SNOBOL 4

By: Bahman Ravaei 790649 Farhad Rad 116703

نیمسال 81-82-2

Introduction to the snobol 4 P.L

• Developed at BELL laboratories in 1962• Is a string of characters (character oriented)• Define other data type• Consist of a sequence of statement• Array and Table have more flexibility • Function calls can be made recursively• Link list node and complex number are

possible programmer define data type

four basic type of statements:

i) the Assignment statementii) the Pattern matching

statementsiii) the Replacement statementiv) the End statement

Assignment statements:

• the simple type of assignment:variable = value

example:v = 5

• the value may be given an experssion example:

W = 14 + (16 - 10)which assigns the value 20 to the variable w

• “BLANK are required around arithmetic operators”

• The value may be string of characters example:

v = ‘dog’

Arithmetic: i) Integers

ii) Real

arithmetic operation is “addition +” ,”subtraction -” ,”multiplication * ” ,”division / ” and “exponentiation ** or !”

blanks are required between the binary operators and their operands ,Unary operator such as the minus sign must be adjacent to their operands.

example: Q2 = -P / -N

Predicates of arithmetic operations:

-Unary operators -Exponentiation -Multiplication and Division -Addition and Subtraction

All arithmetic operations associate to the left except exponentiation .

example : 2 ** 3 ** 5is equivalent to :

2 ** (3 ** 5)

Example :m = 4n = 5p = n * m / (n - 1)

assign the value 5 to pExample :

pi = 3.14159mul = 2. * pi * 5.

assign Real value to pi and mul

Pattern matching statements:

Define : The operation of examining for occurrence of specified substring

Pattern matching have two type: i ) the pattern matching statement ii ) the replacement statement

the pattern matching statement:

• the pattern matching statement has the form :

subject patterntwo fields separated by at least one blank

example: trade = ‘programmer ’ trade ‘gram’

• Pattern can an expression

row = ‘k’ no. = 20

‘kk2429’ row no. + 4

pattern is equivalent by ‘k24’

Replacement statements

• The replacement statement has the form

subject pattern = object

• If the pattern matching operation succeeds the subject string is modified by replacing the matched substring by the object .example:

WORD = “GRID”WORD ‘I’ = ‘OU’

thus WORD is equivalent GROUD

If pattern not found the statement failed example:

WORD ‘AB’ = ‘OU’example:

HAND = ‘AC4DAHDKS’RANK = 4SUIT = ‘D’HAND RANK SUIT = ‘AS’

first RANK and SUIT concatenate that it’s 4D and later HAND ‘4D’ = ‘AS’

example :HAND RANK SUIT =

Patterns • Two operation available for

constructing complex patterns:i) alternation ii) concatenation

alternation is indicated by expression of the form:

P1 | P2example:

KEYWORD = ‘COMPUTER’ | ‘PROGRAM’KEYWORD = KEYWORD | ‘ALGORITM’

can use KEYWORD to pattern field:

TEXT = ‘PROGRAMMING ALGORITM FOR COMPUTERS ‘

TEXT KEYWORD =after matching:TEXT = ‘MING ALGORITM FOR COMPUTERS’

Concatenation• Concatenation of two patterns ,P1

and P2 ,is specified in the same way as the concatenation of two strings:

P1 P2example:

BASE = ‘BINARY’ | ‘DECIMAL’ | ‘HEX’SCALE = ‘FIXED’ | ‘FLOAT’ATTRIBUTE = SCALE BASE

Array: • Arrays are created by function ARRAY

.ARRAY (p , e)

example :board = ARRAY (‘3,3’ , ‘x’)x x X

X x x

x x x

Example :A1 = ARRAY ( 5 )A2 = ARRAY (5 , A1)

A2 A1

Flow of control

A SNOBOL program is a sequence of statements terminated by an end statement . labels and goto provide to control the flow of the program.

Example of label:START TEXT = INPUT

the end statement is distinguished by the label END

Transfer to a labelled statement is specified in the goto field which may appear at the end of a statement and is separated from the rest of the statement by a colon ‘ : ’.

Two types of transfers can be specified in the goto field :

i ) conditionalii ) unconditional

conditional A conditional transfer consists of a

label enclosed within parentheses preceded by an F or S corresponding to Failure

or Success.example :

TEXT = INPUT :F(DONE)example :

LOOP PUNCH = INPUT S:LOOPEND

we can use F and S in one statementexample:

COLOR = ‘RED’ | ‘GREEN’ | ‘BLUE’BRIGHT TEXT COLOR = :S(BRIGHT)F(BLAND)...BLAND …….

unconditional

An unconditional transfer is indicated by the absence of an F or S before the enclosing parentheses .

example:LOOP PUNCH = INPUT :F(END)

OUTPUT = PUNCH :(LOOP)END

Indirect reference Indirect referencing is indicated by

the unary operator $.examples:i) MONTH = ‘APRIL’

$MONTH = ‘CRUEL’is equivalent:

APRIL = ‘CRUEL’ii) WORD = ‘RUN’

$(WORD ‘:’) = $(WORD ‘:’) + 1iii) $(‘A’ | ‘B’)

iv) N = N + 1 :($(‘PHASE’ N))

Functions:

i) primitive functionsii) predicatesiii) defined functions

Primitive function

Many snobol procedure are invoked by function built into the system ,called primitive functions. Few example of primitive function:i) SIZE (string)

APE = ‘SIMIAN’OUTPUT = SIZE(APE)

ii) DUPL (string ,integer)DUPL(‘/*’,5)

iii) REPLACE( TEXT,CH1,CH2)STATEMENT = ‘A(I,J) = A(I,J) + 3’OUTPUT = REPLACE(STATEMENT,’()’,’<>’)

print the line :A<I,J> = A<I,J> + 3

Predicate: A function or operation that returns the

NULL string if a given condition is satisfied.

Example: i) LE(n1 , n2 ) PUNCH = LE( SIZE(TEXT),80) TEXT ii) LT( n1 , n2 ) ADD N = LT( N , 50 ) N+1 iii) DIFFER ( st1 , st2 ) OUTPUT = DIFFER ( FIRST , SECOND ) FIRST

SECOND iv) LGT ( st1 , st2 )

Defined function:

A function defined by Programmer. The primitive function DEFINE to specify

the function name , formal arguments , local variables, and the entry point of the function.

Example:

DEFINE ( ‘ DELETE ( STRING , CHAR )’ , ‘D1’ ) D1 STRING CHAR = :S ( D1 )

DELETE = STRING : ( RETURN )

Example:

DEFINE ( ‘ DELETE ( STRING , CHAR )’ ) DELETE STRING CHAR = :F ( FRETURN)

D2 STRING CHAR = :S ( D1 )

DELETE = STRING : ( RETURN )

Programmer define data type:

The primitive function DATA can be used to difine the new data type.

Example: DATA( ‘ NODE ( VALUE , LINK)’)

P = NODE (‘ S ‘ , ) P = NODE (‘ T ‘ , P ) P = LINK ( P )

P ‘S ’ NULL ‘T‘

Program format: Example: OUTPUT = ‘ THE TOTAL NUMBER OF OCCURRENCE IS’

+ SUM< N >

Example: X = 2 ; Y = 3 ; Z = 10

* : THIS IS A COMMENT

End