Chapter 1

ISBN 0-321-49362-1

Chapter 7

Expressions andAssignmentStatements

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Chapter 7 Topics

•Introduction

•Arithmetic Expressions

•Overloaded Operators

•Type Conversions

•Relational and Boolean Expressions

•Short-Circuit Evaluation

•Assignment Statements

•Mixed-Mode Assignment

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Introduction

•Expressions are the fundamental means ofspecifying computations in a programminglanguage

•To understand expression evaluation, needto be familiar with the orders of operatorand operand evaluation

•Essence of imperative languages isdominant role of assignment statements

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Arithmetic Expressions

•Arithmetic evaluation was one of themotivations for the development of the firstprogramming languages

•Arithmetic expressions consist ofoperators, operands, parentheses, andfunction calls

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Arithmetic Expressions: Design Issues

•Operator precedence rules?

•Operator associativity rules?

•Order of operand evaluation?

•Operand evaluation side effects?

•Operator overloading?

•Type mixing in expressions?

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Arithmetic Expressions: Operators

•A unary operator has one operand

•A binary operator has two operands

•A ternary operator has three operands

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Arithmetic Expressions: OperatorPrecedence Rules

•The operator precedence rules forexpression evaluation define the order inwhich operators in an expression that aredıfferent from one another are evaluated

•Typical precedence levels

– parentheses

– unary operators

– ** (power operator)

– *, /

– +, -

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Arithmetic Expressions: OperatorAssociativity Rule

•The operator associativity rules for expressionevaluation define the order in which adjacentidentical operators are evaluated

•Typical associativity rules

–Left to right, except **, which is right to left

–Sometimes unary operators associate right to left

•APL is different; all operators have equalprecedence and all operators associate right to left

•Precedence and associativity rules can be overridenwith parentheses

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Arithmetic Expressions: ConditionalExpressions

•C-based languages (e.g., C, C++)

–An example:

average = (count == 0)? 0 : sum / count

–Evaluates as if written like

if (count == 0)

average = 0

else

average = sum /count

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Arithmetic Expressions: Potentials forSide Effects

•Functional side effects: when a function changes atwo-way parameter or a non-local variable

•Problem with functional side effects:

–When a function referenced in an expression altersanother operand of the expression; e.g., for a parameterchange:

a = 10;

/* assume that fun changes its parameter */

b = a + fun(a);

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Overloaded Operators

•Use of an operator for more than onepurpose is called operator overloading

•Some are common (e.g., + for int andfloat)

•C++, Ada, Fortran 95, and C# allow user-defined overloaded operators

•Potential problems:

–Readability may suffer. Same operator, differentfunctionality.

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Type Conversions

•A narrowing conversion is one that convertsan object to a type that cannot include allof the values of the original type e.g.,float to int

•A widening conversion is one in which anobject is converted to a type that caninclude at least approximations to all of thevalues of the original typee.g., int to float

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Type Conversions: Mixed Mode

•A mixed-mode expression is one that hasoperands of different types

•A coercion is an implicit type conversion

•Disadvantage of coercions:

–They decrease in the type error detection ability of thecompiler

•In most languages, all numeric types are coercedin expressions, using widening conversions

•In Ada, there are virtually no coercions inexpressions

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Explicit Type Conversions

•Done by the user explicitly

•Called casting in C-based languages

•Examples

–C: (int)angle

–Ada: Float (Sum)

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Type Conversions: Errors in Expressions

•Causes

–Inherent limitations of arithmetice.g., division by zero

–Limitations of computer arithmetice.g. overflow

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Relational and Boolean Expressions

•<, >, >=, <=,

•!=, ==, .NE., <>

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Relational and Boolean Expressions

•Boolean Expressions

–Operands are Boolean and the result is Boolean

–Example operators

FORTRAN 90C Ada

and && and

or || or

not ! not

xor

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Relational and Boolean Expressions: NoBoolean Type in C

•C uses int type with 0 for false andnonzero for true

•One odd characteristic of C’s expressions:a < b < c is a legal expression, but theresult is not what you might expect:

–Left operator is evaluated, producing 0 or 1

–The evaluation result is then compared with thethird operand (i.e., c)

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Short Circuit Evaluation

•An expression in which the result isdetermined without evaluating all of theoperands and/or operators

•Example: (13*a) * (b/13–1)

If a is zero, there is no need to evaluate (b/13-1)

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Short Circuit Evaluation (continued)

•C, C++, and Java: use short-circuit evaluation forthe usual Boolean operators (&& and ||), but alsoprovide bitwise Boolean operators that are notshort circuit (& and |)

•Short-circuit evaluation exposes the potentialproblem of side effects in expressionse.g. (a > b) || (b++ / 3)

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Assignment Statements

•The general syntax

<target_var> <assign_operator> <expression>

•The assignment operator

= FORTRAN, BASIC, the C-based languages

:= ALGOL, Pascal, Ada

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Assignment Statements: ConditionalTargets

•Conditional targets (Perl)($flag ? $total : $subtotal) = 0

Which is equivalent to

if ($flag){

$total = 0

} else {

$subtotal = 0

}

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Assignment Statements: CompoundOperators

a = a + b

is written as

a += b

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Assignment Statements: UnaryAssignment Operators

•Combine increment and decrementoperations with assignment

•Examples

sum = ++count (pre-increment)

sum = count++ (post-increment)

count++

-count++ (count incremented then negated)

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Assignment as an Expression

•In C, C++, and Java, the assignmentstatement produces a result and can beused as operands

•An example:

while ((ch = getchar())!= EOF){…}

ch = getchar() is carried out; the result(assigned to ch) is used as a conditionalvalue for the while statement

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List Assignments

•Perl and Ruby support list assignments

e.g.,

($first, $second, $third) = (20, 30, 40);

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Mixed-Mode Assignment

•Example

int a, b;

float c;

c = a / b;

•In Fortran, C, and C++, any numeric typevalue can be assigned to any numerictype variable

•In Java, only widening assignmentcoercions are done

•In Ada, there is no assignment coercion

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Summary

•Expressions

•Operator precedence and associativity

•Operator overloading

•Mixed-type expressions

•Various forms of assignment