 High Performance Scientific Computing   AMath 483/583 Class Notes   Spring Quarter, 2011

Fortran modules

Optimizing code

# Fortran Input / Output¶

## Formats vs. unformatted¶

print or write statements for output and read statements for input can specify a format or can be unformatted.

For example,

`print *, 'x = ', x`

is an unformatted print statement that prints a character string followed by the value of a variable x. The format used to print x (e.g. the number of digits shown, the number of spaces in front) will be chosen by the compiler based on what type of variable x is.

The statements:

```i = 4
x = 2.d0 / 3.d0
print *, 'i = ', i, ' and x = ', x```

yield:

```i =            4  and x =   0.666666666666667
```

The * in the print statement tells the compiler to choose the format.

To have more control over the format, a formatted print statement can be used. A format can be placed directly in the statement in place of the * , or can be written separately with a label, and the label number used in the print statement.

For example, if we wish to display the integer i in a field of 3 spaces and print x in scientific notation with 12 digits of the mantissa displayed, in a field that is 18 digits wide, we could do

```    print 600, i, x
600 format('i = ',i3,' and x = ', e17.10)```

This yields:

```i =   4 and x =  0.6666666667E+00
```

The 4 is right-justified in a field of 3 characters after the ‘i = ‘ string.

Note that if the number doesn’t fit in the field, asterisks will be printed instead!

```i = 4000
print 600, i, x
```

gives:

`i = *** and x =  0.6666666667E+00`

Instead of using a label and writing the format on a separate line, it can be put directly in the print statement, though this is often hard to read. The above print statement can be written as:

```print "('i = ',i3,' and x = ', e17.10)", i, x
```

## Writing to a file¶

Instead of printing directly to the terminal, we often want to write results out to a file. This can be done using the open statement to open a file and attach it to a particular unit number, and then use the write statement to write to this unit:

```open(unit=20, file='output.txt')
write(20,*) i, x
close(20)```

This would do an unformatted write to the file ‘output.txt’ instead of writing to the terminal. The * in the write statement can be replaced by a format, or a format label, as in the print statement.

There are many other optional arguments to the open command.

Unit numbers should generally be larger than 6. By default, unit 6 refers to the terminal for output, so

`write(6,*) i, x`

is the same as

`print *, i, x`

## Reading input¶

Unformatted read:

```print *, "Please input n... "
read *, n```

Reading from a file:

```open(unit=21, file="infile.txt")
read(21,*) n
close(21)```