Compiler: Programming Language= Assignments and statements

COMPILERS: PROGRAMMING
LANGUAGES
ASSIGNMENTS - STATEMENTS.
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Presentation By: Geo S. Mariyan
(Master in Computer Science)

Assignment - Introduction
• Assignment is to "assign a pre declared variable with
some value or by the value of another variable of the
same data type. This is done anywhere in body of the
program.
For E.g. : n=9 or m=5 then, n=5 !
assigns value 5 to n
• Initialization is telling compiler to allocate a
memory to the initialized variable so that compiler can
know that this variable is going to store some value.
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ASSIGNMENT – Symbolic Representation
• ASSIGNMENT: The Basic Operation for changing the binding of
the value to the data object.
• Assignment also returns the value, which is a data object
containing the copy of a value assigned.
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Ex : int a; Initializing 'a' as of type int
Assignment means proving a particular value to
any variable.
Ex: int a=2; a in assigned a value to 2.
This is purely assignment and assignment
can be done n no. of times in a program. There is no
such restriction on it.
Assignment throws away the old value of a
variable and replacing it with a new one. It can be done
anywhere in the whole program.
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l- and r- values
• An lvalue can appear on the left side of an assignment
operator, whereas an rvalue can appear on the right side.
• "lvalue" and "rvalue" are so named because of where each
of them can appear in an assignment operation.
• ex: int a, b;
• a = 4;
• b = a;
• In the third line of that example, "b" is the lvalue, and "a"
is the rvalue, whereas it was the lvalue in line.
• This illustrates an important point: An lvalue can also be
an rvalue.
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Assignment Operators
• An assignment operator is the operator used
to assign a new value to a variable, property, event or
indexer element.
• Assignment operators can also be used for logical
operations such as bitwise logical operations or
operations on integral operands and Boolean operands.
• Unlike in C++, assignment operators in C# cannot be
overloaded directly, but the user-defined types can
overload the operators like +, -, /, etc. This allows the
assignment operator to be used with those types.
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Characteristics of Assignment Operators:
• The different assignment operators are based on the type of
operation performed between two operands such as addition
(+=), subtraction, (-=), etc. The meaning of the operator
symbol used depends on the type of the operands.
• Assignment operators are right-associative, which means they
are grouped from right to left.
• Although assignment using assignment operator (a += b)
achieves the same result as that without ( =a +b), the
difference between the two ways is that unlike in the latter
example, "a" is evaluated only once.
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Practice with Assignment Operators
The assignment operator expects the type of both
the left- and right-hand side to be the same for
successful assignment.
Example:
i += 24
i *= x
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Variant forms of Assignment
•Augmented Assignment
•Chained Assignment
•Parallel Assignment
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• Augmented assignment: The case where the assigned
value depends on a previous one is so common that many
imperative languages, most notably C and the majority of
its descendants, provide special operators
called Augmented assignment
ex: *=, so a = 2*a can instead be written as a *= 2
• Chained assignment: A statement like w = x = y = z is
called a chained assignment in which the value of z is
assigned to multiple variables w, x, and y. Chained
assignments are often used to initialize multiple variables, as
in
•example: a = b = c = d = f = 0
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Parallel Assignment:
Some programming languages, such
as Go, JavaScript (since, Maple, Perl, Python, REBOL, Ruby,
and Windows Power Shell allow several variables to be
assigned in parallel, with syntax like:
ex: a, b := 0, 1
which simultaneously assigns 0 to a and 1 to b. This is most
often known as parallel assignment.
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Assignment statements
• Once a variable is declared, it can be assigned values
multiple times in a program; for example:
int num; // num declared,
uninitialized
num = 3; // num assigned 3
num = num + 1; // num assigned 3 + 1 (4)
num = 15 / num; // num assigned 15 / 4
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Implementation of Assignment:
• Compiler can implement the assignment A := B
• The Compiler generates code to compute the r-value and B
into some register r-value of B into some register r
• If the data types of A and B are incompatible, the compiler
generates code to convert the r-value of B to a type
appropriate for A.
• If necessary, the compiler generates the code to determine
the l-value of A.
(if A is in primitive type , no code is needed)
• Finally, the compiler generates code to store the
contents of register r into the l-values of A.
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Assignment compatibility
• When a variable is declared, the data type in the
declaration indicates the nature of the values the variable is
capable of storing.
• For example, an int variable can store a whole number, a
char variable can store a character, and a double variable
can store a real number.
• The value assigned to the variable must be compatible with
its data type.
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•Java is a strongly-typed language.
•This means that, for the most part, you can only
assign a value of a particular data type to a
variable of the same type
•Some examples:
int x = -2; // this is fine; -2 is an integer
char c = ‘2’; // this is OK also
x = 2.5; // syntax error: 2.5 is a double value,
not an int
c = 2; // also an error; 2 is an int value, not a
char
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Assignment Statement Variety:
= FORTRAN, C, C++, Java
= Can be bad if it is overloaded for the
relational operator for equality.
• Example
C: a=b=c
• In the C statement A = B = C, the value of C is
assigned to B, and the value of B is assigned to A.
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Statements:
Definition :
A statement in programming context is a
single complete programming instruction that is
written as code.
The Elementary action of evaluation,
assignment and control of evaluation orders are
specified by the statements of the programming
language .
Statements can have diverse forms and
languages.
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Function Call Statement
•Certain function calls, such as the MESSAGE function,
can be a statement on their own. They are known as
function call statements
•
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The Statement Separator
A single programming statement may span
several code lines; one code line may consist of
multiple statements.
Therefore, the C compiler must be able to
detect when one statement ends and another
statement begins.
[ <statement> { ; <statement> } ]
Example : Integer Num:=10;
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Assignment Statement
In computer programming, an assignment
statement sets and/or re-sets the value stored in the
storage location(s) denoted by a variable name; in other
words, it copies a value into the variable. In most
imperative programming languages, the assignment
statement (or expression) is a fundamental construct.
The syntax of an assignment statement is almost as easy
as the syntax of the function call statement. The syntax is
as follows:
<variable> := <expression>
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Assignment Statement
• Assignment statements can be in the following two forms
1) x := op y
2) x := y op z
• First statement op is a unary operation. Essential unary
operations are unary logical negation, shift operators and
conversion operators.
• Second statement op is a binary arithmetic or logical
operator.
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Assignment Operator
• An assignment operator is the operator used
to assign a new value to a variable, property, event or
indexer element in C# programming language.
• Assignment operators can also be used for logical
operations such as bitwise logical operations or operations
on integral operands and Boolean operands.
• The “colon equals” (:=) is known as the assignment
operator. You use it to assign a value or an expression that
evaluates to a value to a variable.
• :=
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Jump Statements
• The jump statements are the goto statement, the
continue statement, the break statement, and the
return statement.
• The source statement like if and while-do cause jump in the
control flow through three address code so any statement in
three address code can be given label to make it the target of a
jump.
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Indexed Assignment
• Indexed assignment of the form A:=B[I] and A[I]:=B.
the first statement sets A to the value in the location I
memory units beyond location B.
• In the later statement A [I]:=B, sets the location I units
beyond A to the value of B.
• In Both instructions A, B, and I are assumed to refer
data objects and will represented by pointers to the
symbol table.
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Address and Pointer Assignment
• Address and pointer assignment
x := &y
x := *y
*x := y
• First statement, sets the value of x to be
the location of y.
• In x := *y, here y is a pointer or
temporary whose r-value is a location. The r-value of
x is made equal to the contents of that location.
• *x := y sets the r-value of the object
pointed to by a to the r-value of y.
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Assignment Operations
• Most basic statements for initializing variables
• General syntax:
• variable = expression;
• Example:
• length = 25;
• Expression
• Any combination of constants and variables that
can be evaluated to yield a result
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•All variables used in an expression must
previously have been given valid values.
•Must be a variable listed immediately to the
left of the equal sign.
•Has lower precedence than any other
arithmetic operator.
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Assignment Statements: Compound
Operators
•A shorthand method of specifying a
commonly needed form of assignment.
•Introduced in ALGOL; adopted by C
Example:
a = a + b is written as a += b
•Unary assignment operators in C-based
languages combine increment and decrement
operations with assignment.
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Examples:
• sum = ++count (count incremented,
then assigned to sum)
• sum = count++ (count assigned to sum,
then incremented)
• count++ (count incremented)
• -count++ (count incremented then negated)
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Assignment as an Expression
• In C, C++, and Java, the assignment statement
produces a result and can be used as operands
• An example:
while ((ch=getchar())!= EndOfFile){…}
ch = getchar() is carried out; the result
(assigned to ch) is used as a conditional value for the
while statement
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Simple and Compound Statements
• Simple Statement: A Simple statement is one which
does not contain any augmented statement.
• Read, Assignment and goto are the three examples of
simple statements.
• In certain languages, a program may be regarded
as a sequence of simple statements. (e.g., BASIC,
SNOBOL)
• Compound Statement: A Compound Statement is one
that may contain one or more embedded statements.
• The logical IF in FORTRAN
• IF (condition) statement
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Compound statements: The Compound Statement
enable computation that logically belong together and
be grouped together syntactically.
• This ability of grouping together helps make program
readable.
Example of Compound statement:
1. If condition then statement
2. If condition then statement else statement
3. While condition do statement
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Explanation:
• The if-then and if-then-else statements allows all actions
that apply for the value of condition to be grouped
together.
• The compound statements permits the sequence of the
statements to be used wherever one may appear.
• The while statements is to establish the loops with
arbitrary loop control conditions.
• In compound statement in other languages the syntax
may be widely different.
•Note*: The test for the condition is performed
before the statements allowing the loops to be
executed zero times if necessary.
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Types of Statements
1. Conditional Statements.
2. Sequential Statements.
3. Control Statements.
4.Structural Statements.
5. Declaration Statements.
6.Input Statements.
7. Output Statements.
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Conditional Statements:
• In Conditional statements are features of
a programming language, which perform different
computations or actions depending on whether a
programmer specified boolean condition evaluates to true
or false.
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Conditional statement is a set of rules
performed if a certain condition is met. Below are some
examples of conditional statements in programming.
this last example will print if it is greater than 10, less than 10
or equal to 10.

Sequential Statement:
• In most languages , control automatically flows from one
statement to the next.
• Certain statements such as goto, break, call and return
deliberately alter the flow of control.
• There has been considerable debate as to what statements
are best suited for creating easy to understand flow-of-
control patterns in programs.
• The Sequential statement has been the focus and it permits
the arbitrary flow-of-control patterns, including those that
are difficult to comprehend.
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Control Structure:
• A control statement is a statement that determines whether
other statements will be executed.
• An if statement decides whether to execute
another statement, or decides which of two statements to
execute.
• A loop decides how many times to execute another statement.
• while loops test whether a condition is true before executing
the controlled statement.
• do-while loops test whether a condition is true after executing
controlled statement.
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•The for loops are (typically) used to execute the controlled
statement a given number of times.
•A switch statement decides which of several statements to
execute.

Structural Statements:
• Certain Statements such as END serve to group Simple
statements into structures.
• Following the structured program theorem:
• "Sequence: Ordered statements or subroutines
executed in sequence.
• "Selection: One or a number of statements is
executed depending on the state of the program. This is
usually expressed with keywords such as if..then..else.
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Iteration: a statement or block is executed until the program
reaches a certain state, or operations have been applied to
every element of a collection. This is usually expressed with
keywords such as while, repeat, for or do..until
Recursion: a statement is executed by repeatedly calling itself
until termination conditions are met.

Declaration Statements:
• These statements generally produce no executable
code.
• Their semantic effect is to uniform the compiler about
the attributes of names encountered in the program .
• The compiler implements’ a declaration statement by
storing the attribute information contained therein in
the symbol table and then consulting this information
whenever declared names are used in program.
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•Declaration of a variable serves two purposes: It associates a
type and an identifier (or name) with the variable. The type
allows the compiler to interpret statements correctly.
•Declarations are most commonly used for functions, variables,
constants, and classes, but can also be used for other entities
such as enumerations and type definitions.

Input & Output Statements:
• These statements serve the role as their names .
• In many languages they are implemented by library subroutines
made available by operating system to control input/output
devices.
• It convert data to human readable form.
• In variety of languages we see formats, which are strings of
information about how data is to be interpreted or Expressed
(Read or Write)
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Compiler: Programming Language= Assignments and statements

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