SUMMARY OF CHAPTER 5

SUMMARY OF INPUT AND OUTPUT

1. Outputting Terms
The main built-in predicate provided for outputting terms is write/1. The write/1 predicate takes a single argument, which must be a valid Prolog term. Evaluating the predicate causes the term to be written to the current output stream, which by default is the user's screen. The built-in predicate nl/0, It takes no arguments. Evaluating a nl goal causes a new line to be output to the current output stream. for examples;

?- write(26),nl.
26
yes

2. Inputting Terms
The built-in predicate read/1 is provided to input terms. It takes a single argument, which must be a variable. Evaluating it causes the next term to be read from the current input stream,
which by default is the user's keyboard. When a read goal is evaluated, the input term is unified with the argument variable. If the variable is unbound (which is usually the case) it is bound to the input value. for example;

?- read(X).
: 26.
X = 26

If the argument variable is already bound (which for most users is far more likely to occur by mistake than by design), the goal succeeds if and only if the input term is identical to the previously bound value.

?- X=fred,read(X
: fred.
X = fred

3. Input and Output Using Characters
Although input and output of terms is straightforward, the use of quotes and full stops can be cumbersome and is not always suitable. A much better approach for problems of this kind is to input a character at a time.
All printing characters and many non-printing characters (such as space and tab) have a corresponding ASCII (American Standard Code for Information Interchange) value, which is an integer from 0 to 255. The table below gives the numerical ASCII values corresponding to the main printable characters and some others.

4. Outputting Characters
Characters are output using the built-in predicate put/1. The predicate takes a single argument, which must be a number from 0 to 255 or an expression that evaluates to an integer in that range. Evaluating a put goal causes a single character to be output to the current output stream. This is the character corresponding to the numerical value (ASCII value) of its argument, for example

?- put(97),nl.
a
yes

5. Inputting Characters
The get0 predicate takes a single argument, which must be a variable. Evaluating a get0 goal causes a character to be read from the current input stream. The variable is then unified with the ASCII value of this character. Assuming the argument variable is unbound (which will usually be the case), it is bound to the ASCII value of the input character.

?- get0(N).
: a
N = 97

The get predicate takes a single argument, which must be a variable. The variable is then unified with the ASCII value of this character in the same way as for get0.

6. Input and Output Using Files

Prolog takes all input from the current input stream and writes all output to the current output stream. By default both of these are the stream named user, denoting the user's terminal.

The user may open and close input and output streams associated with any number of named files but there can only be one current input stream and one current output stream at any time. Note that no file can be open for both input and output at the same time (except user) and that the user input and output streams

cannot be closed.

7. file Output: Changing the Current Output Stream

The current output stream can be changed using the tell/1 predicate. This takes a single argument, which is an atom or variable representing a file name. Evaluating a tell goal causes the named file to become the current outputstream.

If the file is not already open, a file with the specified name is first created.

The default current output stream is user, i.e. the user's terminal. This value can be restored either by using the told predicate or by tell(user).

The built-in predicate told/0 takes no arguments. Evaluating a told goal causes the current output file to be closed and the current output stream to be reset to user, i.e. the user's terminal.

The built-in predicate telling/1 takes one argument, which must be a variable and will normally be unbound. Evaluating a telling goal causes the variable to be bound to the name of the current output stream.

8. File Input: Changing the Current Input Stream

The current input stream can be changed using the see/1 predicate. This takes a single argument, which is an atom or variable representing a file name, e.g. see('myfile.txt').

Evaluating a see goal causes the named file to become the current input stream. If the file is not already open it is first opened (for read access only). If it is not possible to open a file with the given name, an error will be generated.

Note that the file corresponding to the previous current input stream remains open when a new current input stream is selected. Only the current input stream can be closed (using the seen predicate described below).

The default current input stream is user, i.e. the user's terminal. This value can be restored either by using the seen predicate or by see(user).

The built-in predicate seen/0 takes no arguments. Evaluating a see goal causes the current input file to be closed and the current input stream to be reset to user, i.e. the user's terminal.

The built-in predicate seeing/1 takes one argument, which must be a variable and will normally be unbound. Evaluating a seeing goal causes the variable to be bound to the name of the current input stream.

11.Reading from Files: End of File

If the end of file is encountered when evaluating the goal read(X), variable X will be bound to the atom end_of_file.

12.Reading from Files: End of Record

Typically the end of a line of input at the user's terminal will be indicated byt the character with ASCII value 13. The end of a record in a file will generally be indicated by two ASCII values: 13 followed by 10. The following program shows how to read in a series of characters from the keyboard and print them out, one per line.