TUTORIAL 4 - Summary Chapter 5

Input and Output in ProLog

Outputting Terms
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. Evaluating a nl goal causes a new line to be output to the current output stream.

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

Note that atoms that have to be quoted on input (e.g. 'Paul', 'hello world') are not quoted when output using write. If it is important to output the quotes, the writeq/1 predicate can be used. It is identical to write/1 except that atoms that need quotes for input are output between quotes (other atoms are not).

?- writeq('a string of characters'),nl.
'a string of characters'
yes
?-writeq(dog),nl.
dog
yes
?- writeq('dog'),nl.
dog
yes

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. 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. Note that for input from the keyboard (only) a prompt character such as a colon will usually be displayed to indicate that user input is required. It may be necessary
to press the 'return' key before Prolog will accept the input. Both of these do not apply to input from files.
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.

?- read(X).
: jim.
X = jim
?- read(X).
: 26.
X = 26
?- read(X).
: mypred(a,b,c).
X = mypred(a,b,c)
?- read(Z).
: [a,b,mypred(p,q,r),[z,y,x]].
Z = [a,b,mypred(p,q,r),[z,y,x]]
?- read(Y).
: 'a string of characters'.
Y = 'a string of characters'

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).
: jim.
no
?- X=fred,read(X).
: fred.
X = fred

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.

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
?- put(122),nl.
z
yes
?- put(64),nl.
@
yes

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.
Note that for input from the keyboard (only) a prompt character such as a colon
will usually be displayed to indicate that user input is required. It may be necessary
to press the 'return' key before Prolog will accept the input. Both of these also
apply to the get predicate described below but do not apply to input from files.
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
?- get0(N).
: Z
N = 90
74 Logic Programming With Prolog
?- get0(M)
: )
M = 41

If the argument variable is already bound, the goal succeeds if and only if it has a numerical value that is equal to the ASCII value of the input character.

?- get0(X).
: a
X = 97
?- M is 41,get0(M).
: )
M = 41
?- M=dog,get0(M).
: )
no
?- M=41.001,get0(M).
: )
no

The get predicate takes a single argument, which must be a variable. Evaluating
a get goal causes the next non-white-space character (i.e. character with an ASCII
value less than or equal to 32) to be read from the current input stream. The
variable is then unified with the ASCII value of this character in the same way as
for get0.

?- get(X).
: Z
X = 90
?- get(M).
: Z
M = 90