CptS 355 Programming Language Design

1
CptS 355Programming Language Design
Values, Expressions, Conversions

• Roger Ray
• WSU Vancouver, Spring 2001

values.ppt
2
Arithmetic Expressions

• Their evaluation was one of the motivations for
  the development of the first programming
  languages
• They consist of operators, operands, parentheses,
  and function calls
• Design choices are surprisingly varied!

?Advice
3
Operator Syntax

• Terminology
• A unary operator has one operand
• A binary operator has two operands
• A ternary operator has three operands
• The operator precedence rules define the order in
  which adjacent operators of different precedence
  levels are evaluated. Typical levels
• function calls
• parentheses
• unary operators
• (if the language supports it)
• , /
• , -
• gt, lt, ...
• The operator associativity rules define the order
  in which adjacent operators with the same
  precedence level are evaluated.
• What are some common rules?

C, C, and Java have over 50 operators and 17
different levels of precedence
?Advice
4
Side Effects

• When a function referenced in an
  expressionalters another operand of the
  expression
• Solution 1 Write the language definition to
  disallow functional side effects
• No two-way parameters in functions
• No nonlocal references in functions
• Solution 2 Write the language definition to
  specify operand evaluation order
• Eval operands left-to-right

a 10 b a fun(a) / Assume that fun
returns 1, but alters a to 2 /
Discussion
?Advice
Discussion
5
Unusual Expressions

• Conditional (If-Then-Else) expressions
• Sequence (next value) expressions

average (count 0)? 0 sum / count
C/Java
average (printf ("here we are"), sum/count)
C/Java
?Advice
6
Operator Overloading

• Some is common
• for int and float
• Some is surprising
• for numbers and text (Java)
• Some is potential trouble
• / for int and float (C, Java)
• Some is extensible
• C and Ada allow user-defined overloads

Discussion
?Advice
7
Type Conversions

• A mixed-mode expression has operands of different
  types
• A narrowing conversion may lose information
• A widening conversion preserves information (at
  least approximately)
• An implicit type conversion is often called a
  coercion
• An explicit conversion is often called a cast

Examples
Discussion
?Advice
8
Boolean Expressions

• Comparison symbols used vary among languages
• "not equal" !, /, .NE., ltgt,
• Boolean operators also vary
• FORTRAN 77 FORTRAN 90 C Ada
• .AND. and and
• .OR. or or
• .NOT. not ! not
• What does "short circuit" mean?
• C/Java yes
• Pascal no
• Ada explicit choice
• C/Java Oddities
• C has no boolean type
• a lt b lt c is not what you might expect
• similar bit operations

Discussion
?Advice
9
Assignment Oddities

• Dual meaning
• Multiple targets
• Conditional targets
• Compound assignment operators
• Unary assignment operators
• Statement or operator?
• Implicit conversions

x y z
PL/1
x y z
Algol
a, b 10 x, y y, x
PL/1 published
(a b) ? x y 0
C
C
sum next
i
C
while ((c getchar()) ! EOF)
C
i x
C
?Advice