isbn 0-321-33025-0 chapter 7 expressions and assignment statements

ISBN 0-321-33025-0

Chapter 7

Expressions and Assignment Statements

Copyright © 2006 Addison-Wesley. All rights reserved. 1-2

Chapter 7 Topics

• Introduction• Arithmetic Expressions• Overloaded Operators• Type Conversions• Relational and Boolean Expressions• Short-Circuit Evaluation• Assignment Statements• Mixed-Mode Assignment


Introduction

• Expressions are the fundamental means of specifying computations in a programming language

• Expressions are motivation for early language

imperative language

dominant role: assignment

must understand: • order of operators• operand evaluation


Arithmetic Expressions: Design Issues

• Design issues for arithmetic expressions– operator precedence rules– operator associativity rules– order of operand evaluation– operand evaluation side effects– operator overloading– mode mixing expressions

more interesting


Arithmetic Expressions: Operators

operators

unary (1 operand)binary (2 operands)ternary (3 operands)

infix - mostprefix - Perl

typical precedence:• parentheses• unary operators• ** (if the language supports it, Fortran, Ruby, VB and Ada do)• *, /, %• +, -

Typical associativity rulesLeft to right, except **, which is right to left, and assignmentSometimes unary operators associate right to left (e.g., in FORTRAN and C-based, ++ and --, also =)

• APL is different; all operators have equal precedence and all operators associate right to left, precedence and associativity rules can be overridden with parentheses would this be more or less confusing?


Arithmetic Expressions: Conditional Expressions

• Conditional Expressions– C-based languages (e.g., C, C++)– Also in Perl, JavaScript and Ruby– An example:

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

– Evaluates as if written likeif (count == 0)

average = 0

else

average = sum /count


Arithmetic Expressions: Potentials for Side Effects

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

• Problem with functional side effects: – When a function referenced in an expression alters another

operand of the same expression; e.g., for a parameter change:

a = 10; /* assume that fun changes its parameter, e.g., to 15 */

b = a + fun(a); • What value will b have after the above statements are

executed?


Functional Side Effects

Two possible solutions to the problem1. Write the language definition to disallow

functional side effects• No two-way parameters in functions• No non-local references in functions• Advantage: it works!• Disadvantage: inflexibility of two-way

parameters and non-local references

2. Write the language definition to demand that operand evaluation order be fixed• Disadvantage: limits some compiler

optimizations** compilers draw graphs to detect when values are created/modified


Referential Transparency

• A program has the property of referential transparency if any two expressions in the program with same value can be substituted for one another anywhere, without affecting the action of the program.

result1 = (fun(a) + b / (fun(a) – c);

temp = fun(a);

result2 = (temp + b) / (temp – c);

• If fun has no side effects, result1 and result2 are equal. If fun modifies b or c, then not equal, violates referential transparency.

• Functional languages are referentially transparent, easier to understand, more like math functions.

Copyright © 2006 Addison-Wesley. All rights reserved.

Overloaded Operators

• & as address vs bitwise AND

ptr = &y;if (x & y == 0) …• - unary vs binaryz = -x + y;z = x – -y;

• Integer vs floating point division

avg = sum/count;• Pascal avoided:avg = sum div count;

would this be a good idea in C++/Java?


Overloaded Operators (continued)

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

• Ruby allows overloading all methods, includes all operators

• Overloading was not included in Java• Potential problems:

– Users can define nonsense operations– Readability may suffer, even when the

operators make sense

Should it have been?


Type Conversions – dangerous?• Widening Conversions: can include at least approximations to all of

the values of the original type – byte to short, int, long, float or double

– short to int, long, float or double

– char to int, long, float or double

– int to long, float or double

– long to float or double

– float to double

• Narrowing conversions: cannot include all of the values of the original type – short to byte or char

– char to byte or short

– int to byte, short, or char

– long to byte, short, char or int

– float to byte, short, char, int or long

– double to byte, short, char, int, long or float


Type conversions (continued)

• Even widening conversions may lose accuracy. Example: integers stored in 32 bits, 9 digits of precision. Floating point values also stored in 32 bits, only about 7 digits of precision (because of space used for exponent).

Conversions should be used with care! Warnings shouldnot be ignored…


Type Conversions: Mixed Mode

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

• A coercion is an implicit type conversion• Disadvantage of coercions:

– They decrease the type error detection ability of the compiler

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

• In Ada, there are virtually no coercions in expressions


Explicit Type Conversions

• Explicit Type Conversions• Called casting in C-based language• Examples

– C: (int) angle– Ada: Float (sum)

Note that Ada’s syntax is similar to function calls


Time now…

• To do some Perl!


Relational and Boolean Expressions

• Relational Expressions– Use relational operators and operands of

various types– Evaluate to some Boolean representation

How do you say “not equal”?!=

/=#

<>

.NE.

Do you want to coerce that? !=====eql?

Leave me like youfind me! (JavaScript/Perl)

Ruby


Relational and Boolean Expressions• Boolean Expressions

– Operands are Boolean and the result is Boolean

– AND and OR have equal precedence in some languages, not others (AND higher in C++)

FORTRAN 77 FORTRAN 90 C Ada

.AND. and && and .OR. or || or

.NOT. not ! not

xor


Relational and Boolean Expressions• C has no Boolean type (0=false, >0 =

true)• SO: a < b < c is a legal expression• but is the result what you want????

– (a < b) => produces 0 or 1– (0 < c) OR (1 < c) Who cares whether 0 < c ???

• Some error detection is lost if no Boolean type

How is a < b < c evaluated by compiler in Java?


Relational and Boolean Expressions: Operator Precedence

• Precedence of C-based operatorspostfix ++, --

unary +, -, prefix ++, --, !

*,/,%

binary +, -

<, >, <=, >=

==, !=

&&

|| Why should || be lower than &&?


Short Circuit Evaluation

• An expression in which the result is determined without evaluating all of the operands and/or operators

• (a >= 0) && (b < 10) • Problem with non-short-circuit evaluation:

index = 0;while (index < length) && (LIST[index] != value)

index++;– When index=length, LIST [index] will cause an

indexing problem (assuming LIST has length -1 elements)


Short Circuit Evaluation (continued)• C, C++, and Java: use short-circuit evaluation for

the usual Boolean operators (&& and ||), but also provide bitwise Boolean operators that are not short circuit (& and |)

• Ruby, Perl and Python are short-circuit evaluated• Short-circuit is a choice in Ada• Short-circuit evaluation exposes the potential

problem of side effects in expressions e.g. (a > b) || (b++ / 3)

will b be incremented or not?


Assignment Statements

• The general syntax<target_var> <assign_operator> <expression>

• The assignment operator= FORTRAN, BASIC, PL/I, C, C++, Java:= ALGOLs, Pascal, Ada

• = can be bad when it is overloaded for the relational operator for equality

• confusing for beginners even when it’s not overloaded


Assignment Statements: Conditional Targets

• Conditional targets (C, C++, and Java)(flag)? total : subtotal = 0

Which is equivalent to

if (flag)

total = 0

else

subtotal = 0


Assignment Statements: Shorthand Syntax

• Adding two values:a = a + b

is written asa += b

– Introduced in ALGOL; adopted by C, Perl, JavaScript, Python and Ruby

• Incrementing or decrementing:sum = ++count (count incremented, added to sum)

sum = count++ (count incremented, added to sum)

count++ (count incremented)

-count++ (count incremented then negated)– Included in C-based languages, Perl and JavaScript


Assignment as an Expression

• In C, C++, Java, Perl and JavaScript the assignment statement produces a result and can be used as operand

• An example: while ((ch = getchar())!= EOF){…}

ch = getchar() is carried out; the result (assigned to ch) is used as a conditional value for the while statement; must be in (); can be harder to read


List Assignments

• Some recent languages like Perl and Ruby provide multiple-target, multiple-source assignments

• In Perl:($first, $second, $third) = (20, 30, 30);($first, second) = ($second, $first);excess values on right ignoredexcess values on left set to undef

How useful is this feature?


Mixed-Mode Assignment

• Assignment statements can also be mixed-mode, for exampleint a, b;

float c;

c = a / b;

• Coercion takes place after rhs evaluated• Fortran, C, C++ and Perl use coercion

rules for mixed mode assignment• In Java, only widening assignment

coercions are done (increases reliability)• In Ada, there is no assignment coercion


Time now…

• To do some Perl!

isbn 0-321-33025-0 chapter 7 expressions and assignment statements

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