ICON

1
ICON
2
Icon

• Icon is a very-high-level, typeless, expression
  language withbacktracking.
• "Icon" does not mean "picture".

3
Whats Icon good for?

• Tools
• Prototyping
• Trying out ideas
• Quick programming
• Text processing

4
Topics of Discussion

• Program structure
• Data types
• Control constructs

5
Program structure

• Files compiled together
• Declarations
• Procedures
• Variables
• Records

6
Hello, world.

• Must begin with procedure
• must conclude with end
• must have a procedure main
• procedure main()write("Hello, world")end

7
Procedures

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

8
Procedure Parameters

• Given in a list enclosed in parentheses
• procedure main()
• greet("Hello, ", "world ")
• end
• procedure greet(what,who)
• writes(what)
• write(who)
• end
• Produces
• Hello, world

9
Typelessness

• procedure main()
• x "Example "
• writes(x)
• x 1
• write(x)
• end

10
Variable Values

• ICON has both values and variables that have
  values
• procedure main( )
• line "Hello, world "
• write (line)
• end
• assigns the string value " Hello, world " to the
  identifier line, which is a variable

11
String Variables

• May be written literally
• line "Hello, world"
• all 256 characters may occur in a string
• empty string, given by " " contains no characters
  and its length is zero

12
Strings

• s"ab"
• ss"cd"
• write(s)
• "abcd".
• s"abc"
• write(s)
• s"a"
• write(s)
• s""
• write(s)
• 3
• 1
• 0

13
Strings

• s"find"
• write(s3)
• s4 "e"
• write(s)
• will write out
• n
• fine

14
Identifiers

• Must begin with a letter or underscore
• May be followed with other letters, digits and
  underscore
• Upper and lowercase are distinct
• comp Label test10 entry_value
• There is no declaration for identifier line
• Scope declarations are possible but optional
• ICON has no type or storage declarations
• Correctness of types is checked when operations
  are performed
• Storage for values in provided automatically

15
Comments

• The character in a program signals the
  beginning of a comment
• Both the number sign and the remaining characters
  in the line are ignored
• End of line terminates comment
• If appears in quotes, it will be displayed
• write ("Apt 23 ")

16
read ( )

• Reads a line
• write (read( ))
• reads a line and displays it
• This will read an expression in your program and
  write it
• You will need to do some work on the line
• Try
• lineread( )
• write(line)

17
Expression success or failure

• Many expressions in ICON will either succeed or
  fail
• If succeeds, produces a value, if fails produces
  no value
• read( ), failure occurs when end of line or file
  is reached
• What happens at end of line or end of file?
• the statement
• lineread( )
• failure does not affect the execution of the next
  line so write(line) displays current line

18
Control Structures

• While
• while line read( ) do
• write(line)
• repeatedly evaluates read( ) in a loop
• each time read succeeds the value is assigned to
  line and write(line) is evaluated to display that
  value
• when read( ) fails the assignment operator fails
  and loop terminates
• success or failure of the expression that follows
  while controls evaluation of the expression that
  follows do
• while and do are reserved words

19
Control Structures

• If
• if count 0 then sign 1 else sign -1
• else clause is optional
• What about if-then-else ambiguity?
• if find("Mr. ", line) then
• if find("Mrs. ",line)
• then mmmm1
• else mrmr1
• If there is a"dangling else" in nested
  if-then-else expressions, then else is grouped
  with nearest preceding if
  
  if find("Mr. ",line) then
  
  if find("Mrs. ",line) then mmmm1
  else mrmr1
• Control structures in ICON are expressions

20
Control Structures

• Case statement
• case expr of
• case-clause1
• case-clause2
• .
• each case-clause has the form
• expr1 expr2
• there is an optional default clause
• default expr3

21
Case

• case s of
• "begin" depthdepth1
• "end" depthdepth-1
• expression fail if s is neither "begin" or "end"
• case i of
• j1 write("high")
• j-1 write("low")
• j write("equal")
• default write("out of range")
• displays one of four strings, depending on the
  relative values of i and j

22
find ( ) and match( )

• find (string1, string2)
• succeeds if string1 occurs in string2
• match ( string1, string2)
• succeeds only if string1 is an initial substring
  that occurs in the beginning of string2
• find("on", "slow motion") - succeeds
• find("on", "noise") - fails
• match("on", "slow motion") - fails
• match("slo", "slow motion") - succeeds

23

• If an expression that fails is an argument in
  another expression, the other expression fails
  also, since there is no value for its argument
• value of find("on", "slow motion") is 10
• procedure countm(s)
• procedure produces a count of the lines that
  begin with the value of s
• count 0
• while line read( ) do
• if match(s,line) then count count1
• return count
• end
• return is a reserved word that indicate values to
  be returned from a procedure

24
find( )

• The call countm("") returns a count of the
  comment lines in an ICON program
• if i find(s1,s2) then write(i)
• writes the location of s1 in s2 if s1 is in s2
• find("i", "mills")
• returns a value of 2
• find("l", "mills")
• returns a value of 3

25
Generators

• ICON allows expressions to produce more than one
  result
• these are called generators
• example
• every i find("l","mills") do writes(i)
• displays 3 4

26
Elementary generators

• every i 1 to 10 do e
• equivalent of a for loop. The expression 1 to 10
  is a generator that generates the integers 1, 2,
  ..., 10. Each of the values is assigned to
  variable i and
• the expression e is evaluated.
• every i 1 to 10 j 1 to 10 do e
• behaves like two nested for loops.
• for i 1, j will iterate from one to 10, then
• for i 2, j will go from 1 to 10, and so on.
• every i 1 to 10 j 1 to 10 i j do e

27
Control constructs

• Expression language
• "Statements" are expressions
• Expressions are generators
• Expressions generate sequences of values
• Expressions generate their sequences of values by
  backtracking

28
Backtracking

• expressions can succeed or fail
• backtracking into an expression generates its
  next value
• backtracking is "cut off" between statements and
  in some other contexts

29
Relational operators

• a relational operator is a binary operator
• it fails if the relation does not hold
• it returns the right operand if it succeeds
• a than b and b is less than c

30
While expressions

• while e1 do e2while e1
• at most one value from each (e1 and e2) per
  iteration
• e2 may succeed or fail
• restarts e1 from the beginning
• fails when e1 fails

31
Example File copy

• procedure main()while write(read())end

32
Every expressions

• every e1 do e2every e1
• generates all values for e1
• for each value of e1, evaluate e2 generate at
  most one value for e2
• e2 may succeed or fail
• fails when e1 fails

33
Why file copycouldnt use every

• while write(read())
• read() returns one line from the input
• read() does not generate another line when backed
  into

34
Example write tableof max. values

• procedure main()writes(" ") every writes(" ",1
  to 5) write()every i 1 to 5 do
  writes(i) every j 1 to 5 do writes("
  ",max2(i,j)) write()end

35
Example max2(x,y)

• procedure max2(x,y)x x
• procedure max2(x,y)x

36
Example table of max.values

• 1 2 3 4 51 1 2 3 4 52 2 2 3 4 53 3 3 3 4
  54 4 4 4 4 55 5 5 5 5 5

37
If expressions

• if e1 then e2 else e3if e1 then e2
• if e1 succeeds, behave like e2 if e1 fails,
  behave like e3
• do not generate more than onevalue from e1

38
Stack

• Stack
• push(Stack,"")
• apop(Stack)
• push(Stack,-)
• aget(Stack)

39
Using case

• procedure get_priority(Stack)
• sypop(Stack)
• push(Stack,sy) get() pops the stack, so return
  value to stack
• case sy of
• "" return -1
• "" return 3
• "(" return 0
• "" return 2
• "/" return 2
• "" return 1
• "-" return 1
• end

40
Blocks

• while priority incoming do
• sympop(opStack)
• writes(sym)
• priorityget_priority(opStack)

41
Programming Assignment 1

• Produce an ICON program that converts infix to
  postfix.
• Assume
• input is an arithmetic statement
• a(b-c)
• output is the statement in postfix form
• abc-
• no spaces in statement
• operands are single characters
• operators are - / ( )
• Due 10/5/2001