So far we have only showed
free format input/output. This uses a set of default rules for how to output
values of different types (integers, reals, characters, etc.).
A D V E R T I S E M E N T
programmer wants to specify some particular input or output format, e.g. how
many decimals in real numbers. For this purpose Fortran 77 has the format
statement. The same format statements are used for both input and output.
A simple example demonstrates
how this works. Say you have an integer variable you want to print in a field 4
characters wide and a real number you want to print in fixed point notation with
3 decimal places.
write(*, 900) i, x
900 format (I4,F8.3)
The format label 900 is chosen
somewhat arbitrarily, but it is common practice to number format statements with
higher numbers than the control flow labels. After the keyword
format follows the
format codes enclosed in parenthesis. The code I4 stands for an integer with
width four, while F8.3 means that the number should be printed using fixed point
notation with field width 8 and 3 decimal places.
The format statement may be
located anywhere within the program unit. There are two programming styles:
Either the format statement follows directly after the read/write statement, or
all the format statements are grouped together at the end of the (sub-)program.
Common format codes
The most common format code letters are:
A - text string
D - double precision numbers, exponent notation
E - real numbers, exponent notation
F - real numbers, fixed point format
I - integer
X - horizontal skip (space)
/ - vertical skip (newline)
The format code F (and similarly
D, E) has the general form Fw.d where w is an integer constant
denoting the field width and d is an integer constant denoting the number
of significant digits.
For integers only the field
width is specified, so the syntax is
character strings can be specified as
Aw but the
field width is often dropped.
If a number or string does not
fill up the entire field width, spaces will be added. Usually the text will be
adjusted to the right, but the exact rules vary among the different format
For horizontal spacing, the nX code is often used. This means n horizontal
spaces. If n is omitted, n=1 is assumed. For vertical spacing (newlines),
use the code /. Each
slash corresponds to one newline. Note that each read or write statement by
default ends with a newline (here Fortran differs from C).
This piece of Fortran code
x = 0.025
write(*,100) 'x=', x
100 format (A,F)
write(*,110) 'x=', x
110 format (A,F5.3)
write(*,120) 'x=', x
120 format (A,E)
write(*,130) 'x=', x
130 format (A,E8.1)
produces the following output when we run it:
Note how blanks are
automatically padded on the left and that the default field width for real
numbers is usually 14. We see that Fortran 77 follows the rounding rule that
digits 0-4 are rounded downwards while 5-9 is rounded upwards.
In this example each write
statement used a different format statement. But it is perfectly fine to use the
same format statement from many different write statements. In fact, this is one
of the main advantages of using format statements. This feature is handy when
you print tables for instance, and want each row to have the same format.
Format strings in read/write statements
Instead of specifying the format code in a separate format
statement, one can give the format code in the read/write statement directly.
For example, the statement
write (*,'(A, F8.3)') 'The answer is x = ', x
is equivalent to
write (*,990) 'The answer is x = ', x
990 format (A, F8.3)
Sometimes text strings are given in the format statements,
e.g. the following version is also equivalent:
write (*,999) x
999 format ('The answer is x = ', F8.3)
Implicit loops and repeat counts
Now let us do a more complicated example. Say you have a
two-dimensional array of integers and want to print the upper left 5 by 10
submatrix with 10 values each on 5 rows. Here is how:
do 10 i = 1, 5
write(*,1000) (a(i,j), j=1,10)
1000 format (I6)
We have an explicit do loop over the rows and an
implicit loop over the column index j.
Often a format statement involves repetition, for example
50 format (2X, I3, 2X, I3, 2X, I3, 2X,
There is a shorthand notation for this:
950 format (4(2X, I3))
It is also possible to allow
repetition without explicitly stating how many times the format should be
repeated. Suppose you have a vector where you want to print the first 50
elements, with ten elements on each line. Here is one way:
write(*,1010) (x(i), i=1,50)
1010 format (10I6)
The format statements says ten
numbers should be printed. But in the write statement we try to print 50
numbers. So after the ten first numbers have been printed, the same format
statement is automatically used for the next ten numbers and so on.