1. Expressions and Assignment Statements
Chapter 7
Expressions and Assignment Statements

2. Chapter 7 Topics Introduction Arithmetic Expressions
Overloaded Operators
Type Conversions
Relational and Boolean Expressions
Short-Circuit Evaluation
Assignment Statements
Mixed-Mode Assignment
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3. Introduction
Expressions are the fundamental means of specifying computations in a programming language
Expressions are motivation for early language
dominant role: assignment
imperative language
must understand:
order of operators
operand evaluation
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4. 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
mode mixing expressions
more
interesting
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5. Arithmetic Expressions: Operators
infix - most
prefix - Scheme
unary (1 operand)
binary (2 operands)
ternary (3 operands)
operators
Typical associativity rules
Left to right, except assignment and **, which are right to left
Sometimes unary operators associate right to left (e.g., in FORTRAN and C-based, ++ and --, also =)
typical precedence:
parentheses
unary operators
** (if the language supports it,
Fortran, Ruby, VB and Ada do)
*, /, %
+, -
APL is different; all operators have equal precedence and all operators associate right to left, precedence and associativity rules can be overridden with parentheses
would this be more or less confusing?
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6. Arithmetic Expressions: Conditional Expressions
C-based languages (e.g., C, C++)
Also in Perl, JavaScript and Ruby
An example:
average = (count == 0)? 0 : sum / count
Evaluates as if written like
if (count == 0)
average = 0
else
average = sum /count
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7. 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 same expression; e.g., for a parameter change:
a = 10;
b = a + fun(a);
void fun(int& a) { a = 15; }
What value will b have after the above statements are executed?
The order in which operands in an expression are evaluated is unspecified in C++. The only guarantee is that they will all be completely evaluated at the next sequence point.
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8. Functional Side Effects
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
Write the language definition to demand that operand evaluation order be fixed
Disadvantage: limits some compiler optimizations*
* compilers draw graphs to detect when values are created/modified
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9. Referential Transparency
A program has the property of referential transparency if any two expressions in the program with same value can be substituted for one another anywhere, 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 and result2 are equal. If fun modifies b or c, then not equal, violates referential transparency.
Functional languages are referentially transparent, easier to reason about, more like math functions.
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10. Overloaded Operators & as address vs bitwise AND ptr = &y;
if (x & y == 0) …
- unary vs binary
z = -x + y;
z = x – -y;
Integer vs floating point division
avg = sum/count;
Pascal avoided:
avg = sum div count;
would this be a good idea
in C++/Java?
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11. Overloaded Operators (continued)
C++, Ruby, Ada and C# allow user-defined overloaded operators
Overloading was not included in Java
Potential problems:
Users can define nonsense operations
Readability may suffer, even when the operators make sense
Should it have been?
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12. Type Conversions
Widening Conversions: can include at least approximations to all of the values of the original type
byte to short, int, long, float or double
short to int, long, float or double
char to int, long, float or double
int to long, float or double
long to float or double
float to double
Narrowing conversions: cannot include all of the values of the original type
short to byte or char
char to byte or short
int to byte, short, or char
long to byte, short, char or int
float to byte, short, char, int or long
double to byte, short, char, int, long or float
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13. Type conversions – dangerous?
Even widening conversions may lose accuracy. Example: integers stored in 32 bits, 9 digits of precision. Floating point values also stored in 32 bits, only about 7 digits of precision (because of space used for exponent).
Conversions should be used with care! Warnings should
not just be ignored…
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14. 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 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
Ada is strongly typed
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15. Explicit Type Conversions
Called casting in C-based language
Examples
C: (int) angle
Ada: Float (sum)
Note that Ada’s syntax is similar to function calls – may be better because casting does have associated code (convert bit patterns)
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16. Relational and Boolean Expressions
Relational Expressions
Use relational operators and operands of various types
Evaluate to some Boolean representation
.NE. /=
How do you say “not equal”? # !=
<>
Do you want to coerce that?
!==
===
eql?
Leave me like you
find me!
(JavaScript/Perl)
Ruby
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17. Relational and Boolean Expressions
Operands are Boolean and the result is Boolean
AND and OR have equal precedence in some languages, not others (AND higher in C++)
FORTRAN FORTRAN C Ada
.AND and && and
.OR or || or
.NOT not ! not
xor
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18. Relational and Boolean Expressions - review
C has no Boolean type (0=false, >0 = true)
SO: a < b < c is a legal expression
but is the result what you want????
(a < b) => produces 0 or 1
(0 < c) OR (1 < c) Who cares whether 0 < c ???
Some error detection is lost if no Boolean type
How is a < b < c evaluated by compiler in Java?
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19. Short Circuit Evaluation - review
An expression in which the result is determined without evaluating all of the operands and/or operators
(a >= 0) && (b < 10)
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 last index of LIST is length - 1)
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20. 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 |)
Ruby, Perl and Python are short-circuit evaluated
Short-circuit is a choice in Ada
Short-circuit evaluation exposes the potential problem of side effects in expressions e.g. (a > b) || (b++ / 3)
will b be incremented or not?
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21. Assignment Statements
The general syntax
<target_var> <assign_operator> <expression>
The assignment operator
= FORTRAN, BASIC, PL/I, C, C++, Java
:= ALGOLs, Pascal, Ada
= can be bad when it is overloaded for the relational operator for equality
confusing for beginners even when it’s not overloaded
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22. Assignment Statements: Conditional Targets
Conditional targets (C, C++, and Java) (flag)? total : subtotal = 0
Which is equivalent to
if (flag)
total = 0
else
subtotal = 0
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23. Assignment Statements: Shorthand Syntax
Adding two values:
a = a + b
is written as
a += b
Introduced in ALGOL; adopted by C, Perl, JavaScript, Python and Ruby
Incrementing or decrementing:
int count=4;
int sum=++count; // increment then use
int count2=4;
int sum2=count2++; // use then increment
cout << sum << " " << sum2 << endl; // 5 4
count++ (count incremented)
-count++ (count incremented then negated)
Included in C-based languages, Perl and JavaScript
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24. Assignment as an Expression
In C, C++, Java, Perl and JavaScript the assignment statement produces a result and can be used as operand
An example:
while ((ch = getchar())!= EOF){…}
ch = getchar() is carried out; the result (assigned to ch) is used as a conditional value for the while statement; must be in (); can be harder to read
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25. List Assignments
Some recent languages like Perl and Ruby provide multiple-target, multiple-source assignments
In Perl:
($first, $second, $third) = (20, 30, 30);
($first, $second) = ($second, $first);
excess values on right ignored
excess values on left set to undef
How useful is this feature?
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26. Mixed-Mode Assignment
Assignment statements can also be mixed-mode, for example
int a, b;
float c;
c = a / b;
Coercion takes place after rhs evaluated
Fortran, C, C++ and Perl use coercion rules for mixed mode assignment
In Java, only widening assignment coercions are done (increases reliability)
In Ada, there is no assignment coercion
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