Programming Languages (5): Expressions & Assignment Statements

Expressions & Assignment Statements

Fundamental means of specifying computations in a programming language

Arithmetic Expressions

Expressions that consist of operators, operands, parentheses, and function calls

Unary Operator

An operator that has one operand

Binary Operator

An operator that has two operands

Ternary Operator

An operator that has three operands

Operator Associativity Rules

(Arithmetic Expression) Design issue where it defines the ORDER in which ADJACENT operators with the SAME PRECEDENCE are evaluated

Overloaded Operators

(Arithmetic Expression) Design issue where an operator is used for more than one purpose

Operator Precedence, Operator Associativity, Operand Evaluation Order, Operand Evaluation Side Effects, Operator Overloading, Type Mixing in Expressions, Conditional Expressions

What are possible Arithmetic Expression Design Issues?

Functional Side Effects

(Side Effect) When a function CHANGES a TWO-WAY parameter or a NON-LOCAL variable

[Write the language definition to disallow functional side effects] OR [Write the language definition to demand that operand evaluation order be fixed]

What are possible solutions to Functional Side Effects?

Referential Transparency

A property programs may have if ANY TWO EXPRESSIONS in the program that have the same value CAN BE SUBSTITUTED for one another anywhere in the program, without affecting the action of the program.

Narrow Conversion

(Coercion Type) where it converts an object to a type that CANNOT INCLUDE all of the values of the original type

Widening Conversion

(Coercion Type) where it converts an object to a type that CAN INCLUDE AT LEAST APPROXIMATIONS to all values of the original type

Coercion

Implicit type of conversion

Mixed Mode

(Coercion Type) where it has operands of different types

Explicit Type Conversions

Conversion that gets the value of a variable and converting it to the data type of the variable BUT DOES NOT CHANGE THE VARIABLE’S DATATYPE. Also called “Casting'“ or “Type-Casting” in C-based languages.

Relational Expressions

(Expressions) of user-relational operators and operands of various types. Are used to evaluate some Boolean representation.

Boolean Expressions

(Expressions) where operands are Boolean and the result is Boolean

False (C89 has no Boolean type—it uses “int” type with 0 for false and nonzero for true.)

[TRUE/FALSE] C89 has a Boolean type—it uses “int” type with nonzero for false and 0 for true.

Short Circuit Evaluation

(Expression) in which the result is determined without evaluating all of the operands and/or operators

<target_var> <assign_operator> <expression>

What’s the general syntax of Assignment Statements?

Compound Assignment Operators

A shorthand method of specifying a commonly needed form of assignment

a += b

(Compound Assignment Operators) How would the assignment statement “a = a + b” be shortened?

Unary Assignment Operators

COMBINE INCREMENT and DECREMENT operations with assignment

C-based languages, Perl, JavaScript

What are some PL’s that allow Assignment as an Expression?

Perl, Ruby, Lua

Which PL’s allow multiple-target multiple-source assignments?

TRUE

[TRUE/FALSE] Fortran, C, Perl, and C++ let any numeric type value be assigned to any numeric type value while Java and C# only let widening assignment coercions be done.