expressions arithmetic expressions -...

Expressions and Assignments

Fundamentals of Programming

Languages

Flow Control

! Sequence of Execution

" Within an Expression

# Functions that evaluate data objects and return a value

" Among program Statements

# Complete syntactic unit or sentence

" Between program Units

# Different Reference Environments

# Single in/out points (data stored on stacks)

Expressions

! Fundamental means of specifying computations in a programming language

! Motivation behind programming languages

! Arithmetic Expressions

" Return a numeric value

! Predicate Expressions

" Return TRUE or FALSE

! Postfix, Infix and Prefix Expressions Styles

Arithmetic Expressions

! Key Issues to Understand

" What are Operators?

" Operator Precedence Rules

" Operator Associativity Rules

" Order of Operand Evaluation

" Restrictions on operand evaluation side effects

" User-defined operator overloading

" Mode mixing in expressions


! Operators

" Evaluate operands and return a single value

" Unary – single operand

" Binary – two operands

" Ternary – three operands

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

count;

! Operator Precedence Rules

" Defines the order in which adjacent operators of

different precedence level are evaluated.

" Operators are adjacent of they are separated by at

most one operand


! The Usual Precedence Levels:()

Unary operators --, ++, new, delete, cast, not, *, dereference

Exponentiation ** or ^

Unary +, -

*, /, %

+, -

Shift operators <<< >>> <<= >>=

Inequality comparisons <, <=, >, >=

Equality Comparisons ==, !=

AND

OR

throw exception

comma

! Operator Associativity

" order in which adjacent operators of the same

precedence level are evaluated.

! Typical Associativity Rules

" Left to right evaluation for binary operators

# Except exponentiation (**) which is right to left

# Except prefix ++, -- and unary +, -

" Unary operators associate right to left

! Precedence and Associativity can be

overridden with parenthesis.



! Operand Evaluation Order

" Variables

# Fetch the value from memory

" Constants

# Fetch the value from memory

# Can be included in machine code instruction (+1)

" Parenthesized Expressions

# Evaluate enclosed operands and operators first

" Function References

# Hmmmmm? Very interesting……


! Side Effects of Evaluation Order - Functions" Function changes a two-way parameter

(reference)" Function changes a global variable" Function used in an expression alters another

operand used in the expression." Compute the result of the expression:

int fun(int &a){return a=a * 3;}main(){

int b = 6;

int c = b + fun(b);}


! Solutions are in language definitions:

" Disallow functional side effects

# No two-way parameters in functions

# No nonlocal references in functions

# This is very restrictive to programmers

" Demand operand evaluation order be fixed

# Limits compiler optimizations

# C - functions always evaluated first


! Side Effects of Evaluation Order - Overflow

" Assume integer range of:

–32,678 to 32,676

" Compute the result of the expression

25,000 + 20,000 – 15,000

" Left to right evaluation results in overflow

" Right to left evaluation gives correct value


! Operator Overloading

" The operation executed by the operator symbol

is dependent on the operands (type, number,

etc.)

" Examples:

# Integer addition

int + int

# Floating point addition

float + float

# String concatenation

string + string


! Operator Overloading

" Reduces compiler ability to perform error checking.

" Loss of readability

" Avoid need to overload by introducing new symbols

# div for integer division in Pascal

" User-defined overloading decreases readability

farther, but provides tremendous programming

flexibility.

Type Conversions

! Narrowing Conversion" Converts an object to one that cannot include all of

the values of the original type. (float to int)

! Widening Conversion" Converts an object into a type that can include all of

the values of the original types. (int to float)

! Mixed-Mode Expression" Expression with operand of different types

! Coercion" An implicit type conversion

! Cast" An explicit type conversion

Type Conversions

! Decreases compiler type error detection ability

! Numeric types are frequently coerced in expressions using widening.

! Avoid coercion – use casting

" Increased reliability and readability

Predicate Expressions

! Relational Expressions

" Relate or compare two objects (data items)

" Returns a Boolean value (true/false)

" Operator symbols can vary:

not equal to: !=, /=, .NE., <>, #

" Side Effects:

# A legal expresion in C:

A < B < C

# Evaluates as:

(A < B) is true or false so we get

true < C or false < C

Predicate Expressions

! Boolean Expressions

" Operands are Booleans

" Result is Boolean

" ‘C’ has no Boolean type

# Uses int

# FALSE is zero

# TRUE is non-zero

Short Circuit Evaluation

! Terminating the evaluation of an expression as soon as the result can be determined.

! Boolean Expressions

(a >= 0) and (c < b)

! C, C++, and Java:

" use short-circuit evaluation for Boolean

operators (&& and ||)

" provide bitwise Boolean operators that are not

short circuit (& and |)


! Important in loop control evaluation.

" Prevents index overflow

" But – AND & OR usually have low precedence

! Can expose side effects in code:

(a > b) || (b++ / 3)


" Search through an array list[] to find key

int list[maxIndex];

i = 0; . . .

while ((i < maxIndex) && (list[i]<>key))

{ i = i + 1; }

! Linked list with zero or one node: if (head==NULL || head.getNext()==NULL)

versus

if (head.getNext()==NULL || head==NULL)

Assignment Statements

! Operator Symbol

" FORTRAN, BASIC, PL/I, C, C++, Java

=" ALGOLs, Pascal, Ada

:=

" PL/I also uses = for relational equality

A = B = C;


! Multiple Targets (PL/I)

a, b = 10;

! Compound Assignment Operators (C, C++, Java)

sum += next;

! Unary Assignment Operators (C, C++, Java)

a++;

! Arithmetic Binary Operator (C, C++, Java)

a = b * (c = d * 2 + 1) + 1

! Conditional Targets (C, C++, Java)

(first == true) ? total : subtotal = 0


" Assignment as an expression

" In C, C++, and Java,

# assignment statement produces a result

# can be used as operands in an expression

while ((ch = getchar() != EOF) { ... }

" Has wicked-bad & extremely evil expression side effectint x = 4;

if ( x = 2)

cout << “x is 2!”;

else

cout << “x is not 2!”;


! Mixed-Mode Assignment

int = float

" In FORTRAN, C, and C++, # any numeric value can be assigned to any numeric scalar

variable; whatever conversion is necessary is done

" In Pascal, # integers can be assigned to reals, but reals cannot be

assigned to integers (the programmer must specify whether the conversion from real to integer is truncated or rounded)

" In Java, # only widening assignment coercions are done

" In Ada, # there is no assignment coercion

expressions arithmetic expressions -...

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