Software II: Principles of

Programming Languages
Lecture 7 – Expressions and
Assignment Statements

Why Expressions?
• Expressions are the fundamental means of
specifying computations in a programming
language
• To understand expression evaluation, need to
be familiar with the orders of operator and
operand evaluation
• Essence of imperative languages is dominant
role of assignment statements

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 Arithmetic Expressions
• Arithmetic evaluation was one of the
motivations for the development of the first
programming languages
• Arithmetic expressions consist of operators,
operands, parentheses, and function calls

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?
– Type mixing in expressions?

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Arithmetic Expressions: Operators
• A unary operator has one operand
• A binary operator has two operands
• A ternary operator has three operands

Arithmetic Expressions: Operator

Precedence Rules
• The operator precedence rules for expression
evaluation define the order in which “adjacent”
operators of different precedence levels are
evaluated
• Typical precedence levels
– parentheses
– unary operators
– ** (if the language supports it)
– *, /
– +, -

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 Arithmetic Expressions: Operator
Associativity Rule
• The operator associativity rules for expression evaluation
define the order in which adjacent operators with the same
precedence level are evaluated
• Typical associativity rules
• Left to right, except **, which is right to left
• Sometimes unary operators associate right to left (e.g.,
in FORTRAN)
• APL is different; all operators have equal precedence and all
operators associate right to left
• Precedence and associativity rules can be overriden with
parentheses

Expressions in Ruby and Scheme

• Ruby
– All arithmetic, relational, and assignment
operators, as well as array indexing, shifts, and bit-
wise logic operators, are implemented as methods
– One result of this is that these operators can all be
overridden by application programs
• Scheme (and Common LISP)
– All arithmetic and logic operations are by
explicitly called subprograms
– a + b * c is coded as (+ a (* b c))

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 Arithmetic Expressions: Conditional
Expressions
• Conditional Expressions
• Appears in C-based languages (e.g., C, C++)
• An example:
average = (count == 0)? 0 : sum / count
• Evaluates as if written as follows:
if (count == 0)
average = 0
else
average = sum /count

Arithmetic Expressions: Operand

Evaluation Order
• Operand evaluation order
1. Variables: fetch the value from memory
2. Constants: sometimes a fetch from memory;
sometimes the constant is in the machine
language instruction
3. Parenthesized expressions: evaluate all operands
and operators first
4. The most interesting case is when an operand is a
function call

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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 expression; e.g., for
a parameter change:
a = 10;
/* assume that fun changes its
parameter */
b = a + fun(&a);

Functional Side Effects

1. Two possible solutions to the problem
– 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 one-way parameters and lack of
non-local references
2. Write the language definition to demand that operand
evaluation order be fixed
– Disadvantage: limits some compiler optimizations
– Java requires that operands appear to be evaluated in left-to-
right order

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 Referential Transparency
• A program has the property of referential
transparency 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
result1 = (fun(a) + b) / (fun(a) – c);
temp = fun(a);
result2 = (temp + b) / (temp – c);
• If fun has no side effects, result1 = result2
• Otherwise, not, and referential transparency is
violated

Referential Transparency (continued)

• Advantage of referential transparency
– Semantics of a program is much easier to understand if
it has referential transparency
• Because they do not have variables, programs in
pure functional languages are referentially
transparent
– Functions cannot have state, which would be stored in
local variables
– If a function uses an outside value, it must be a
constant (there are no variables). So, the value of a
function depends only on its parameters

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 Overloaded Operators
• Use of an operator for more than one purpose
is called operator overloading
• Some are common (e.g., + for int and float)
• Some are potential trouble (e.g., * in C and
C++)
– Loss of compiler error detection (omission of an
operand should be a detectable error)
– Some loss of readability

Overloaded Operators (continued)

• C++, C#, and F# allow user-defined
overloaded operators
– When sensibly used, such operators can be an aid
to readability (avoid method calls, expressions
appear natural)
– Potential problems:
• Users can define nonsense operations
• Readability may suffer, even when the operators make
sense

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 Type Conversions
• A narrowing conversion is one that converts
an object to a type that cannot include all of
the values of the original type e.g., float to
int
• A widening conversion is one in which an
object is converted to a type that can include at
least approximations to all of the values of the
original type e.g., int to float

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 in 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
• In ML and F#, there are no coercions in expressions

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 Explicit Type Conversions
• Called casting in C-based languages
• Examples
– C: (int)angle
– F#: float(sum)
• Note that F#’s syntax is similar to that of
function calls

Errors in Expressions
• Causes
– Inherent limitations of arithmetic
e.g., division by zero
– Limitations of computer arithmetic
e.g. overflow
• Often ignored by the run-time system

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 Relational Operators
• Use relational operators and operands of
various types
• Evaluate to some Boolean representation
• Operator symbols used vary somewhat among
languages (!=, /=, ~=, .NE., <>, #)

Other Relational Operators

• JavaScript and PHP have two additional
relational operator, === and !==
• Similar to their cousins, == and !=, except that
they do not coerce their operands
• Ruby uses == for equality relation operator
that uses coercions and eql? for those that
do not

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 Boolean Expressions
• Operands are Boolean and the result is Boolean
• Example operators (&& || ! AND OR NOT)
• C89 has no Boolean type--it uses int type with 0
for false and nonzero for true
• One odd characteristic of C’s expressions:
– a < b < c is a legal expression, but the result is not what
you might expect:
– Left operator is evaluated, producing 0 or 1
– The evaluation result is then compared with the third
operand (i.e., c)

Short Circuit Evaluation

• An expression in which the result is determined
without evaluating all of the operands and/or
operators
• Example:
(13 * a) * (b / 13 – 1)
• If a is zero, there is no need to evaluate
(b /13 - 1)

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 Short Circuit Evaluation
• 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 is length - 1
long)

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 |)
• All logic operators in Ruby, Perl, ML, F#, and
Python are short-circuit evaluated
• Ada: programmer can specify either (short-circuit
is specified with and then and or else)

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 Short Circuit Evaluation (continued)
• Short-circuit evaluation exposes the potential
problem of side effects in expressions
• Examples
(a > b) || (b++ / 3)
(c != 0 && (c = getchar()) != '\n')

Assignment Statements
• The general syntax
<target_var> <assign_operator>
<expression>
• The assignment operator
= Fortran, BASIC, the C-based languages
:= Ada, Pascal
= can be bad when it is overloaded for the relational operator
for equality (that’s why the C-based languages use == as the
relational operator)

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 Assignment Statements: Conditional
Targets
Conditional targets (Perl)
($flag ? $total : $subtotal) = 0

Which is equivalent to
if ($flag){
$total = 0
} else {
$subtotal = 0
}

Assignment Statements: Compound

Assignment Operators
• A shorthand method of specifying a
commonly needed form of assignment
• Introduced in ALGOL; adopted by C and the
C-based languages
• Example
a = a + b
• can be written as
a += b

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 Assignment Statements: Unary
Assignment Operators
• Unary assignment operators in C-based
languages combine increment and
decrement operations with assignment
• Examples
count = 5;
sum = ++count; // sum = 6
sum = count++ // sum = 6, count = 7
count++ // count = 8
-count++ // count = 9

Assignment as an Expression
• In the C-based languages, Perl, and JavaScript, the
assignment statement produces a result and can be
used as an operand
while ((ch = getchar())!= EOF){…}
ch = getchar() is carried out; the result (assigned
to ch) is used as a conditional value for the while
statement
• Disadvantage: another kind of expression side effect

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 Multiple Assignments
• Perl, Ruby, and Lua allow multiple-target
multiple-source assignments
($first, $second, $third) = (20, 30, 40);
• Also, the following is legal and performs an
interchange:
($first, $second) = ($second, $first);

Assignment in Functional Languages

• Identifiers in functional languages are only
names of values
• ML
– Names are bound to values with val
val fruit = apples + oranges;
– If another val for fruit follows, it is a new and
different name
• F#
– F#’s let is like ML’s val, except let also creates
a new scope

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 Mixed-Mode Assignment
• Assignment statements can also be mixed-
mode
• In Fortran, C, Perl, and C++, any numeric type
value can be assigned to any numeric type
variable
• In Java and C#, only widening assignment
coercions are done
• In Ada, there is no assignment coercion

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