Summary of Chapter 5
After reading this chapter you should be able to:
Use the built-in predicates that read from and write to either the user'sterminal (keyboard and screen) or a file, both term by term and characterby-character in your own programs. Use ASCII values to manipulate strings of characters
Introduction
Prolog has facilities to enable input and output either of terms or of characters. Using terms is simpler and will be described first. Initially, it will be assumed that all output is to the user's screen and all input is from the user's keyboard.
5.1 Outputting Terms
The main built-in predicate provided for outputting terms is write/1, which has already been used many times in this book. 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 meaning of current output stream will be explained in Sections 5.7 and 5.8. At present it can simply be taken to mean the user's screen.)
5.2 Inputting Terms
In the input stream, the term must be followed by a dot ('.') and at least one white space character, such as space or newline. The dot and white space characters are read in but are not considered part of the term.
5.3 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. 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.
5.4 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.
5.5 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.
5.6 Using Characters: Examples
The first example shows how to read in a series of characters from the keyboardfinishing 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.)
5.7 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 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.
5.8 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 output stream. 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.
5.9 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. 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.
5.9.1 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.
5.9.2 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.
Use the built-in predicates that read from and write to either the user'sterminal (keyboard and screen) or a file, both term by term and characterby-character in your own programs. Use ASCII values to manipulate strings of characters
Introduction
Prolog has facilities to enable input and output either of terms or of characters. Using terms is simpler and will be described first. Initially, it will be assumed that all output is to the user's screen and all input is from the user's keyboard.
5.1 Outputting Terms
The main built-in predicate provided for outputting terms is write/1, which has already been used many times in this book. 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 meaning of current output stream will be explained in Sections 5.7 and 5.8. At present it can simply be taken to mean the user's screen.)
5.2 Inputting Terms
In the input stream, the term must be followed by a dot ('.') and at least one white space character, such as space or newline. The dot and white space characters are read in but are not considered part of the term.
5.3 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. 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.
5.4 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.
5.5 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.
5.6 Using Characters: Examples
The first example shows how to read in a series of characters from the keyboardfinishing 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.)
5.7 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 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.
5.8 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 output stream. 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.
5.9 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. 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.
5.9.1 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.
5.9.2 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.
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