real or double) the generic version SQRT(real) DSQRT(double) CSQRT(complex) hyperbolic sine SINH(real or double) the generic version SINH(real) DSINH(double)

46 Math Functions there are also similar functions for
arcsine, arccosine, arctangent (ASIN, ACOS, ATAN) hyperbolic sine, cosine, tangent (SINH, COSH, TANH) complex conjugate (CONJ) base10 logarithms (LOG10)

47 Fortran Statements The executable statements in Fortran assignment (=)
branching (GOTO) comparison (IF) looping (DO) subroutine invocation (CALL)

48 Fortran Assignment The simple assignment statement stores the result of computations into a variable DIMENSION A(10,10) A(I,10) = 2.0 * PI * R**2 INTEGER A A = A + 1

49 Fortran Branching Fortran includes a GOTO statement
In modern languages this is considered very bad its use was essential in Fortran 66 its predecessors Fortran 77 introduced control statements that lessened the need for the GOTO IF (I .EQ. 0) GO TO 100 A = 4.0 * AINIT GOTO 200 100 B = 52.0 200 C = B * A

50 Fortran Branching The Fortran GOTO always branched to a Fortran statement that contained a label in columns 1-5 The labels varied from 1 to 99999 Variations of the go to statement are assigned goto computed goto Spaces are ignored in Fortran code before 90 GOTO and GO TO are equivalent Excessive use of the goto (required in 66 and before) leads to difficult to understand code

51 Fortran Branching Assigned goto ASSIGN 100 TO TARGET … GOTO TARGET
100 CONTINUE

52 Fortran Branching Computed goto
Operates much like a case or switch statement in other languages GOTO (100,200,300,400),IGO 100 CONTINUE GOTO 500 200 CONTINUE 500 CONTINUE

53 Fortran Continue The CONTINUE statement is a do-nothing statement and is frequently used as a marker for labels It is used most frequently with DO loops

54 Fortran IF The IF statement is used to perform logical decisions
The oldest form is the 3-way if (also called arithmetic if) The logical if appeared in Fortran IV/66 The more modern if-then-else appeared in Fortran 77

55 Fortran 3-way If The 3-way if statement tested a numerical value against zero It branched to one of three labels depending on the result IF (RADIUS) 10,20,30 10 CONTINUE GOTO 100 20 CONTINUE 30 CONTINUE 100 CONTINUE IF (ABS(RADIUS-EPS)) 10,10,20 10 CONTINUE GOTO 100 20 CONTINUE 100 CONTINUE

56 Fortran Logical If The logical if statement performed a test using the logical operators .EQ., .NE., .LT., .LE., .GT., .GE. .AND., .OR., .NOT. If result is true then a single statement is executed IF (ISTART .EQ. 50) GOTO 100 100 CONTINUE IF (IMODE .EQ. 2) A = SQRT(CVALUE) LOGICAL QUICK QUICK = .TRUE. IF (QUICK) STEP=0.5 IF (.NOT. QUICK) STEP = 0.01

57 Fortran Logical If LOGICAL QUICK QUICK = .TRUE. IF (QUICK) STEP=0.5
IF (.NOT. QUICK) STEP = 0.01 IF (QUICK .AND. (ABS(XVALUE – EPS) .LT )) GOTO 1000

58 Fortran Modern If Fortran 77 introduced the modern if statement (so-called structured programming) The test operated the same as the logical if Greatly reduced the need for using the goto statement Includes then clause else clause else if clause

59 Fortran Modern If This form eliminates the goto statements from the previous example LOGICAL QUICK QUICK = .TRUE. IF (QUICK) THEN STEP=0.5 ELSE STEP = 0.01 ENDIF IF (QUICK .AND. (ABS(XVALUE – EPS) .LT )) THEN END IF

60 Fortran Modern If This form reduces the need for the computed goto
IF (MODE .EQ. 0) THEN ELSE IF (MODE .EQ. 1) THEN ELSEIF (MODE .EQ. 2) THEN ELSE END IF

61 Fortran Looping The DO statement is the mechanism for looping in Fortran The do loop is the only “official” looping mechanism in Fortran through 77 Here I is the control variable it is normally an integer but can be real 1 is the start value 10 is the end value 2 is the increment value everything to the 100 label is part of the loop DO 100 I=1,10,2 100 CONTINUE

62 Fortran Looping The labeled statement can be any statement not just continue Loop may be nested nested loops can share the same label – very bad form DO 100 I=1,10,2 DO 100 J=1,5,1 100 A(I,J) = VALUE DO 200 I=1,10,2 DO 100 J=1,5,1 A(I,J) = VALUE 100 CONTINUE 200 CONTINUE

63 Fortran Looping Before Fortran 77 a do loop would always execute at least once – despite the parameters The increment may be negative, if not specified it is assumed to be 1 DO 100 I=10,1,2 100 CONTINUE

64 Fortran Looping WARNING: Through Fortran 77 there is an extended do loop can jump out of loop to code outside the loop that code can jump back into the loop valid as long as code does not modify the control variable no need to ever use it – use a subroutine instead DO 100 I=1,100 GOTO 1000 99 CONTINUE 100 CONTINUE 1000 CONTINUE GOTO 99 DO 100 I=1,100 CALL XYZ 100 CONTINUE

65 Fortran Looping Fortran 77 introduced a form of the do loop that does not require labels The indented spacing shown is not required DO I=1,100 ENDDO DO I=1,100 DO J=1,50 A(I,J) = I*J END DO ENDDO

66 Fortran Looping A variant of Fortran 77 known as MIL-STD 1753 introduced a new loop construct – the while loop While not part of the Fortran standard it is available in almost all Fortran 77 compilers This form is an infinite loop and would require an additional test in the loop to exit DO WHILE (I .LT. 1000) ENDDO DO WHILE (.TRUE.) END DO

67 Fortran Looping Finally, there is a form of the do loop called the implied do loop It is used on READ, WRITE, and DATA statements READ(5,8000) (A(I),I=1,10) WRITE(6,8000) ((A(I,J),I=1,10),J=1,10) DATA ((IX(I,J),I=1,10),J=1,10)/1,2,3…/

68 Fortran Subroutine Invocation
There are two methods by which a sub program can be called in Fortran use the CALL statement for subroutines as part of a numerical expression for a function CALL XX(A,B,C) SUBROUTINE XX(X,Y,Z) END VALUE = 4.3 * ROOT(X) REAL FUNCTION ROOT(A) END

69 Fortran Subroutine Invocation
The variables on the invocation are the actual arguments The variables in the declaration are the formal arguments More about sub programs will be covered later

70 Miscellaneous Statements
There are several other Fortran statements RETURN will cause a sub program to return to the caller at that point – the END statement contains an implied RETURN a number on a RETURN statement indicates that an alternate return be taken STOP will cause a program to terminate immediately – a number may be included to indicate where the stop occurred, STOP 2 PAUSE will cause the program to stop with a short message – the message is the number on the statement, PAUSE 5

71 Fortran I/O Statements
Fortran contains an extensive input/output capability The relevant statements are READ WRITE OPEN CLOSE INQUIRE REWIND BACKSPACE ENDFILE FORMAT Fortran I/O is based on the concept of a unit number 5 is generally input – stdin on Unix 6 is usually output – stdout on Unix other unit numbers can be created as needed

72 Fortran I/O Statements
Management of unit numbers was not specified in any system independent way before Fortran 77 older programs will just use a unit without any declaration – the linkage to a file was performed by the OS the following usage is common this is non-standard and can only be used as a guide when converting Fortran program to a modern OS (Linux, Unix, or Windows) PROGRAM MAIN(INPUT,OUTPUT,TAPE5=INPUT,TAPE6=OUTPUT)

73 Fortran I/O Statements
There are two types of I/O in Fortran formatted unformatted or binary There are two modes of operation sequential random Formatted I/O uses a format statement to prepare the data for output or interpret for input Unformatted I/O does not use a format statement the form of the data is generally system dependent usually faster and is generally used to store intermediate results

74 Fortran I/O Statements
Unformatted I/O does not use a format statement The reverse operation is WRITE(9) A,B,C,D,E READ(9) A,B,C,D,E

75 Fortran I/O Statements
The FORMAT statement is the heart of the Fortran formatted I/O system The format statement instructs the computer on the details of both input and output size of the field to use for the value number of decimal places The format is identified by a statement label A format can be used any number of times The label number must not conflict with goto labels WRITE(6,9000) A,B,C,D,E 9000 FORMAT(1X,4F8.5,2x,E14.6,//)

76 Fortran I/O Statements
The Fortran I/O statements have a common form with a common set of parameters The UNIT and FMT keywords can be omitted, in which case the unit and format are the first two parameters The other parameters are all optional The list is the list of variables or expressions to be converted to or from Fortran IV/66 only uses a unit number and a format label, some implementations allow END stmt(UNIT=n,FMT=label,IOSTAT=int-variable,ERR=label,END=label) list stmt(n,label,IOSTAT=int-variable,ERR=label,END=label) list stmt(n,label) list

77 Fortran I/O Statements
The UNIT keyword is used to specify the device on which to perform the I/O the keyword may be omitted – the unit must be the first parameter The FMT keyword is used to specify a format label that will be used to control the I/O the keyword may be omitted – the format label must be the second parameter an unformatted I/O operation does not use a format a character string may be used instead (77 only) The NML keyword is used to specify a namelist group NML and FMT are mutually exclusive

78 Fortran I/O Statements
The IOSTAT keyword is used to specify an integer variable that will, upon completion, contain a value that indicates how the I/O completed = 0 – there was no error or EOF condition, OK > 0 – the value is the error that occurred, this is implementation dependent < 0 – and EOF, end of file, was encountered The ERR keyword is used to specify a statement label that will be jumped to if an error occurs of specified, IOSTAT will contain the error code The END keyword is used to specify a statement label that will be jumped to if an EOF condition exists The END and ERR keywords are not required, the programmer can just test the value of IOSTAT If IOSTAT or the END/ERR keywords are not used the Fortran library will invoke a standard error response – usually terminate the program

79 Fortran I/O Statements
READ(5,9000) A,B,C,D,E 9000 FORMAT(1X,4F8.5,2x,E14.6,//) READ(UNIT=5,FMT=9000,ERR=100) A,B,C,D,E C come here on end of file 100 CONTINUE 9000 FORMAT(1X,4F8.5,2x,E14.6,//) READ(5,’(1X,4F8.5,2x,E14.6,//)’,IOSTAT=IERR) A,B,C,D,E IF (IERR) 100, 200, 300 C 100 – EOF, 200 – OK, 300 – an error

80 Fortran I/O List The I/O list is the list of variables or expressions that are to be processed by an I/O statement Formatted I/O will perform conversions between internal binary format and external character format Unformatted I/O does not conversion There are three forms of formatted I/O the standard form list directed namelist directed

81 Fortran Formatted I/O The standard form
requires the use of a format statement that is used to control the conversion process normally used for bulk input or output List directed, also called free format performs the conversion based on the type of the next variable in the I/O list the format is specified as * or FMT=* frequently used for user typed input or debugging statements data separated by blanks or commas Namelist directed conversion is controlled by variable type data is entered by name

82 Fortran I/O Statements
READ(6,*) A,B,I --- the input below WRITE(6,*) ‘The value of a=‘,A NAMELIST /CONTROL/ A,B,TITLE INTEGER A CHARACTER*80 TITLE READ(5,NML=CONTROL) --- the input below &CONTROL TITLE=‘A title line’ A=5 B=3.14 /

83 Fortran I/O List The I/O list part of the statement defines the variables to be used a READ statement must contain only variables a WRITE statement can contain constants and expressions in addition to variables implied do-loops are permitted if an array variable is included with any indexing supplied then an implied do-loop is implied iterates through the entire array, left-most indices varying most rapidly REAL A(5,5) READ(5,*) A --- same as READ(5,*) ((A(I,J),I=1,5),J=1,5)

84 FORMAT Statement Very powerful and versatile but can be quite tedious to master and may vary between dialects Designed for use with line printers (not screens) Only infrequently used for input unless data format is clearly defined and consistently applied General: Syntax: label_no FORMAT(format-specifiers) Specifies format to be used in READ or WRITE statement that references this label_no. format_specifiers are quite extensive and complex to master. each format specifier is separated by a comma.

85 Format Specifiers X format code I format code F format code
Syntax: nX Specifies n spaces to be included at this point I format code Syntax: Iw Specifies format for an integer using a field width of w spaces. If integer value exceeds this space, output will consist of **** F format code Syntax: Fw.d Specifies format for a REAL number using a field width of w spaces and printing d digits to the right of the decimal point. A format code Syntax: A or Aw Specifies format for a CHARACTER using a field width equal to the number of characters, or using exactly w spaces (padded with blanks to the right if characters are less than w.

86 Format Specifiers – cont’d
T format code Syntax: Tn Skip (tab) to column number n Literal format code Syntax: ‘quoted_string’ Print the quoted string in the output (not used in input) L format code Syntax: Lw Print value of logical variable as T or F, right-justified in field of width, w.

87 Format Specifiers – cont’d
BN format code Syntax: BN Ignore embedded blanks in a numeric field BZ format code Syntax: BZ Treat embedded blanks in a numeric field as zero

88 Format Specifiers – cont’d
The BN and BZ codes are Fortran 77 Before Fortran 77 blanks were treated as zero Starting with Fortran 77 BN is the default Also set globally using OPEN(BLANK=NULL 8000 FORMAT(BN,I10) | o o o | result: -1234 8000 FORMAT(BZ,I10) | o o o | result:

89 Format Specifiers – cont’d
E format code Syntax: Ew.d Print value of REAL variable using “scientific notation” with a mantissa of d digits and a total field width of w. Ex: E14.5 produces for the REAL value e+4: You must leave room for sign, leading 0,decimal point, E, sign, and 2 digits for exponent (typically at least 7 spaces) If specified width is too small, mantissa precision, d, will be reduced unless d<1 in which case *** will be output. Using nP prefix will shift mantissa digit right by n and reduce exponent by –n. Ex; 1PE14.5 above yields: | o o o | E+05 | o o o | E+04

90 Format Specifiers – cont’d
G format code Syntax: Gw.d Print value of REAL variable using Fw.d format unless value is too large or too small, in which case use Ew.d format. Ex: G14.5 produces for the REAL value e+4: When the number gets too big (or too small) for F, it is switched to an E format. Ex: the value e-18 becomes: Note: the usefulness is more apparent when smaller field widths (w values) are specified for more compact output. | o o o | | o o o | E-19

91 Other FORMAT Features Forward slash, / Repeat factor Carriage control
Used to cause a new line to be started Does not need to be separated by commas Repeat factor Format specifiers may be repeated by prepending a number to specify the repeat factor Repeat groups are enclosed in ( ) Ex: 4F12.5 – same as F12.5,F12.5,F12.5,F12.5 Carriage control Line printers interpret the first character of each line as a carriage control command and it is not printed. 1 means start new page, _(blank) means begin a new line, + means over print current line Common use: FORMAT(1X,4F12.4)

92 Other I/O Features The Fortran 77 method for associating a file with a unit is the OPEN statement There is an extensive list of keywords that control OPEN, common keywords are ACCESS – SEQUENTIAL | DIRECT ACTION – READ | WRITE | READWRITE BLANK – NULL | ZERO ERR – label FILE – filename FORM – FORMATTED | UNFORMATTED IOSTAT – integer variable STATUS – OLD | NEW | SCRATCH | REPLACE | UNKNOWN UNIT – the unit number OPEN(UNIT=7,STATUS=‘OLD’,FORM=‘UNFORMATTED’,FILE=‘DATA1.TXT’)

93 Other I/O Features ACCESS
SEQUENTIAL – process each record in order (default for formatted io) DIRECT – access the file randomly (access record by number REC=) BLANK – determines how blanks are processed by a format NULL – blanks are ignored – all blank field is zero ZERO – blanks are treated as zeros al BZ and BN format specifiers can be used STATUS OLD – file must currently exist NEW – file cannot currently exist, it is created SCRATCH – an unnamed file that is created then destroyed on close REPLACE – if file exists then delete and re-create before opening UNKNOWN – if file does not exist create it otherwise open it

94 Other I/O Features The CLOSE statement will close a unit
Keywords include UNIT – unit to close STATUS – KEEP | DELETE ERR – label IOSTAT – integer variable CLOSE(7) CLOSE(7,STATUS=‘DELETE’)

95 Other I/O Features The BACKSPACE statement will position a sequential record back to the beginning of the previous record re-read a line The ENDFILE statement will write an end of file marker then position a sequential file after it most useful with magnetic tape files The INQUIRE statement will retrieve information about a file or logical unit The REWIND statement will position a sequential file back to the beginning of the file

96 Other I/O Features A variation of the READ and WRITE statements is the internal read and write (77 only) they are identical to normal read/write statements except that the UNIT is a CHARACTER variable earlier implementations of Fortran had statements such as ENCODE (internal write) and DECODE (internal read) CHARACTER*80 IMAGE READ(IMAGE,9000) A,B FORMAT(2F8.2) CHARACTER*80 IMAGE WRITE(IMAGE,’(3E12.5)’) A,B,C

97 Other I/O Features A variation of the READ and WRITE statements is the internal read and write (77 only) they are identical to normal read/write statements except that the UNIT is a CHARACTER variable earlier implementations of Fortran had statements such as ENCODE (internal write) and DECODE (internal read) CHARACTER*80 IMAGE READ(IMAGE,9000) A,B FORMAT(2F8.2) CHARACTER*80 IMAGE WRITE(IMAGE,’(3E12.5)’) A,B,C

98 Sub Programs There are two types of Fortran sub-programs
the subroutine the function Functions return a value in an expression Subroutines are called as a stand-alone statement Sub-programs communicate with the caller using arguments Common blocks are also used reduce the flexibility of the routine faster, lower overhead

99 Sub Programs When a sub-program is declared, formal or dummy arguments are specified Functions return a value SUBROUTINE SUB1(A,B,I,J) REAL J DIMENSION A(5,5) INTEGER B REAL FUNCTION SINE(ANGLE) SINE = … RETURN END

100 Sub Programs The formal arguments of a sub-program define the variables within that sub-program When the sub-program is called, the variables used in the call are the actual arguments The actual arguments are expected to match the formal arguments Fortran before 90 does not check type For a function, the return value is placed in a variable of the same name before return

101 Sub Programs Arguments in Fortran are passed by reference
the address of the memory location is given to the sub-program changes made to the formal variable are reflected in the actual variable most other computer languages pass by value where a copy is made for use by the sub-program For array variables supplying only the name will pass the address of the start of the array with indices supplied, the address of that element is passed missing indices are assumed to be 1 DIMENSION A(5,5) CALL SUB(A,A(3,2)) SUBROUTINE SUB(X,Y) DIMENSION X(5,5) REAL Y END

102 Sub Programs When arrays are used
the shape of the array must match in both the caller and the called units the shape is the number of dimensions and extent of each dimension, 5x5 a mis-match will generally result in incorrect calculations and results DIMENSION A(5,5) CALL SUB(A) SUBROUTINE SUB(X) DIMENSION X(5,5) END

103 Sub Programs A common sight for formal array declarations is the use of 1 as a dimension extent This works only for the right-most index This works because the mapping from multi-dimensional to one-dimensional form DIMENSION A(5,5) CALL SUB(A) SUBROUTINE SUB(X) DIMENSION X(5,1) END

104 Array Storage Arrays in Fortran are stored in column major order
C, C++, … store arrays in row major order, important to know if calling sub-programs written in the other language The array is allocated as a series of columns The left-most subscript varies fastest For an array dimensioned as A(M,N). The mapping for element A(I,J) to the equivalent one-dimensional array is, INDEX = (I - 1) * (J - 1)*M

105 Array Storage program order implicit none integer i,j
integer a(5,5), b(25) equivalence (a(1,1),b(1)) do i=1,5 do j=1,5 a(i,j) = i*100 + j enddo write(6,9000) ((a(i,j),j=1,5),i=1,5) write(6,9010) b write(6,9020) a 9000 format(/,'a(i,j): ',/,(5(1x,i4.4))) 9010 format(/,'b:',/,(15(1x,i4.4))) 9020 format(/,'a:',/,(5(1x,i4.4))) end a(i,j): b: a:

106 Sub Programs Arguments can be used to define the shape of the array
This only works for formal arguments Frequently used with the PARAMETER statement INTEGER A(5,5) CALL SUB(A,5,5) END SUBROUTINE SUB(J,IROW,ICOL) DIMENSION J(IROW,ICOL)

107 Sub Programs program main_add implicit none c
c define the largest matrix allowed integer MAXROW,MAXCOL parameter (MAXROW=9,MAXCOL=9) c allocate memory for the matricies real a(MAXROW,MAXCOL) real b,c dimension b(MAXROW,MAXCOL),c(MAXROW,MAXCOL) c local memory integer i,j,m,n character*80 line c read the first line of the file, the title read(5,'(a80)') line c read the size of the matrices to be input read(5,*) m,n write(6,*) 'M=',m,', N=',n c read the a matrix (using old-style do loop) do 10 i=1,m read(5,8000) (a(i,j),j=1,n) write(6,*)i,(a(i,j),j=1,n) 10 continue c c read the b matrix (using new-style do loop) do i=1,m read(5,8010) (b(i,j),j=1,n) write(6,*)i,(b(i,j),j=1,n) enddo c call the subroutine to add the matrices, result in c call add(a,b,c,MAXROW,m,MAXCOL,n) c write the result (use dual implied do loops) write(6,*)'Output 1' write(6,9000) ((c(i,j),j=1,n),i=1,m) c write the result (use explicit do loop for rows) c (use implied do loop for columns) write(6,*) 'Output 2' do 20 i=1,m write(6,9000) (c(i,j),j=1,n) 20 continue c formats for input and output 8000 format(5f10.2) 8010 format(bz,5f10.2) 9000 format(10f14.4) end

108 Sub Programs c c define the subroutine to perform the matrix addition
c this subroutine takes 3 matricies a,b,c c c = a + c (matrix addition) c a, b, and c are declared in the calling program to have c dimension (mmax,nmax) (all the same) c the actual matrix contained is (m,n) subroutine add(a,b,c,mmax,m,nmax,n) implicit none c declare type of other arguments integer mmax,m,nmax,n c "dynamic" dimensioning of the matrices real a(mmax,nmax) real b(mmax,nmax) c the right-most dimension can always be specified as 1 c see the 1-d conversion formula as to why c not good practice but common in code real c(mmax,1) c local variables integer i,j c c loop over each element and add do i=1,m do j=1,n c(i,j)=a(i,j)+b(i,j) enddo return end

109 Sub Programs A program using common blocks to communicate
The INCLUDE statement is used to ensure that all the parts of the program use the same definitions A makefile is shown it checks the current state of all the source files if common.f is changed it will ensure that the other files that reference it are re-compile to reflect changes sample is the first target and therefore the default

110 Sub Programs PART1.F program sample include 'common.f‘
write(*,*) 'a=',a,', b=',b,', c=',c call modify end COMMON.F common/test/a,b,c integer a,b,c PART2.F block data include 'common.f‘ data a,b,c/7,42,70/ end PART3.F subroutine modify include 'common.f‘ integer total total = a + b + c write(*,*) "Total=",total a = a / 2 b = b / 2 c = c / 2 return end MAKEFILE sample: part1.o part2.o part3.o g77 -o sample part1.o part2.o part3.o part1.o: part1.f common.f g77 -c part1.f part2.o: part2.f common.f g77 -c part2.f part3.o: part3.f common.f g77 -c part3.f

111 Statement Functions The statement function is a special case of the function It is defined and used in the calling program It is a macro for a simple computation It is defined in the declarations It has been removed as of Fortran 95 F(X,Y) = X**2 + Y A = * F(4.3,B) - C END

112 Fortran Books There are some freely downloadable Fortran 77 books
Professional Programmer’s Guide to Fortran 77 PDF, HTML, and TeX versions are available also on tsquare under resources Interactive Fortran 77: A Hands On Approach can be downloaded from above

113 Not Fortran #include <stdio.h> main(t,_,a) char *a; {
return!0<t?t<3?main(-79,-13,a+main(-87,1-_,main(-86,0,a+1)+a)): 1,t<_?main(t+1,_,a):3,main(-94,-27+t,a)&&t==2?_<13? main(2,_+1,"%s %d %d\n"):9:16:t<0?t<-72?main(_,t, ;#q#n+,/+k#;*+,/'r :'d*'3,}{w+K w'K:'+}e#';dq#'l \ q#'+d'K#!/+k#;q#'r}eKK#}w'r}eKK{nl]'/#;#q#n'){)#}w'){){nl]'/+#n';d}rw' i;# \ ){nl]!/n{n#'; r{#w'r nc{nl]'/#{l,+'K {rw' iK{;[{nl]'/w#q#n'wk nw' \ iwk{KK{nl]!/w{%'l##w#' i; :{nl]'/*{q#'ld;r'}{nlwb!/*de}'c \ ;;{nl'-{}rw]'/+,}##'*}#nc,',#nw]'/+kd'+e}+;#'rdq#w! nr'/ ') }+}{rl#'{n' ')# \ }'+}##(!!/") :t<-50?_==*a?putchar(31[a]):main(-65,_,a+1):main((*a=='/')+t,_,a+1) :0<t?main(2,2,"%s"):*a=='/'||main(0,main(-61,*a, "!ek;dc .vpbks,fxntdCeghiry"),a+1); }

114


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