The language

The language on which the compiler works is a psuedo-code that implements all the constructs of structured programming: sequence, selection, iteration. The language has four different data types: Intero (equals to int of C), Reale (equals to double of C), Carattere (equals to char of C), and Stringa (equals to an array of char of C), furthermore it doesn't support any type of structured data type.

BNF of the grammar

<statements> ::= <statement> "\n" | <statement> "\n" <statements>

<statement> ::= <type> <id> | <id> " <- " <val> | <io> | "MENTRE " <conditions> " ALLORA <!\n" <statements> "!>" | "SE " <conditions> " ALLORA <!\n" <statements> "!> " <else> |

<else> ::= "ALTRIMENTI <!\n" <statements> "!>" |

<conditions> ::= <condition> <logicop> <conditions> | <condition>

<condition> ::= <val> <compare> <val>

<operation> ::= <mathop> <val> <val>

<io> ::= "LEGGI -> " <id> | "STAMPA <- " <val>

<val> ::= <id> | <value> | <operation>

<id> ::= [a-zA-Z][_a-zA-Z0-9]*

<type> ::= "Intero" | "Reale" | "Carattere" | "Stringa"

<value> ::= 0|-?[1-9][0-9]* | (0|-?[1-9][0-9]*)\.?[0-9]* | ['].['] | ["][^"]*["] 

<compare> ::= "<" | "<=" | ">" | ">=" | "=" | "!="

<mathop> ::= "+" | "-" | "*" | "/" | "%"

<logicop> ::= "AND" | "OR"

Code example

Intero a
Intero b
Intero r
LEGGI -> a
LEGGI -> b
SE a < b ALLORA <!
	r <- a
	a <- b
	b <- r
!>
MENTRE b != 0 ALLORA <!
	r <- % a b
	a <- b
	b <- r
!>
STAMPA <- "The GCD is "
STAMPA <- a
STAMPA <- "\n"

Functioning

MENTRE

MENTRE <condition> ALLORA <!
	[block]
!>

The construct is the equivalent of a preconditioned while, where the condition is at first evaluated and if it's true the block is executed.

SE-ALTRIMENTI

SE <condition> ALLORA <!
	[true block]
!> ALTRIMENTI <!
	[false block]
!>

The construct is the equivalent of an if-then-else, it can be used also an if-then, by not writing "ALTRIMENTI".

I/O

LEGGI -> <id>
STAMPA <- <val>

The input is given with the keyword "LEGGI", while the output with "STAMPA", which permits the concatenation with the operator +, only between Stringa and Carattere:

Stringa a
Stringa b
LEGGI -> a
LEGGI -> b
STAMPA <- + a b

Conditions

In the conditions of MENTRE and SE constructs, more conditions can be concatenated with the logical operator AND and OR. It is not possible to establish a priority with parenthesis, so the evaluation is done in short-circuit from left to right.

MENTRE a < 0 AND continue = 1 <!
	[blocco]
!>

The comparing operators available are: <, <=, >, >=, =, !=.

Arithmetical operators

The arithmetical operators use on Intero, Reale or Carattere have the classic functioning:

• + : sum;
• - : difference;
• * : product;
• / : division;
• % : module;

If these are use on Stringa, they have the following behavior:

• + : concatenation

+ <val>.Carattere/<val>.Stringa <val>.Carattere/<val>.Stringa

• - : it returns the Stringa with n chars removed from right

- <val>.Stringa <val>.Intero

• * : it returns the char in the specified position, if the index is not valid, it returns 0

* <val>.Stringa <val>.Intero

• / : it returns the Stringa with n chars removed from left

/ <val>.Stringa <val>.Intero

• % : it returns 1 if the Stringa or Carattere given is a substring

% <val>.Stringa <val>.Carattere/<val>.Stringa

Types compatibility

Assignments

The valid assignments, considering the first as the left value and the second as the right, are:

• Intero: Intero, Reale, Carattere;
• Reale: Intero, Reale, Carattere;
• Carattere: Intero, Reale, Carattere;
• Stringa: Carattere, Stringa.

Comparisons

The valid comparisons, considering the first as the left value and the second as the right, are:

• Intero: Intero, Reale, Carattere;
• Reale: Intero, Reale, Carattere;
• Carattere: Intero, Reale, Carattere;
• Stringa: Stringa.

Operations

The operations define the final data type of the expressions, so, considering the cases where the data types are different:

• Intero-Carattere: evaluated as Intero;
• Reale-Intero/Carattere : evaluated as Reale;
• Stringa-Carattere: evaluated as Stringa in case of +, in case of % as Intero;
• Stringa-Intero: evaluated as Stringa in case of *, as Stringa in case of - or /.