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Interactive input and output

Interactive input and output

Topics

• I/O Overview.

• FORMAT Statement.

• Edit Descriptors.

• Input/Output Lists.

• Namelist.

• Non-Advancing I/O.

I/O Overview

Background

• Data may be input or output:
  • default form, or

  • user defined form - formatting.

FORMAT Statement

• Used to read or write data in a form other than the default format.

• I/O is controlled using edit descriptors.

• A labelled statement - WRITE and READ statements reference the format statement label number

• General form:

where list is a list of edit descriptors drawn from:

I,F,E,ES,EN,D,G,L,A,H,T,TL,TR,X,P,

BN,BZ,SP,SS,S,/,:,'

Examples:

WRITE(*,100) i,j

READ(*,FMT=200) x,y

WRITE(*,200) x,y

100 FORMAT(2I)

200 FORMAT(2F10.6)

• Format may be placed directly in I/O statements:

WRITE(*,'(2F12.6)') x,y

Edit Descriptors

• Specify exactly how values should be converted into a character string.

• Only the following are covered.

• The following terms are used in the edit descriptor definitions:
  • a - repeat count

  • w - width of field - total number of characters

  • m - number of digits

  • d - digits to right of decimal point

  • e - number of digits in exponent

I

• I - integer data

• General forms: I, Iw, Iw.m, aI, aIw, aIw.m

• If w is too small or if w => m then w asterisks is printed

• Examples:

• I6.4 specifies a total of 6 characters including a sign and leading spaces, if necessary, but at least 4 digits thus:

would ouput two spaces followed by 0056

F

• Real - fixed point form.

• F, Fw, Fw.d, aF, aFw, aFw.d

• If no decimal point is supplied d digits are read as the fractional part.

• Examples: F10.5 F12.6 5F14.7

  where F12.6 specifies a total of 12 characters with 6 digits following the decimal point, thus:

would output:

^^^12.600000 -131.456780

E, ES, EN

• Floating point notation for real data.

• E, Ew, Ew.d, aE, aEw, aEw.d

• w includes signs and decimal point.

• Specifies a number in the form S0.XXXESXX where S signifies a sign, X digits and E identifies the exponent.

• The most significant digit is placed to the right of the decimal point e.g. 0.123E-2

• EN - exponent is divisible by 3.

• ES - most significant digit is placed to right of the decimal point.

A, X, L

• Input or output of single characters or strings
  • A, Aw, aAw

  • If w is greater than required then characters are right justified and leading spaces inserted.

  • On input the string does not need to be in quotes

• Skip specified number of characters
  • X, aX

• Logical data
  • L, aL

  • On input T, F, .TRUE., FALSE. are acceptable

  • On output T or F will be written

Special characters

• / take a new line

• : terminate I/O if list is exhausted

• () group descriptors - usually for repetition, for example 4(2X,F12.6)

• ` output the specified character string, for example (`Area of circle =', F12.6)

Input/Output Lists

Implied DO Loop

• Useful for I/O with arrays.

• General form:

• Similar to the indexed DO loop.

• Examples:

REAL, DIMENSION(10) :: a

REAL, DIMENSION(10,10) :: b

READ(*,*) (a(j),j=1,10)

WRITE(*,*) (a(j), j=10,1,-1)

WRITE(*,*) ((b(i,j),i=1,10), j=1,10)

The last WRITE satatement prints b(1,1), b(2,1), b(3,1), ...etc.

Namelist

• Facility for grouping variables for I/O.

• Complex rules govern use - input, pointers and allocatable arrays are not covered.

• Most useful for outputting debugging information.

• Use NAMELIST statement to group variables.

• General form:

Examples

• For example:

• The list may extended within the same scoping unit by repeating the group_name on another statement, as follows:

• Example:

tues = 3

sat = 1

WRITE(*,NML=week)

will produce:

&WEEK MON=2, TUES=3, SAT=1.../

• The output record has the general form.

• This form is required for input.

• Arrays may also be specified:

NAMELIST /group/items

items(1) = 1

...

WRITE(*,NML=group)

would produce:

&group items(1)=1, items(2:10)=0 /

Non-Advancing I/O

• Inhibit normal advance to next record.

• Read long records in sections.

• Response appears on the same line as prompt.

• Use ADVANCE keyword:

WRITE(*,*, ADVANCE='NO') ...

• Optional Keywords
  • EOR=label

  • SIZE=integer-variable

• Padding with blanks (OPEN statement)

Examples

• First:

READ(*,*) I

This would appear as

value? 10

• Second:

INTEGER :: length

bb: DO

WRITE(*,*, ADVANCE='NO') 'File? '

READ(*,*,ADVANCE='NO',EOR=20,&

SIZE=length) filename

OPEN(10,FILE=filename(1:LENGTH))

EXIT bb

20 WRITE(*,*)'error...'

END DO