CHAPTER 6

EXPRESSIONS AND ASSIGNMENT STATEMENTS

INTRO:

• Expressions are the fundamental means of specifying computations in a programming language. It is important for programmers to understand the syntax and the semantics of expressions.
• To understand expression evaluation, it is necessary to be familiar with the orders of operator and operand evaluation. The operator evaluation order of expressions is governed by the associativity and precedence rules of the language.
• The essence of the imperative programming languages is the dominant role of assignment statements. The purpose of assignment statement is to change the value of a variable. So an integral part of all imperative languages is the concept of variables whose values change during program execution.

ARITHMETIC EXPRESSIONS

• Most of the characteristics of arithmetic expressions in programming languages were inherited from conventions that had evolved in mathematics. In programming languages, arithmetic expressions consist of operators, operands, parentheses, and function calls.
• The operators can be unary, meaning they have single operand, or binary, meaning they have two operands.
• C, C++, and Java include a ternary operator, which has three operands.
• In most imperative languages binary operators are infix which mend they appear between their operands.
• The purpose of an arithmetic expression is to specify an arithmetic computation. The two actions of this are:
• fetching the operands, usually from memory,
• executing the arithmetic operations on those operands

OPERATOR EVALUATION ORDER

PRECEDENCE

• The order of an expression depends on the order of evaluation of the operators
• The operator precedence rules for expression evaluation define the order in which the operators of different precedence levels are evaluated. These rules are based on the hierarchy of operator priorities.
• Unary addition is called identity operator because it usually has no associated operation and thus has no effect on its operand.

ASSOCIATIVITY

• Ex. A – B + C – D
• When an expression contains two adjacent occurrences of operators with the same level of precedence, the question of which operator is evaluated first is answered by the associativity rules of the language.
• An operator can either have left or right associativity, meaning that the leftmost occurrence is evaluated first or the rightmost occurrence is evaluated first, respectively.
• A – B + C in FORTRAN the left opertor is evaluated first.
• Exponentiation in FORTRAN: A ** B ** C the right operator is evaluated first.
• A ** B ** C, in Ada is illegal, but, (A ** B) ** C is legal.
• The associativity rules of some imperative languages are:

Language Associativity Rule

FORTRAN Left: *, /, +, -

Right: **

Pascal Left: All

C Left: postfix ++, postfix --, *, /, %, binary +, binary –

Right: prefix ++, prefix --, unary +, unary –

C++ Left: *, /, %, binary +, binary –

Right: ++, --, unary -, unary +

Ada Left: all except **

Non-associative: **

PARENTHESES

• Programmers can alter the precedence and associativity rules by placing parentheses in expressions
• (A + B) * C here the addition is done first.

CONDITIONAL EXPRESSIONS

• Sometimes if-then-else statements are used to perform a conditional expression assignment.
• The ternary operator, ?:, which is part of C, C++, and Java is used to form conditional expressions.

If (count = 0)

Then average := 0

Else average := sum / count

Can become:

Average = (count == 0) ? 0 : sum / count;

OPERAND EVALUATION ORDER

SIDE EFFECTS

• A side effect of a function, called a functional side effect, occurs when the function changes either one of its parameters or a global variable. (A global variable is declared outside the function but is accessible in the function.)
• Ex. A + FUN(A)

if FUN does not have the side effect of changing A, then the order of evaluation of the two operands, A and FUN(A), has no effect on the value of expression. But, if FUN changes A, there is an effect.

OVERLOADED OPERATORS

• Operator overloading is the multiple use of operators.
• Ex., the “+” is used for addition and for string concatenation in Java.
• The “&’ in C is a problem and overloading the minus operator is also a problem.

TYPE CONVERSIONS

• Type conversions are either narrowing or widening.
• A narrowing conversion converts a value to a type that cannot store even approximations of all the values of the original type.
• Ex., converting a double to float (double is larger than float)
• A widening conversion converts a value to a type that can include at least approximations of all of the values of the original type. Ex., converting an int to a float

COERCION IN EXPRESSIONS

• One of the design decisions concerning arithmetic expressions is whether an operator can have operands of different types. Languages that do allow such expressions, which are called mixed-mode expressions, must define conventions for implicit operand type conversions, called coercions, because computers usually do not have binary operations that take operands of different types.
• Coercions defined as an implicit type conversion that is initiated by the compiler.

RELATIONAL AND BOOLEAN EXPRESSIONS

RELATIONAL EXPRESSIONS

• A relational operator is an operator that compares the value if its two operands.
• A relational expression has two operands and one relational operator. The value of a relational expression is Boolean, except when Boolean is not a type in the language. The relational operators are usually overloaded for a variety of types
• The syntax of the relational operators in some languages:

Operation Pascal Ada C FORTRAN77

Equal = = == .EQ.

Not equal <> /= != .NE.

Greater than > > > .GT.

Less than < < < .LT.

Greater than or equal >= >= >= .GE.

Less than or equal <= <= <= .LE.

BOOLEAN EXPRESSIONS

• Boolean expressions consist of Boolean variables, Boolean constants, relational expressions, and Boolean operators.
• The operators include those for the AND, OR, and NOT operations, and sometimes for exclusive OR and equivalence.
• The Boolean operators are evaluated like the arithmetic operators, in a hierarchical precedence order.

SHORT-CIRCUIT EVALUATION

• A short-circuit evaluation of an expression is one in which the result is determined without evaluating all of the operands and/or operators.
• Ex., (13 * A) * (B / 13 – 1) is independent of the value (B / 13 – 1) if a is 0. Because 0*x = 0 for any x. So if A is 0 there is no need to evaluate (B / 13 – 1).
• Short-circuit evaluation of expressions exposes the problem of allowing side effects in expressions.

ASSIGNMENT STATEMENTS

• The assignment statement is one of the most central constructs in imperative languages. It provides the mechanism by which the user can dynamically change the bindings of values to variables.

SIMPLE ASSIGNMENTS

• The general syntax of simple assignments is;

<target_variable> <assignment_operator> <expression>

• FORTRAN, BASIC, PL/1, C, C++, and Java use the equal sign for the assignment operator. This can lead to confusion if the equal sign is also used as a relational operator, as it is in PL/1 and BASIC.

COMPOUND ASSIGNMENT OPERATORS

• A compound assignment operator is a shorthand method of specifying a commonly needed form of assignment. The form of assignment that can be abbreviated with this technique has the destination variable also appearing as the first operand in the expression on the right side as in,

a = a + b

• Compound assignment operators were introduced by ALGOL 68
• The syntax of C's compound assignment operators is the desired binary operator to the = operator.

For example, sum += value;

Is equivalent to sum = sum + value;

UNARY ASSIGNMENT OERATORS

• C, C++, and Java include 2 special unary arithmetic operators that are actually abbreviated assignments. They combine increment and decrement operators with assignment.
• ++ used for increment
• -- used for decrement

ASSIGNMENT AS AN EXPRESSION

• C, C++, and Java the assignment statement produces a result, which is the same as the value assigned to the target. So, it can be used as an expression and as an operand in other expressions.
• The disadvantage of allowing assignment statements to be operands in expressions is that it provides yet another kind of expression side effect. This type of side effect can lead to expressions that are difficult to read and understand.