1. Chapter 7 : Expression and Assignment Statements
Introduction
Arithmetic Expressions
Overloaded operators
Type Conversions
Relational and Boolean Expressions
Short-Circuit Evaluation
Assignment Statements
Mixed-Mode Assignment.

2. Expression and assignment statements
This chapter will focus on expression and assignment statements.
The semantic rules that determine the order of evaluation of operators in expression are discussed first.
This is followed by the potential problems of implementation-defined operand evaluation order when expiration can have side effects.

3. Overload operators ,both predefined and user-defined ,are then discussed ,along with their effects on expression in programs.
Next ,mixed-mode expressions are discussed and evaluated .
Relational and Boolean expressions are then discussed .
Finally ,the assignment statement from its simplest form to all of its variations , is covered , including assignments as expression and mixed-mode assignments

4. Introduction
Expressions are the fundamental means of specifying computations in a programming language.
It is crucial for a programmer to understand both the syntax and semantics of expressions.
Methods of describing the syntax of expirations were described in chapter 3

5. Introduction
In this chapter, we focus on the semantics of expression that is, what they mean, which is determined by how they are evaluated
To understand expression evaluation, it is necessary to be familiar with the order of operator and operand evaluation.
The operator evaluation order of expressions is governed by the associativity and procedure rules of the language.
The essence of an imperative programming language is the dominant role of assignment statements.
The purpose of an assignment statement is to change the value of a variable.

6. Chapter 7 Arithmetic Expressions
Their evaluation was one of the motivations for the development of the first programming languages
Arithmetic expressions consist of operators, operands, parentheses, and function calls

7. Chapter 7 Design issues for arithmetic expressions:
What are the operator precedence rules?
What are the operator associatively rules?
What is the order of operand evaluation?
Are there restrictions on operand evaluation side effects?
Does the language allow user-defined operator overloading?
What mode mixing is allowed in expressions?

8. In programming languages, arithmetic expression consist of operators, operands, parentheses, and function calls
A unary operator has one operand
A binary operator has two operands
A ternary operator has three operands
C,C++,and Java include a ternary operator which has three operands .
Most imperative programming languages ,binary operators are infix ,which means that they appear between their operands.
One exception is perl which has some operators that are prefix ,which means that they appear precede their operands.

9. Operator precedence
Def: The operator precedence rules for expression evaluation define the order in which “adjacent” operators of different precedence levels are evaluated (“adjacent” means they are separated by at most one operand)
Typical precedence levels
parentheses
unary operators
** (if the language supports it)
*, /
+, -

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

11. - Typical associatively rules:
- Left to right, except **, which is right to left
- Sometimes unary operators associate right to
left (e.g., FORTRAN)
- APL is different; all operators have equal
precedence and all operators associate right to
left
Precedence and associatively rules can be
overridden with parentheses

12. Operand evaluation order
- The process:
1. Variables: just fetch the value
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. Function references: The case of most interest!
- Order of evaluation is crucial
Functional side effects - when a function changes
a two-way parameter or a nonlocal variable

13. The 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;
b = a + fun(&a);
/* Assume that fun changes its parameter */

14. Two Possible Solutions to the Problem:
1. Write the language definition to disallow
functional side effects
- No two-way parameters in functions
- No nonlocal references in functions
- Advantage: it works!
- Disadvantage: Programmers want the flexibility
of two-way parameters (what about C?) and
nonlocal references
2. Write the language definition to demand that
operand evaluation order be fixed
- Disadvantage: limits some compiler
optimizations

15. Operator Overloading: Multiple use of an operator
Conditional Expressions
- C, C++, and Java (?:)
e.g.
average = (count == 0)? 0 : sum / count;
Operator Overloading: Multiple use of an operator
- Generally thought to be acceptable, as long as readability and /or don’t suffer
- Some is common (e.g., + for int and float)
- Some is potential trouble (e.g., * in C and C++)
- Loss of compiler error detection (omission of
an operand should be a detectable error)
- Can be avoided by introduction of new symbols
(e.g., Pascal’s div)

16. - C++ and Ada allow user-defined overloaded
operators
Potential problems:
- Users can define nonsense operations
- Readability may suffer

17. Implicit Type Conversions
Def: A narrowing conversion is one that converts
an object to a type that cannot include all of
the values of the original type
Def: 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
Def: A mixed-mode expression is one that has
operands of different types
Def: A coercion is an implicit type conversion

18. - The 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 Modula-2 and Ada, there are virtually no
coercions in expressions

19. Explicit Type Conversions
- Often called casts
e.g.
Ada:
FLOAT(INDEX) -- INDEX is INTEGER type C:
(int)speed /* speed is float type */

20. Errors in Expressions
- Caused by:
- Inherent limitations of arithmetic
e.g. division by zero
- Limitations of computer arithmetic
e.g. overflow
- Such errors are often ignored by the run-time
system

21. Relational Expressions
A relational operator is an operator that compares the values of its two operands.
A relational expression has two operands and one relational operator.
The vale of a relational expression is Boolean ,except when Boolean is not a type in the language.
Use relational operators and operands of various types
Evaluate to some Boolean representation
Operator symbols used vary somewhat among
languages (!=, /=, .NE., <>, #)
The relational operator always have lower precedence than the arithmetic operator.

22. Boolean Expressions: consist of boolean variables ,boolean constants ,relational expressions ,and boolean operator.
- Operands are boolean and the result is boolean
- Operators:
FORTRAN FORTRAN C Ada
.AND and && and
.OR or || or
.NOT not ! not
xor
- C 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

23. Precedence of All Operators:
Pascal: not, unary -
*, /, div, mod, and
+, -, or
relops
Ada: **
*, /, mod, rem
unary -, not
+, -, &
and, or, xor
C, C++, and Java have over 50 operators and 17
different levels of precedence

24. Short Circuit Evaluation: is one in which the result is determind without evaluating all the operands or operators
E.g.
(13*A)*(b/13-1)
Pascal: does not use short-circuit evaluation
Problem: table look-up
index := 1;
while (index <= length) and
(LIST[index] <> value) do
index := index + 1

25. 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 |)
Ada: programmer can specify either (short-circuit
is specified with and then and or else)
FORTRAN 77: short circuit, but any side-affected
place must be set to undefined
Short-circuit evaluation exposes the potential
problem of side effects in expressions
e.g. (a > b) || (b++ / 3)

26. Assignment Statements
The operator symbol:
1. = FORTRAN, BASIC, PL/I, C, C++, Java
2. := ALGOLs, Pascal, Modula-2, Ada
= can be bad if it is overloaded for the
relational operator for equality
e.g. (PL/I) A = B = C;
Note difference from C

27. More complicated assignments:
1. Multiple targets (PL/I)
A, B = 10
2. Conditional targets (C, C++, and Java)
(first = true) ? total : subtotal = 0
3. Compound assignment operators (C, C++, and
Java)
sum += next;

28. More complicated assignments:
4. Unary assignment operators (C, C++, and Java)
a++;
C, C++, and Java treat = as an arithmetic binary
operator
e.g.
a = b * (c = d * 2 + 1) + 1
This is inherited from ALGOL 68

29. Assignment as an Expression
- In C, C++, and Java, the assignment statement
produces a result
- So, they can be used as operands in
expressions
e.g. while ((ch = getchar() != EOF) { ... }
Disadvantage
- Another kind of expression side effect

30. Mixed-Mode Assignment
- In FORTRAN, C, and C++, any numeric value can
be assigned to any numeric scalar variable;
whatever conversion is necessary is done
- In Pascal, integers can be assigned to reals, but
reals cannot be assigned to integers (the
programmer must specify whether the conversion
from real to integer is truncated or rounded)
- In Java, only widening assignment coercions are
done
- In Ada, there is no assignment coercion

31. Q21: Consider the following C program
int fun(int * I) {
*I + = 5;
return 4; }
void main( )
int x;
x = x + fun(&x);
What is the value of x after the assignment
statement in main assuming:
a- operands are evaluated left to right.
b- operands are evaluated right to left.

32. int fun(int * I) {
*I + = 5;
return 4; }
void main( )
int x=3;
x = x + fun(&x);
left to right = 7
right to left = 12

33. Q19 : Should an optimizing compiler for C or C++
be allowed to change the order of subexpressions
in Boolean expression ? Why or why not ?

34. I think yes, Consider :
a < b < c . This statement is legal. What it will
compare is a < b . The answer will be either 0 or
1. Now this number will be compared with c. Thus
there is no comparison at all between b and c.

35. Question 20 : It is the same question for Ada

36. void main() int i = 10, j = 10, sum1, sum2; sum1 = ( i/2) + fun(&i);
Q15: Let the C function be defined as :
int fun(int * k) {
*k += 4;
return 3* (*k) - 1; }
Suppose fun is used in a program as follows:
void main()
int i = 10, j = 10,
sum1, sum2;
sum1 = ( i/2) + fun(&i);
sum2 = fun(&j) + ( j/20);
Determine the values of sum1 and sum2 by running
the program on a computer. Explain the results ?

37. #include<stdio.h> int fun(int * k) { *k += 4;
return 3* (*k) - 1; }
void main()
int i = 10, j = 10,
sum1, sum2;
sum1 = ( i/2) + fun(&i);
sum2 = fun(&j) + ( j/20); 46 41