Goal

• To understand the difference between reference model languages and value model.
• To appreciate the ramification of operators with side-effects.
• To understand iterators.
  

What will be covered ?

• L-Values, R-Values, 
• Reference vs Value model of variables
• Immutiable objects
• Assignment operators
• Side-effects
• The principle of orthogonality

Code:

• Arrays in Perl
• Arrays in Python
• Python code--Generators
• Ruby code--Blocks & Iterators
• Python Class
• self is a convention in python
• Ruby Class
• Iterating with First Class functions
• Continuations using Ruby
  

• Values and References

• Examples
• Diagram

• L-Value/R-Value 

• L-value is an expression that refers to storage area 
• Original definition: The object that appears on the left hand side.
• Distinguish the differences between R-values and L-values in C++
• the L-value of a variable refers to the "container"
• R-value is an expression that denotes value
• Immutable objects only have R-values
• Under the value model, the context of an expression determines whether is it an L or R Value.
  
• Examples
• Arrays in Perl
• Arrays in Python
• Immutiable objects -- 

• Expressions -- and -- assignment 

• What is the pure form of an expression?
  
  
• "="  in C,C++,C# and Java
• is both an expression and has a side effect
• is right associative
  
  
• Alternate assignments and unary operator -- appear to have the same semantics.
  
  int x = 0;
  x = x + 1;
  x = 0;
  x += 1;
  x = 0;
  x++;



• Java code
  
  
static int n = 0;
static int [] A = new int[2];
static int f() {
    if (n == 0) {
        n=1;
    } else {
        n = 0;
    }
    return n;
}
static public void main (String [] arg) {
A[f()] = A[f()] +1;  // 0 1
A[f()] += 1;    // 0 2
A[0] = A[1] = n = 0;
A[f()] += 1;  // 0 1
A[f()] = A[f()] + 1;  // 2 1 }
}

Java evaluates the lhs first.

• Multiway assignments
• In Python, the comma on the rhs defines a tuple of multiple r-values and the comma on the lhs produces  a tuple of l-values

a , b = 15 , 3*25

• Order of Evaluation of Operators and Parameters and Undefined behavior
  

• C,Pascal, ADA::  the order of evaluations of the operands of most operators are not specified in the language.
  
  
• C:: The same for evaluations of  parameters
  
  
• Why is this a problem? 
  
  

Examples::
What is the value of x and a after ::


int x;
x = ++a + a; /* expression 1 */
a = 2;
x = a + ++a; /* expression 2 */




What is printed out after ::
int fun (int a, int b) {
  return a + b;
}
int main(void){
  int a =2;
  int x;
  x = fun (a,a++);
  printf("fun(a,a++): x %d, a %d\n\n",x,a);
  a = 2;
  x = fun (a++,a);
  printf("fun(a++,a): x %d, a %d\n\n",x,a);
 }  


GCC output::

fun(a,a++): x 5, a 3
fun(a++,a): x 4, a 3

• More on Operators with Side Effects

  • These expressions change memory as well as returning a value.

  x = (y = z)
  x++
  --y
  

• In C it is implementation dependent when side effects are applied
  

• What is the evaluation of the following expression containing side-effect operators:
  (Assume all side effect happens as soon as possible.)
  

int n = 3;
int x = ++n * n++ - --n


• Assume left to right evaluation:  (~> 12)

• Unlike C,  Java specifies exactly when the side effects happen.

int i = 0;
i = i++;
System.out.println(i);

• Prints 0 --- 
• Java notes the value of i, increments i then makes the "=" of the noted value.

• "Implementation-defined behavior is defined by the ISO C Standard in section 3.4.1 as:
  
  unspecified behavior where each implementation documents how the choice is made
  
  EXAMPLE An example of implementation-defined behavior is the propagation of the high-order bit when a signed integer is shifted right.
  
  Any code that relies on implementation defined behaviour is only guaranteed to work under a specific platform and/or compiler. Portable programs should try to avoid such behaviour." (from clc-wiki)
  
  
  

• "Unspecified behaviour is defined in ISO C99 in section 3.4.4, as:
  
  behavior where [the standard] provides two or more possibilities and imposes no further requirements on which is chosen in any instance
  
  EXAMPLE An example of unspecified behavior is the order in which the arguments to a function are evaluated.
  
  In general, the careful programmer will avoid writing any code that depends upon the outcome of aspects of C that have unspecified behavior, as the code can easily behave differently when ported from one platform to another, or even when it is built with a different compiler and toolchain on the same platform.
  
  It is even possible that the behavior could change from revision of a compiler to the next, although that is less likely to occur in practice." (from clc-wiki)
  
  
• "Undefined behaviour (UB) is defined by the ISO/ANSI C Standard as:
  
  behavior, upon use of a nonportable or erroneous program construct, of erroneous data, or of indeterminately valued objects, for which this International Standard imposes no requirements
  
  NOTE Possible undefined behavior ranges from ignoring the situation completely with unpredictable results, to behaving during translation or program execution in a documented manner characteristic of the environment (with or without the issuance of a diagnostic message), to terminating a translation or execution (with the issuance of a diagnostic message).
  
  The C99 version of the Standard removed "indeterminately valued objects" from this definition - in C99 the use of indeterminately valued objects is unspecified behaviour." from clc-wiki)

A good reference for C coding standard is the Software Engineering Institude CERT Center.

• What is the primary difference between the conditional operator and the conditional statement?
• What is the primary difference between �=� operator in C/Java/C++ and the �=:� in Pascal ?

Relational and Boolean Expressions

• Relation operator compares two operands and returns a Boolean. 
• Most languages have the type Boolean. 
• C does not have the type boolean. 
• 0 is considered false anything else is true.
• What is the value computed below?

a > b > c  /* a = 10 , b = 9, c = 5  */



Boolean Expressions

• Design issue: How many parentheses are necessary.
• Short circuit evaluation -- lazy evaluation
• Order of Evaluation
• C specifies that short circuit operators are evaluated from left to right.
• Ada and Pascal do not guarantee the order of evaluation.
  
  

Iterators


• Iterators are a separate thread of control
  
•  Used to iterator over collections/containers
  
• True Iterators are found in Python, Ruby, c#
  
• Python code--Generators
• Ruby code--Blocks & Iterators
• Python Class
• self is a convention in python
• C++, Java, Euclid do this using iterator objects.
• Rudy iterators use functional semantics.
• Iterating with First Class functions
• Implementing 'each" for a collection in a Ruby Class