CHAPTER 7 - Expressions and Assignment Statements

Expressions

the fundamental means of specifying computations in a programming language

4 components of arithmetic expressions

Operators -- Operands -- Parentheses -- Function Calls

3 types of operators by operand count

Unary (1 operand) -- Binary (2 operands) -- Ternary (3 operands)

Ternary operator

has three operands -- used in C-based languages as a conditional expression -- syntax: expression_1 ? expression_2 : expression_3

2 actions of arithmetic expression implementation

fetching operands from memory -- executing arithmetic operations on those operands

6 design issues for arithmetic expressions

operator precedence rules -- operator associativity rules -- order of operand evaluation -- restrictions on side effects -- user-defined operator overloading -- mode mixing in expressions

Operator Precedence

defines the order in which operators of different precedence levels are evaluated

Unary Addition (+)

called the identity operator -- no associated operation -- no effect on its operand

Unary Minus (-) in Java/C#

causes implicit conversion of short and byte operands to int type

Operator Associativity

defines the order in which adjacent operators with the same precedence level are evaluated -- can be left or right associative

Typical associativity

left to right -- except ** which is right to left

APL associativity

all operators have equal precedence -- all associate right to left

Overriding precedence and associativity

done with parentheses

Functional side effect

occurs when a function changes one of its parameters or a global variable

2 solutions to functional side effects

write language definition to disallow side effects -- write language definition to demand fixed operand evaluation order

Java operand evaluation order

guarantees left-to-right order -- eliminates side effect ambiguity

Operator Overloading

the use of an operator for more than one purpose

& in C/C++ -- overloading problem

unary: address-of operator -- binary: bitwise AND -- using the same symbol for two unrelated operations harms readability

in Java -- overloading

addition for numbers -- string catenation for strings

Languages allowing user-defined operator overloading

C++ -- C# -- F#

2 potential problems of user-defined operator overloading

users can define nonsense operations -- readability may suffer

Narrowing Conversion

converts an object to a type that cannot include all values of the original type -- example: double to float

Widening Conversion

converts an object to a type that can include at least approximations to all values of the original type -- example: int to float

Coercion in Expressions

implicit type conversion in a mixed-mode expression -- disadvantage: decreases type error detection ability of the compiler

Mixed-mode expression

an expression that has operands of different types

Languages with no coercions in expressions

ML -- F#

Mixed-mode assignment -- Fortran/C/C++

any numeric value can be assigned to any numeric scalar variable

Mixed-mode assignment -- Java/C#

only widening assignment coercions are done

Mixed-mode assignment -- Ada

no assignment coercion

Casts

explicit type conversions in the C-based languages

Relational Operator

an operator that compares the values of its two operands -- result is Boolean

Boolean in C (before C99)

no Boolean type -- uses int -- 0 is false -- nonzero is true

Short-Circuit Evaluation

result is determined without evaluating all operands and/or operators

Equal sign

Assignment operator symbols in ALGOL, Pascal, Ada

Compound Assignment Operators

shorthand where destination variable also appears as first operand on the right side

Unary Assignment Operators (++ and --)

combine increment/decrement with assignment -- can be prefix or postfix

Prefix ++ behavior

increments first then uses the value

Postfix ++ behavior

uses the value first then increments

Assignment as an Expression

treats assignment operator like a binary operator -- has side effect of changing its left operand -- leads to difficult-to-read expressions -- common source of errors in C

Multiple Assignments

languages that support multiple-target multiple-source assignments: Perl -- Ruby -- Lua

Assignment in Functional Languages

identifiers are only names of values -- no mutable assignment