The Prolog Tutorial:

Input / Output

By this stage of the tutorial you should be able to create simple procedures and know a number of the built-in predicates in Prolog.

The aims of this tutorial

• to introduce the basic predicates for input / output.

So far we have come across two input and output predicates, write/1 and read/1. To recap write will print a Prolog term onto the screen and read will get input from the user so long as the user enters a legitimate Prolog term.

These predicates can also be used to read from a file and write to a file.

Type the following at the prompt and see what response you get.

In both case you should have got a response that said X = user. The first predicate, seeing/1, gives the current name of the input stream. In this case your keyboard. The second predicate, telling/1, gives the name of the current output stream. The default is the screen.

By using the predicates, see/1 and tell/1 we can change these defaults. For example type the following -

The input stream has moved from the keyboard to the file. Now type the following -

Repeat this last query a few times and see what response you get. You should notice that read is now gathering input from the file. X is instantiated to the next Prolog term in the file every time we call it. Now type the following -

What has happened here? The predicate seen/0 closes the current input stream and sets it back to the default, the keyboard.

Now try the following -

In this interaction you have moved the output stream to a file called 'test_output.pl', if this file does not exist it will be created. This change in the output stream can be checked by using telling/1. You now use write/1 to enter a fact into this file. After that has been done you have used told/0 to close the current output stream and return it to the default, the screen.

As an exercise you should attempt to 'read' the fact you have placed in 'test_output.pl' by using the predicates you have just learned.

You should note from all this the predicates write/1 and read/1 are quite powerful. There are not the only predicates for input / output. Here are some more that may prove useful.

get - get( X ) will read the next character from the standard input device, skipping non-printing characters and instantiates X to its ASCII code value.

get0 - same as 'get' but doesn't skip non-printing characters.

put - put( X ) will write the character of ASCII code X to the standard output.

What you should have learned from this tutorial -

• We can use read/1 to read any input stream e.g. the user or a file.
• We can use write/2 to write to any output stream.
• To change the output stream use tell/1.
• To change the input stream use see/1.

The built-in predicates that were first introduced in this tutorial were:

see/1, tell/1, seeing/1, telling/1, seen/0, told/0,

get/1, get0/1, put/1

Author - Jer Hayes - 2000