CSC302 2007S, Class 06: Hoare's Examples

Admin:
* Assigned: Homework 3: Removing Recursion.
* Due: Homework 2: Permutation with Repetitions.
* I do not have Wednesday's reading yet.  Sorry.
* Grr ... few reflections on Hoare, revisited.

Overview:
* Examples from Hoare.
* Choosing a language.
* Key design criteria.
* More notes from the readings.

/Examples from Hoare/

//Fortran Syntax//

DO 17 I = 1,10
vs.
DO 17 I = 1 10
vs.
DO17I = 110

Moral: Ignoring whitespace is bad.

//Case Statement//

switch SS = L1,L2,L3;
...
go to SS[i];
L1: Q1; goto L;
L2: Q2; goto L;
L3: Q3;
L:

case i 
{
 Q1, 
 Q2,
 Q3
}

//Assignment in Algol//

x := y;  ALWAYS changes X
x := y+1; NEVER changes X, if X is a reference variable

problem 1: we have pointers explicit in the language

critique of coercion:
  int i;
  float f;
  ...
  f = i * 2;
Fun in C

char *c;
c = 1;
printf("%s", c);

In algol,  c := 1 interpreted as ...
* Store 1 in the thing c points to
In algol, x := y+1 interpreted as
* Store the sum of y and 1 in the thing c points to

/Detour on Coercion/

* What are the numeric coercion rules in C/Java
  * Types: int,long,float,double
  * Operation: Assignment
    i = l; // Won't accept; need to cast b/c possible loss of info
    i = f; // Won't accept; need to cast. b/c possible loss of info
    i = d; // Won't accept; need to cast b/c possible loss of info
    l = i;
    l = f;
    l = d;
    f = i; // Probably won't have to cast b/c no loss of info
    f = l; // Probably won't have to cast b/c no loss of info
    f = d;
    d = i;
    d = l;
    d = f;

/Last Hoare Example/

Suppose matrixmuliply(X,Y,Z) means "X = Y * Z"
matrximultiply(A,A,A)

Problem is "Aliasing"