Department of Mathematics and Computer Science

Math 169 Notes -- Icon

Contents

• A. "Hello World!" in Icon
• B. History
• C. Identifiers
• D. Reserved Words
• E. Output Function
• F. Program Structure
• G. Subprograms
• H. Comments
• I. Expressions
• J. Variables
• K. Input Function
• L. Success or Failures
• M. Control Structures
• ---1. While Loops
• ---2. Until Loops
• ---3. If-then-else
• ---4. Loop Termination
• ---5. Repeat Loops
• ---6. Case Structure
• ---7. Note
• N. Emergency Program Termination
• O. Pattern Matching
• P. Arithmetic Expressions, Binary Operators
• Q. Numerical Comparisons
• R. Repeated Evaluation
• S. Iteration
• T. Alternation and Conjunction
• U. Other Assignment Features
• V. Unary Operators
• W. String Scanning
• X. Csets
• Y. Lists (Arrays), Tables, Sets

A. "Hello World!" in Icon

          procedure main()
	       write("Hello world!")
	  end

B. History

C. Identifiers

D. Reserved Words

Icon reserved words are written with all lower case letters.

E. Output Function

F. Program Structure

Each procedure begins with the reserved word "procedure" and ends with the reserved word "end". The name of the procedure follows the reserved word "procedure" and is an identifier of the language. A matched set of parentheses follows the procedure name and included within the parentheses are any arguments to the procedure.

Even if the procedure has no arguments, the parentheses are nevertheless included.

As in C/C++, The main program is the procedure named "main" with no parameters.

For example,

	procedure main()
		write("Hello world")
	end

G. Subprograms

For example,

	procedure hello()
		write("Hello world")
	end
	procedure main()
		hello()
	end

Procedures can explicitly return a value by using the reserved word "return" before the identifier storing the value. Procedures can explicitly fail by using the reserved word "fail" (or by ending without an explicit return).

For example,

	if count > 0 then
		return count
	else fail

H. Comments

I. Expressions

J. Variables

	procedure main()
		i := 1
		write(i)
		i := "hello"
		write(i)
	end

	1
	hello

Variables can also explicitly be declared as local inside each procedure by using the reserved word "local" followed by the list of identifiers. This line should be the first one in a procedure (after the procedure heading line).

Variables are usually created when a procedure is called and destroyed when it is completed. To keep a variable and its last value alive from one invocation of a procedure to another, the variable can be declared as "static" (in a way similar to declaring variables global or local).

K. Input Function

One can read from a file either by using Unix file redirection when invoking the executable file, or by explicitly opening a file and associating the actual file name with an identifier which is then used as the argument to the "read" function. For example:

procedure main ()
    intext := open("sonnet.inp")
    while line := read(intext) do
	{
	    write(line)
	}
end

L. Success or Failure

Thus, if the input file is empty,

read()	fails,
line := read()	also fails, and
write(read())	also fails.

M. Control Structures

2. "Until" loops are related to Pascal REPEAT loops with the following differences. The reserved word "until" comes first followed by an expression that can fail. The "do" section then follows and it is optional. As in a "while" structure, multiple expressions are grouped by using curley braces.

3. If-then-else structures are used as in Pascal/Fortran-90/C/C++. The "else" clause is optional.

4. A loop can be terminated immediately by invoking the expression "break" (similar to the Fortran-90/95 exit statement). Normally this would be used as part of an if-then-else structure for early termination. Part of the expression in the body of the loop can be skipped by invoking the expression "next" (similar to the Fortran-90/95 cycle statement). This would be used as part of an if-then-else structure, and control is then immediately transfered to the beginning of the loop structure, ignoring any remaining expressions within the body fo the loop.

5. An Icon "repeat" loop is significantly different from the Pascal structure with the same name. The Icon "repeat" loop has no conditional clause (e.g., "until"), and repeats indefinitely whether the expressions in its body succeed or fail. It can only be terminated using a "break" expression.

6. A "case" structure exists, similar to the one in Pascal (or the SELECT CASE structure in Fortran-90/95). The options are grouped as clauses with braces. Each clause has a left side (the evaluation option), a colon, and a right side (the action expression). If the key expression matches one of the options listed, the corresponding action is performed. There is a "default" option if none of the options listed equals the key expression. For example,

	case i of {
		j+1	: write("high")
		j-1	: write("low")
		default	: write("otherwise")
		}

As in other languages, the reserved word "not" can be used before any expression to reverse its success/failure value.

N. Emergency Program Termination

O. Pattern Matching

find(s1,s2)	succeeds if s1 is a substring of s2 occurring anywhere in s2. The resulting value is the location in s2 of the first character of s1.
match(s1,s2)	succeeds if s1 is an initial substring of s2 occurring at the beginning of s2. The resulting value is one plus the length of s1.

find("on", "motion") succeeds (with a value of 5), and
match("on", "motion") fails (given no value), but
match("mo", "motion") succeeds (with a value of 3).

P. Arithmetic Expressions, Binary Operators

+	addition
-	subtraction
*	multiplication
/	division
%	"mod", i.e., remainder after division
^	exponentiation

Q. Numerical Comparisons

<	less than
<=	less than or equal to
=	equal to
>	greater than
>=	greater than or equal to
~=	not equal to

	3 < 5

	n := 4 < 10

Compound comparisons can be written as in standard mathematical expressions, such as:

	1 <= n <= 10

R. Repeated Evaluation

		find(s1,s2) > 10

S. Iteration

One can generate a sequence of numbers by using

		i to j by k 

Thus,

 
		every i:= 1 to 9 by 2 do write(i^2)

T. Alternation and Conjunction

	if i = (0|1) then write("okay")

Conjunction, that is the "and" logical operator, is expressed by the single ampersand, &.

U. Other Assignment Features

Augmented assignments (as in C/C++) are permitted, e.g., i +:= 1 is interpreted as i := i+ 1.

Exchange of values is indicated by an equals sign with colons on both sides, e.g., x :=: y.

V. Unary Operators

?s	produces one of the characters in string s randomly chosen
!s	generates the characters of string s one by one, from first to last, left to right
*x	produces the size of x where x is a string, cset or structure

W. String Scanning

The syntax of use is: expr1 ? expr2
where expr1 is the subject to be scanned
and expr2 does the scanning.

The function move(i) which increments the position in the subject by i characters (if possible, else it fails), can be used in a possible scanning expression. The value of move(i) is the portion of the subject between the old and new positions.

The functions find and match can be used in a scanning expression. In this case, the second argument is omitted and the subject is assumed to be the argument to be searched.

For example

	write(text ? find("the"))

The function tab(i) sets the position in the subject to (right before) the i-th character when i is positive and "returns" the substring from the current position up to the i-th character. If i is 0, tab(i) indicates the right edge of of the subject. If i is negative, tab(i) indicates |i| characters from the right edge of the subject.

X. Csets

Predefined csets include:

&letters	all upper and lowercase letters
&ucase	all uppercase letters
&lcase	all lowercase letters
&digits	the ten single digits
&cset	all 256 characters
&ascii	the first 128 characters in ASCII

++	set union (binary)
**	set intersection (binary)
--	set difference (binary)
~	set complement (unary)
*	set size (unary)

upto(c)	generates the position in the subject in which any character in cset c occurs.
many(c)	produces the position after a sequence of characters in c
any(c)	produces the position after any character in c.

The following is an example of the use of these functions:

procedure main()
    text := "Does anyone have an apple or an ugly orange?"
    text ? {
	while tab(upto(&letters)) do
	    { write("Upto =", upto(&letters))
	      write("many =", many(&letters))
	    write("tab many=", tab(many(&letters)))}
    }
end

The output is:

Upto =1
many =5
tab many=Does
Upto =6
many =12
tab many=anyone
Upto =13
many =17
tab many=have
Upto =18
many =20
tab many=an
Upto =21
many =26
tab many=apple
Upto =27
many =29
tab many=or
Upto =30
many =32
tab many=an
Upto =33
many =37
tab many=ugly
Upto =38
many =44
tab many=orange

Y. Lists (Arrays), Tables, Sets

Thus,

		vector := list(100,0.0)

An empty list (containing no values) can be created by [] or list(0).

A multi-dimensional array can be simulated by created a list of lists. Thus,

		board := list(8)
		every !board := list(5)

Lists can be used as queues and stacks. The operations put(L,x) and get(L) enqueues an element x onto the right end of list L, and dequeues and element from the left end of list L. The operation push(L,x) adds an element x to the left end of list L, and pop(L) removes a value from the left end of L.

A table is a collection of items that can be referenced by "subscripts" that are neither numeric nor ordered. In other words, a table is like an array (list), except that the subscripts can be anything, including arbitrary strings. A table is created by invoking the function table and giving an initial value as the argument. A table automatically expands whenever a new element is referenced. Thus

		words := table(0)

		words["The"] := 1

++	set union
**	set intersection
--	set difference

This page is maintained by Dennis C. Smolarski, S.J. dsmolarski@math.scu.edu
Last updated: 14 May 2002.