Generalized I/O

Mystery of stars

• I/O statements took the form

read *, x, y,z
print *, a,b,c

• Such I/O is said to be List-directed or in free format

• user has no control over the appearance of I/O

• No flexibility over the I/O devices

• Keyboard input

• Display unit output

• * indicates no control or default format

Mystery resolved

• * stands for (*,*)

• In general, it will be (arg1,arg2)

• arg1 used to select the I/O device

– * indicates default devices (keyboard and monitor)

• arg2 controls the I/O formatting

– It is a string constant

– * means default formatting

Files

• I/O through files is the most convenient

• Standard I/O is very painful for large data

• Large amount of data can be stored and retrieved with ease

• Output of one program can be used as input to another

• The programs need not be running at the same time

• Once created they remain - permanent memory

• Files are high level software abstractions of these permanent memory devices

• They are called as secondary memory

Secondary Memory

• permanent, slow and large volume of data

• Some kind of magnetic memory –

• no power required to retain unlike main memory

• different kinds - Hard Disks, Floppy Disc, CDs, Magnetic Tapes (old)

• Random or Sequential access

• details of devices abstracted from application programmers

• Files are the abstractions

• Std. I/O are also files

File Organization

• Fortran programmer's view

• A simple file is a linear sequence of records

• Each record is a meaningful piece of data

• Files are read or written one record at a time

• Each record is typically a line of data

– creation of file determines type of data

• Files can be sequential or random access

• Random access files require keys

File Operations

• Opening a file

• writing or reading a record

• Rewinding a file

• Closing a file

• Valid sequence

– (open (read/write/rewind)* close)*

• File operations translated to OS calls

• They can fail and graceful recovery possible

File Opening

• Simplest example is:

open(unit = 23, file = "input.dat")

• This commands associates the file input.dat with the number 23

• This number can be used for reading and writing onto the file

• Example: read(23, *) N

write(23, *) M

• Unit numbers are system dependent (usually from 1 to 100)

• unit and file are named parameters

• Contrast this with positional parameters

Open attributes

• Many file attributes assume default values in the above

• to override these, additional parameters to be set

• action = READ, WRITE or READWRITE (default)

• status = OLD, NEW, REPLACE (default), SCRATCH, UNKNOWN

• ios = x, x is assigned a value at the end of the call

0 - if the operation is successful

> 0 - otherwise

• access = "sequential" (default) or "direct"

Some more attributes

• form = "formatted" (default) or

"unformatted“

– in unformatted form, data stored in some internal representation

– can not be edited

• recl = length of each record

– required for direct access files

• position = "rewind", "append"

Example

integer:: ierr

open(unit=23, file = "in.dat", status = "old",

action = "Read“, iostat = ierr)

– opening the existing file "in.dat", for read

– ierr gets a value > 0 iff failure in opening

Another Example

integer:: ierr

integer:: ut = 34

open(unit= ut, file = "out.dat", status = "new",

action = "write“, iostat = ierr)

– creating a new file "out.dat

– Status = replace would replace the file

One more Example

open(unit= 15, status = "scratch", iostat = ierr)

– no file name specified - error otherwise

– default action - reading and writing temporary data

– erased after the program terminates

Closing a file

• Close(close_list)

• Close_list

– must specify the unit number

– may specify other clauses

• the number is released and can be reused in other open statements

• repeated opening and closing of non scratch files possible

• scratch file contents lost once they are closed

Reading and Writing

• files contain sequence of records

• Each record is read/written using a single read or write operation

• So we can say that each record occupies a single line

• Every read statement refers to the successive records

• Reference to the file through the unit number

• example:

read(unit = 16, *) x, y, z

write(unit = 25, *) "test output=", a, b, c

• read and write one record to the appropriate file

Read Pointer

• There can be errors while reading a file

• There is a read pointer that indicates the next record to be read

• Every read moves the read pointer to the next record (in sequential access)

Read Error

• The read pointer may be at the end

• use istat clause for detecting end of file (EOF)

Read(16,*, iostat = err) x, y, z

• If err is non zero then there is an error

• Use always iostat clause

• otherwise, program will abort

Other parameters

• For direct access files, use the argument
rec = n

• This reads/writes the nth record

Example:

write(23,*,iostat = err, rec = 15) i,j

– write values of i, j to the record 15 of the associated file

Program bubble_sort

implicit none

integer:: N, i, rt_end, index, temp
! N - the total number of items in the array
! i, j - array index variables

integer, dimension(100):: A
integer:: err,errf, werr

open(unit=1, file = "in.dat", &
status = "old", iostat = err)
if (err/=0) then
print *, "file opening error"
else

outer: do

Program Continued

read (unit =1, iostat = errf, fmt = * ) N
read (1, *, iostat = errf) (A(i),i=1,N)
if (errf /= 0) exit
rt_end = N
inner: do
if (rt_end <= 1) exit
index = 0
do i = 1, (rt_end-1)
if (A(i) > A(i+1)) then
temp = A(i)
A(i) = A(i+1)
A(i+1) = temp
index = i
endif
enddo
rt_end = index

end do inner

Program Continued

open(unit=2, file = "out.dat", &
status = "old", access = "sequential", &

iostat = werr)

print *, (A(i),i=1,N)

write (2, fmt = *) (A(i),i=1,N)

end do outer

endif

close(unit=1)

end program

I/O Formatting

• So far we have used List-Directed I/O Statements (Free format)

print *, A,B,C

read *, A,B

• Fine for small programs and small number of inputs and outputs

• Many data processing applications manipulate large collection of inputs and outputs

– Tabulating marks and results

– preparing mark sheets and reservation charts

• Pretty printing and Formatted input very essential

• Fortran has elaborate features for formatting I/O

Format Strings

• I/O formats can be specified by using format strings

print fs, A, B,C

read fs, C,D

• fs identifies a format string

– a constant string

– a string variable

– a label that labels a format statement containing the format string

• Here is an example format strings

print "I4, I4, I4", A, B, C

• This prints A, B and C in a row with each being given 4 decimal places

Edit Descriptor

• I4 is an edit descriptor

• Format string contains one or more edit descriptors

• Very many edit descriptors that help print different types of variables

• Alternate format strings

Character(len=20):: str

str = "(I4, I4, I4, I4)”

write(*, str) a,b,c,d

read *, 100, a, b, c, d

100 format ("I4,I4,I4,I4")

print "4I4", a, b,c,d

I/O buffer

• I/O done through buffers

• Buffer is a temporary storage space where every line of characters is composed before doing I/O

• Line-oriented I/O

• For reading – input process builds the buffer and gives the buffer content to the program

• Every print statement composes the output in the output buffer completely before sending for printing on the device

• Buffers are read/written as per the format statement

Output Buffer

• Usually 133 characters long (line-printer output)

• First character is a control character

– 1 skip to next page

– Blank single spacing (default when * is used)

– 0 double spacing

– + no space – overwriting

• The rest of the characters are printed

• Make sure that first character is the intended control character

print (*,100) n

100 format (I3)

Behaviour of print statement when n = 25 and n =125

Some descriptors

• Iw - w columns reserved for printing an integer value includes - if the number is negative

• 5Iw - 5 successive w columns reserved for printing/reading 5 values

• Iw.m - minimum m digits need to be presented

• If w not enough then * printed

• If the number is less than m digits - leading zeros added

Format Descriptors

• rFw.d

r - repeat count

w - number of columns for the entire number

(including decimal point and -ve sign)

d - number of columns to the right of the decimal point

• Number appears right-justified

• Trailing zeros if the no. of digits after decimal point is less than d

• rounded off if d is more

• if (w-d-1) is less, then asterisks printed

E-descriptor

rEw.d

Printing is done in E notation

Print "E11.4", A

will print -0.1234E-02 when the value of A is

1.234 * 10 ** -1

Make sure that

w > = d + 7

A-descriptor

rA or rAw

if w is more then A is right justifies
if w is less than only first w characters of A are printed


X descriptor

nX inserts n spaces in the output

T descriptor

Tc moves to the column number c

rLw

used for printing logical variables - w columns are used
output will be either T or F, right justified

Repetition

Repetition can be applied to groups of descriptors

2(I6, 2X, 2F10.2)

is used to print/read two groups, each consisting of an integer (6 digits)
and two floating point numbers of size 10

/ operator used for breaking the line

A single write statement can print on more than one line

Descriptor Association

• print fs, var_list

• association of format descriptor to a variable is done from left to right

• if for every descriptor (except print controls) there is a variable and vice versa, then association is simple

• if less variables then rest of the descriptors discarded

• if more variables then the printing starts from a fresh line for rest of the variables

• format strings for these variables start from right most open parenthesis that does not have repetition count

Example

print (8, 60) j,k,l,m,n

60 format (1X,"The value” = , /, (1X, ‘New Line', 2(3X, I5)))

The output is

1 16 24 (column number not printed)

The value =

New Line 1 2

New Line 3 4

New Line 5

where j - n has the value 1 - 5

Other aspects

• Formatted reading is similar (see the books/manuals for details)

• Format strings can be specified in generalized file-based I/O

• Include the format string in the argument fmt