Discrete Math.: Summary Input and Output

Summary Input and Output

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14.51

built-in predicates that read from and write well for users

terminal (keyboard and screen) or a file, the second term by term and characterby --

characters in your own programs. ASCII value for manipulating character strings. Prolog has a facility to enable both input and output of the term or character. Using simple terms and will be explained first. Initially, it will be assumed that

all the output to the screen the user and all user input is the keyboard. Inputs and outputs use external files.

1.Outputting Terms

predicate takes one argument, which must be a valid Prolog term. Evaluate the causes predicate terms to be written to the output current river, which by default is the user's screen. (The meaning of output current Logic Programming With Prolog streaming)

Examples
?- write(26),nl.
26
yes
?- write('a string of characters'),nl.
a string of characters
yes
?- write([a,b,c,d,[x,y,z]]),nl.
[a,b,c,d,[x,y,z]]
yes
?- write(mypred(a,b,c)),nl.
mypred(a,b,c)
yes
?- write('Example of use of nl'),nl,nl,write('end of example'),nl.
Example of use of nl
end of example
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. (The meaning of current input stream willbe explained in Sections 5.7 and 5.9. At present it can simply be taken to mean theuser's keyboard.)

In the input stream, the term must be followed by a dot ('.') and at least onewhite space character, such as space or newline. The dot and white space

characters are read in but are not considered part of the term.

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.

4.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. For example, it would be tedious to define a predicate (using read) which would read a series of characters from the keyboard and count the number of vowels. A much better approach for problems of this kind is to input a character at a time. To do this it is first necessary to know about the ASCII value of a character.

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.

5. 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 (ASCIIvalue) of its argument.

6. Inputting Characters

Two built-in predicates are provided to input a single character: get0/1 and get/1.

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.

7. Using Characters: Examples

The first example shows how to read in a series of characters from the keyboard finishing with * and to output their corresponding ASCII values one per line (for all characters excluding *).

The predicate readin is defined recursively. It causes a single character to be input and variable X to be bound to its (numerical) ASCII value. The action taken (the process(X) goal) depends on whether or not X has the value 42 signifying a * character. If it has, the evaluation of the goal stops. If not, the value of X is output,

followed by a new line, followed by a further call to readin. This process goes on

indefinitely until a * character is read. (In the example below, the ASCII values of characters P, r, o etc. are correctly shown to be 80, 114, 111 etc.)

Assuming the argument variable is unbound (which will usually be the case), it is bound to the ASCII value of the input character.

8. 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, i.e. keyboard for input and screen for output.

The same facilities available for input and output from and to the user's terminal either term by term or character by character are also available for input and output from and to files (e.g. files on a hard disk or a CD-ROM).

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.

9. 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, e.g. tell('outfile.txt').

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 (any existing file with the same name is deleted).

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

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.

10. 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.

If the end of file is encountered while evaluating the goal get(X) or get0(X), variable X will be bound to a 'special' numerical value. As ASCII values must be in the range 0 to 255 inclusive, this will typically be -1, but may vary from one implementation of Prolog to another.

12.Reading from Files: End of Record

Depending on the version of Prolog used, there may be an incompatibility for

character input between reading the end of a record (i.e. the character(s) that

signify the end of a line) from the user's terminal and from a file.

Typically the end of a line of input at the user's terminal will be indicated by

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.