Download Input and Output

Input and Output
READ
WRITE
OPEN
FORMAT statement
Format statements allow you to control how data are
read or written. Some simple examples:
Int=2; real=4.5678
PRINT ‘(I6,2x,F5.1)’,int,real.
This will print the following on the screen:
2
4.6
(i) Integer in a field of 6 (here 5 spaces + 2)
(ii) Two blanks (2x)
(iii) Real in a field of 5 with 1 digit after the decimal pt
FORMAT statement again
You can define a format either directly by means of:
‘(format-instructions)’
or
“(format-instructions)”
Or by means of a FORMAT statement (useful if you want to
use the same format in several PRINT or WRITE
statements.
PRINT 30, int1, int2, int3, real1, real2, real3
30 FORMAT(1x,3i8,3f10.3)
The 30 is a STATEMENT LABEL
Types of format descriptors
Iw
Integer of width w
Fw.d
Real of width w with d characters after the .
Ow
Octal of width w (write out no. as octal)
Zw
Hexadecimal of width w
Ew.d
Exponential format e.g. 5.319e-09. W is the
total no. chars, d the no. after the decimal point
A or Aw
Character (use A for input and Aw for output)
There are others you may need later on, e.g Lw for logical
Example 1
Print ‘(1x, “x=”,f6.2, “ Y=”, e12.3)’,x,y
Will print out x as a normal real and y in exponential format with
3 digits after the decimal point
The format descriptor nx inserts n blanks
The format descriptor tn tabs to the nth character, e.g.:
Print 75, “John Q. Doe”, ‘CPSC’, Number
75 FORMAT(1x, A11, T16, A4, 2x, I3)
Will produce the output
> John Q. Doe CPSC 141
Example 2
A number BEFORE the format descriptor tells the program to
repeat it. Thus
5I6
means 5 integers of 6 fields
8e10.3
means 8 exponential numbers with 3 digits after
the decimal point
5(I8,2x,f10.3) repeats the pattern within the brackets
Use of a slash within a Format statement forces input or output
from a fresh line, e.g.
Print ‘(2i3/a20)’,int1,int2,characters puts the characters on a
fresh line
Formated READ
This is normally used when reading from a file, but will work
from the screen also:
READ ‘(format-descriptor)’,var1,char1,var2,char2
Note that FORTRAN ignores blanks when reading in variables
under format control – so that, for instance, with a format
statement i6 the following all get read as 2:
2_____ , _2____ , __2___ , ___2__ ,
____2_ , _____2
Reading character input
This is slightly tricky. Some compilers will allow:
Character(len=7)::name
Read*,name
But in general they don’t. You need to use an A format:
Read ‘(a)’,name
Note that it is not necessary to define the length of the character
field – the computer will read in however many characters it
needs to fill the declared variable.
File processing: Open
Before using a file you need an OPEN statement. This has
many options, but you don’t need to know them all now.
OPEN(unit,file=file_name,status=‘XXX’, action=‘YYY’)
Unit is a NUMBER which identifies this file with a channel
that can subsequently be accessed by PRINT, WRITE,
READ
File_name is the name of the file
Status takes values: “OLD”,”NEW”,”REPLACE” or
“UNKNOWN”.
Action takes values ‘READ’, ‘WRITE’, OR ‘READWRITE’
Examples of Open
Open(8,file=‘datasub/file1’)
Opens file1 on channel 8 with unknown status, and
readwrite access
Character(len=20)::filename
Filename=‘datasub/file1’
Open(13,file=filename,status=‘old’,action=‘read’)
Opens file1 on channel 13 with status old, read only
Open(22,file=‘newfile’,status=‘new’,action=‘write’)
Creates a blank file called newfile in the current
directory, with write access only
Avoid using channels 5 or 6 as these are by default the keyboard
and screen respectively
Reading from a file
Very similar to reading from the screen, except you define the
channel:
Real,dimension(100)::indata
Open(8,file=‘mydatafile’,status=‘old’)
Read(8,*) i1,i2,i3,i4
Read(8,’(4i6)’)i1,i2,i3,i4
Read in free-format
Read under format control
Read(8,30)i1,i2,i3,i4
Read(8,’(8f10.3)’)indata
Reads file directly into array
Read(8,’(8f10.3)’)(indata(I),I=1,100)
Writing to a file
The Write command is used to write data to a file, rather then
the PRINT command. WRITE is very similar to READ:
Write(8,*) x1,x2,x3,x4
writes in free format
Write(8,’(2f10.3,5x,2e12.5)’)x1,x2,x3,x4 writes under format
control
Write(8,’(4f10.3)’)x
writes out an array
Write(8,’(4f10.3)’)(x(j),j=1,4)
Other file commands
Close(8)
closes a file (releasing the unit no. for a
subsequent open command
Rewind 8
goes back to the beginning of a file
Backspace 8 goes back one record. Useful if you’re not sure
what kind of data to expect – read the line in as
characters, decide what it is then backspace and
read again as numbers
Error codes
What if you read a file and reach the end of the data? Any
further read statement causes the program to crash. It is
advisable therefore to allow for this with IOSTAT:
Integer :: test
Do
Read(8,900,iostat=test)x1,x2,x3,x4
If(test.eq.0)then
……..
!No problem. Carry on executing program
Else if (test.lt.0) then
Exit
!End of data. Leave do loop
Else
Print*,’error on reading file’;stop !stop executing program
End do
Unformatted read/write
Binary (or unformatted) files are much more compact than text
(or ascii) files and for very large data files these are preferred.
Reading in a binary file is quite easy:
READ(8,iostat=test)array
- there is no format statement
However, you can only read data easily in this way if you used
the corresponding write statement to create the file on the same
computer (or at least type of computer) – since this maps the
file directly to the memory of the computer.
Exercise 1
Write a program to write all the numbers from 1 to 100 into a
file. Choose a sensible format
Write a program to read this file, multiply each number by pi
and write the results out to a new file
Write a program to read your name and address from a file and
write it to the screen in a different format.
Exercise 2
Write a program to read the data files you created in exercise 1
and convert the numbers from decimal degrees to degrees,
minutes and seconds. Put the results into a new file with
suitable explanatory text.