9-Coding.ppt

Coding 2
Coding
 Goal is to implement the design in best
possible manner
 Coding affects testing and maintenance
 As testing and maintenance costs are high,
aim of coding activity should be to write code
that reduces them
 Hence, goal should not be to reduce coding
cost, but testing and maint cost, i.e. make
the job of tester and maintainer easier

Coding 3
Coding…
 Code is read a lot more
 Coders themselves read the code many times for
debugging, extending etc
 Maintainers spend a lot of effort reading and
understanding code
 Other developers read code when they add to
existing code
 Hence, code should be written so it is easy to
understand and read, not easy to write!

Coding 4
Coding…
 Having clear goal for coding will help achieve
them
 Weinberg experiment showed that coders
achieve the goal they set
 Diff coders were given the same problem
 But different objectives were given to diff
programmers – minimize effort, min size, min
memory, maximize clarity, max output clarity
 Final programs for diff programmers generally
satisfied the criteria given to them

Coding 5
Weinberg experiment..
Resulting Rank (1=best)
O1 o2 o3 o4 o5
Minimize Effort (o1)
Minimize prog size (o2)
Minimize memory (o3)
Maximize code clarity (o4)
Maximize output clarity (o5)
1 4 4 5 3
2-3 1 2 3 5
5 2 1 4 4
4 3 3 2 2
2-3 5 5 1 1

Coding 6
Programming Principles
 The main goal of the programmer is write
simple and easy to read programs with few
bugs in it
 Of course, the programmer has to develop it
quickly to keep productivity high
 There are various programming principles
that can help write code that is easier to
understand (and test…)

Coding 7
Structured Programming
 Structured programming started in the
70s, primarily against indiscriminate use
of control constructs like gotos
 Goal was to simplify program structure
so it is easier to argue about programs
 Is now well established and followed

Coding 8
Structured Programming…
 A program has a static structure which is the
ordering of stmts in the code – and this is a
linear ordering
 A program also has dynamic structure –order
in which stmts are executed
 Both dynamic and static structures are
ordering of statements
 Correctness of a program must talk about the
dynamic structure

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 To show a program as correct, we must show that its
dynamic behavior is as expected
 But we must argue about this from the code of the
program, i.e. the static structure
 I.e program behavior arguments are made on the
static code
 This will become easier if the dynamic and static
structures are similar
 Closer correspondence will make it easier to
understand dynamic behavior from static structure
 This is the idea behind structured programming

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 Goal of structured programming is to
write programs whose dynamic
structure is same as static
 I.e. stmts are executed in the same
order in which they are present in code
 As stmts organized linearly, the
objective is to develop programs whose
control flow is linear

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 Meaningful programs cannot be written as
seq of simple stmts
 To achieve the objectives, structured
constructs are to be used
 These are single-entry-single-exit constructs
 With these, execution of the stmts can be in
the order they appear in code
 The dynamic and static order becomes same

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 Each structured construct should also have a
clear behavior
 Then we can compose behavior of stmts to
understand behavior of programs
 Hence, arbitrary single-entry-single-exit
constructs will not help
 It can be shown that a few constructs like
while, if, and sequencing suffice for writing
any type of program

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 SP was promulgated to help formal
verification of programs
 Without linear flow, composition is hard and
verification difficult
 But, SP also helps simplify the control flow of
programs, making them easier to understand
and argue about
 SP is an accepted and standard practice today
– modern languages support it well

Coding 14
Information Hiding
 Software solutions always contain data
structures that hold information
 Programs work on these DS to perform the
functions they want
 In general only some operations are
performed on the information, i.e. the data is
manipulated in a few ways only
 E.g. on a bank’s ledger, only debit, credit,
check cur balance etc are done

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Information Hiding…
 Information hiding – the information should
be hidden; only operations on it should be
exposed
 I.e. data structures are hidden behind the
access functions, which can be used by
programs
 Info hiding reduces coupling
 This practice is a key foundation of OO and
components, and is also widely used today

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Some Programming Practices
 Control constructs: Use only a few
structured constructs (rather than using
a large no of constructs)
 Goto: Use them sparingly, and only
when the alternatives are worse
 Info hiding: Use info hiding
 Use-defined types: use these to make
the programs easier to read

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Some Programming Practices..
 Nesting: Avoid heavy nesting of if-then-
else; if disjoint nesting can be avoided
 Module size: Should not be too large –
generally means low cohesion
 Module interface: make it simple
 Robustness: Handle exceptional
situations
 Side effects: Avoid them, document

Coding 18
Some Programming Practices..
 Empty catch block: always have some default
action rather than empty
 Empty if, while: bad practice
 Read return: should be checked for
robustness
 Return from finally: should not return from
finally
 Correlated parameters: Should check for
compatibility

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Coding Standards
 Programmers spend more time reading code than
writing code
 They read their own code as well as other
programmers code
 Readability is enhanced if some coding conventions
are followed by all
 Coding standards provide these guidelines for
programmers
 Generally are regarding naming, file organization,
statements/declarations, …
 Some Java conventions discussed here

Coding 20
Coding Standards…
 Naming conventions
 Package name should be in lower case
(mypackage, edu.iitk.maths)
 Type names should be nouns and start with
uppercase (Day, DateOfBirth,…)
 Var names should be nouns in lowercase; vars
with large scope should have long names; loop
iterators should be i, j, k…
 Const names should be all caps
 Method names should be verbs starting with lower
case (eg getValue())
 Prefix is should be used for boolean methods

Coding 21
Coding Standards…
 Files
 Source files should have .java extension
 Each file should contain one outer class
and the name should be same as file
 Line length should be less than 80; if
longer continue on another line…

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Coding Standards…
 Statements
 Vars should be initialized where declared in the
smallest possible scope
 Declare related vars together; unrelated vars
should be declared separately
 Class vars should never be declared public
 Loop vars should be initialized just before the loop
 Avoid using break and continue in loops
 Avoid executable stmts in conditionals
 Avoid using the do… while construct

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Coding Standards…
 Commenting and layout
 Single line comments for a block should be
aligned with the code block
 There should be comments for all major
vars explaining what they represent
 A comment block should start with a line
with just /* and end with a line with */
 Trailing comments after stmts should be
short and on the same line

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Coding Process
 Coding starts when specs for modules from
design is available
 Usually modules are assigned to
programmers for coding
 In top-down development, top level modules
are developed first; in bottom-up lower levels
modules
 For coding, developers use different
processes; we discuss some here

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An Incremental Coding Process
 Basic process: Write code for the
module, unit test it, fix the bugs
 It is better to do this incrementally –
write code for part of functionality, then
test it and fix it, then proceed
 I.e. code is built code for a module
incrementally

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Test Driven Development
 This coding process changes the order
of activities in coding
 In TDD, programmer first writes the
test scripts and then writes the code to
pass the test cases in the script
 This is done incrementally
 Is a relatively new approach, and is a
part of the extreme programming (XP)

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TDD…
 In TDD, you write just enough code to pass
the test
 I.e. code is always in sync with the tests and
gets tested by the test cases
 Not true in code first approach, as test cases may
only test part of functionality
 Responsibility to ensure that all functionality
is there is on test case design, not coding
 Help ensure that all code is testable

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TDD…
 Focus shifts to how code will be used as test
cases are written first
 Helps validate user interfaces specified in the
design
 Focuses on usage of code
 Functionality prioritization happens naturally
 Has possibility that special cases for which
test cases are not possible get left out
 Code improvement through refactoring will be
needed to avoid getting a messy code

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Pair Programming
 Also a coding process that has been proposed
as key practice in XP
 Code is written by pair of programmers rather
than individuals
 The pair together design algorithms, data
structures, strategies, etc.
 One person types the code, the other actively
reviews what is being typed
 Errors are pointed out and together solutions are
formulated
 Roles are reversed periodically

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Pair Programming…
 PP has continuous code review, and reviews
are known to be effective
 Better designs of algos/DS/logic/…
 Special conditions are likely to be dealt with
better and not forgotten
 It may, however, result in loss of productivity
 Ownership and accountability issues are also
there
 Effectiveness is not yet fully known

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Source Code Control and Built
 Source code control is an essential step
programmers have to do
 Generally tools like CVS, VSS are used
 A tool consists of repository, which is a
controlled directory structure
 The repository is the official source for all the
code files
 System build is done from the files in the
repository only
 Tool typically provides many commands to
programmers

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Source code control…
 Checkout a file: by this a programmer gets a
local copy that can be modified
 Check in a file: changed files are uploaded in
the repository and change is then available to
all
 Tools maintain complete change history and
all older versions can be recovered
 Source code control is an essential tool for
developing large projects and for coordination

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Verification
 Code has to be verified before it can be used
by others
 Here we discuss only verification of code
written by a programmer (system verification
is discussed in testing)
 There are many different techniques; key
ones – unit testing, inspection, and program
checking
 Program checking can also be used at the
system level

Coding 36
Code Inspections
 The inspection process can be applied to
code with great effectiveness
 Inspections held when code has compiled and
a few tests passed
 Usually static tools are also applied before
inspections
 Inspection team focuses on finding defects
and bugs in code
 Checklists are generally used to focus the
attention on defects

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Code Inspections…
 Some items in a checklist
 Do all pointers point to something
 Are all vars and pointers initialized
 Are all array indexes within bounds
 Will all loops always terminate
 Any security flaws
 Is input data being checked
 Obvious inefficiencies

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Code inspections…
 Are very effective and are widely used in
industry (many require all critical code
segments to be inspected)
 Is also expensive; for non critical code one
person inspection may be used
 Code reading is self inspection
 A structured approach where code is read inside-
out
 Is also very effective

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Unit Testing
 Is testing, except the focus is the module a
programmer has written
 Most often UT is done by the programmer
himself
 UT will require test cases for the module –
will discuss in testing
 UT also requires drivers to be written to
actually execute the module with test cases
 Besides the driver and test cases, tester
needs to know the correct outcome as well

Coding 40
Unit Testing…
 If incremental coding is being done, then
complete UT needs to be automated
 Otherwise, repeatedly doing UT will not be
possible
 There are tools available to help
 They provide the drivers
 Test cases are programmed, with outcomes being
checked in them
 I.e. UT is a script that returns pass/fail

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Unit Testing…
 There are frameworks like Junit that can be
used for testing Java classes
 Each test case is a method which ends with
some assertions
 If assertions hold, the test case pass,
otherwise it fails
 Complete execution and evaluation of the test
cases is automated
 For enhancing the test script, additional test
cases can be added easily

Coding 42
Static Analysis
 These are tools to analyze program sources
and check for problems
 Static analyzer cannot find all bugs and often
cannot be sure of the bugs it finds as it is not
executing the code
 So there is noise in their output
 Many different tools available that use
different techniques
 They are effective in finding bugs like
memory leak, dead code, dangling pointers,..

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Formal Verification
 These approaches aim to prove the
correctness of the program
 I.e. the program implements its specifications
 Require formal specifications for the program,
as well as rules to interpret the program
 Was an active area of research; scalability
issues became the bottleneck
 Used mostly in very critical situations, as an
additional approach

Coding 44
Metrics for Size
 LOC or KLOC
 non-commented, non blank lines is a
standard definition
 Generally only new or modified lines are
counted
 Used heavily, though has shortcomings

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Metrics for Size…
 Halstead’s Volume
 n1: no of distinct operators
 n2: no of distinct operands
 N1: total occurrences of operators
 N2: Total occurrences of operands
 Vocabulary, n = n1 + n2
 Length, N = N1 + N2
 Volume, V = N log2(n)

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Metrics for Complexity
 Cyclomatic Complexity is perhaps the most
widely used measure
 Represents the program by its control flow
graph with e edges, n nodes, and p parts
 Cyclomatic complexity is defined as V(G) = e-
n+p
 This is same as the number of linearly
independent cycles in the graph
 And is same as the number of decisions
(conditionals) in the program plus one

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Cyclomatic complexity example…
1. {
2. i=1;
3. while (i<=n) {
4. J=1;
5. while(j <= i) {
6. If (A[i]<A[j])
7. Swap(A[i], A[j]);
8. J=j+1;}
9. i = i+1;}
10. }

Coding 49
Example…
 V(G) = 10-7+1 = 4
 Independent circuits
1. b c e b
2. b c d e b
3. a b f a
4. a g a
 No of decisions is 3 (while, while, if);
complexity is 3+1 = 4

Coding 50
Complexity metrics…
 Halsteads
 N2/n2 is avg times an operand is used
 If vars are changed frequently, this is
larger
 Ease of reading or writing is defined as
D = (n1*N2)/(2*n2)
 There are others, e.g. live variables,
knot count..

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Complexity metrics…
 The basic use of these is to reduce the
complexity of modules
 One suggestion is that cyclomatic
complexity should be less than 10
 Another use is to identify high
complexity modules and then see if
their logic can be simplified

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Summary
 Goal of coding is to convert a design into
easy to read code with few bugs
 Good programming practices like structured
programming, information hiding, etc can
help
 There are many methods to verify the code of
a module – unit testing and inspections are
most commonly used
 Size and complexity measures are defined
and often used; common ones are LOC and
cyclomatic complexity

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Some Other Topics Related to
Coding

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Common Coding Errors

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Common Coding Errors
 Goal of programmer is to write quality code
with few bugs in it
 Much of effort in developing software goes in
identifying and removing bugs
 Common bugs which occur during coding
directly or indirectly manifest themselves to a
larger damage to the running program
 List of common coding errors can help a
programmer avoid them

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Memory Leaks
 A memory leak is a situation, where the memory is allocated to
the program which is not freed subsequently
 Occurs frequently in the languages which do not have
automatic garbage collection
 Can cause increasing usage of memory which at some point of
time can lead to exceptional halt of the program
 E.g char* foo(int s)
{
Char *output;
If(s>0)
Output=(char*) malloc (size)
If(s==1)
Return NULL /* if s==1 then mem leaked */
Return (output);
}

Coding 57
Freeing an Already Freed Resource
 Programmer tries to free the already freed resource
 May be serious, if some malloc between the two free stmts as
the freed location may get allocated to a new variable, and
subsequent free will deallocate the new variable.
E.g main()
{
char *str;
Str=(char *)malloc(10);
If(global==0)
free(str);
Free(str); /* str is already freed */
}

Coding 58
NULL Dereferencing
 Occurs when we access a location that points to NULL
 Improper initialization and missing the initialization in different
paths leads to the NULL reference error
 Can also be caused by aliases- for e.g two variables refer to
same object , and one is freed and an attempt is made to
dereference the second
 E.g char *ch=NULL;
if (x>0)
{
ch=‘c’;
}
printf(“%c”. *ch); /* ch may be NULL */
*ch=malloc(size)
ch=‘c’; /* ch will be NULL if malloc returns NULL */

Coding 59
NULL Dereferencing (accessing
uninitialized memory)
 NULL dereference is the error of accessing initialized
memory
 Often occurs if data is initialized in most cases, but
most cases do not get covered
E.g switch(i)
{
case 0: s=OBJECT_1; break;
case 1: s=OBJECT_2; break;
}
return (s); /* s not initialized for values other
than 0 or 1 */

Coding 60
Lack of Unique Addresses
 Aliasing creates many problems among them
is violation of unique addresses when we
expect different addresses.
 For e.g in the string concatenation function,
we expect source and destination addresses
to be different.
 strcat (src , destn );
/* In above function , if src is aliased to
destn , then we may get a runtime
error */

Coding 61
Synchronization Errors
 Possible when there are multiple threads which are accessing
some common resources, in a parallel program
 three categories of synchronization errors: deadlocks, race
condition, live lock
 Deadlock example:
Thread 1:
synchronized (A){
synchronized (B){ }
}
Thread 2:
synchronized (B){
synchronized (C){ }
}
Thread 3:
synchronized (C){
synchronized (A){ }
}

Coding 62
Synchronization Errors…
 Race condition occurs when two threads access same resource
and result depends on the order of the execution
E.g class reservation
{
int seats_remaining ;
public int reserve (int x)
{
if (x <= seats_remaining ) {
seats_remaining -=x;
return 1;
}
return 0;
}
}
 Inconsistent synchronization represents the situation where there
is a mix of locked and unlocked accesses to some shared
variables, pariticularly if the access involves updates

Coding 63
Buffer overflow
 It is a security vulnerability, can be exploited by
executing arbitrary code by a malicious user
E.g
char s1 [1024];
void mygets ( char *str ){
int ch;
while (ch= getchar () != ‘n’ && ch != ‘0 ’)
*( str ++)= ch;
*str = ‘0 ’;
}
main (){
char s2 [4];
mygets (s2 );
}
 If we have malicious code in s1 and if return address of
mygets() is replaced by this address by overflowing the buffer
s2, then we can have the code in s1 executed

Coding 64
Other common type of errors
 Array index out of bounds, care needs to be taken to see that
the array index values are not negative and do not exceed their
bounds
 Enumerated data types can lead to overflow and underflow
errors, care should be taken while assuming the values of such
types
typedef enum {A, B,C, D} grade ;
void foo( grade x){
int l,m;
l= GLOBAL_ARRAY [x -1];
/* Underflow when A */
m= GLOBAL_ARRAY [x +1];
/* Overflow when D and size of array is 4 */
}

Coding 65
Other common type of errors..
 Arithmetic exceptions, include errors like divide by zero and
floating point exceptions
 Off by one errors like starting variable at 1 instead of starting at
0 or vice versa, writing <= N instead of < N or vice versa etc.
 String handling errors like failure of string handling
functions e.g strcpy, sprintf, gets etc.
 Illegal use of & instead of &&, arises if non short
circuit logic (like & or |) is used instead of short
circuit logic (&& or ||)
E.g if(object!null &object.getTitle()!=null)
/* Here second operation can cause a null dereference */

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Background
 Ultimate goal of verification techniques is to make
programs correct by removing errors
 Proving a program as correct while developing it may
result in more reliable program that can be proved
more easily
 Any proof technique must begin with a formal
specification of the program
 Stating the goal of the program is not sufficient
 Input conditions in which the program is to be
invoked (pre conditions) and expected valid results
(post conditions) should also be mentioned.

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Axiomatic Approach
 Axiomatic method (Floyd-Hoare proof method) is one
method for proving correctness
 Goal is to take the program and construct a sequence
of assertions, and the rules and axioms about
statements and operations in the program
 Basic assertion about a program in Hoare’s notation
P{S}Q
P – precondition, Q – post condition,
S- program segment
 These assertions are about the values taken by the
variables in the program before and after its
execution

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Axiomatic approach…
 Some rules and axioms are necessary in
order to prove the theorem of the form
P{S}Q
 Let us consider a simple programming
language which deals with integers and
has types of statements 1) assignment
2) conditional 3)iterative statement

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Axiom of Assignment
 Consider an assignment statement of the form x:=f
where x is an identifier, f is an expression without any
side affects
 Any assertion that is true about x after the assignment
must be true of the expression f before the assignment
 The axiom is stated as
P is the post condition of the program segment
containing only the assignment statement
is an assertion obtained by substituting f for all
occurrences of x in the assertion P.
P
f
x
Px
f }
:
{ 
x
f
P

Coding 71
Rule of composition
 Rule for sequential composition where two
statements S1 and S2 are executed in sequence
P {S1 }Q,Q {S2 }R
P{S1;S2}R
 Using this rule if we can prove P{S1}Q and Q{S2}R,
we can claim that if before execution the pre-
condition P holds, then after execution of the
program segment S1;S2 the post condition R will
hold
 In other words, from the proofs of simple statements,
proofs of programs (sequence of statements) will be
constructed

Coding 72
Rule for Alternate Statement
 Rule for an If statement, entire If statement
is treated as one construct
 Two types of If statement, one with an else
and one without. The rules for both are
P ∩ B{S}Q, P ∩ ~ B  Q
______________________
P { if B then S} Q
P ∩ B{S}Q, P ∩ B{S2} Q
______________________
P { if B then S1 else S2} Q

Coding 73
Rule for Alternate Statement
 If we can show that starting in the state
where P∩B is true and executing S1or starting
in a state where P∩~B is true and executing the
statement S2,both lead to the post condition Q
 Hence, the statement: if-then-else statement is
executed with pre-condition P, the post condition
Q will be hold after the execution of the statement,
can be inferred.
 Similarly the if-then statement

Coding 74
Rules of Consequence
 Used to prove new theorems from the ones we have
already proved
P {S} R, RQ
_____________
P {S} Q
P R, R{S}Q
_____________
P {S} Q
 If the execution of a program ensures that an assertion Q is true after
execution of a program, then it also ensures that every assertion
logically implied by Q is also true after execution
 If a pre-condition ensures that a post condition is true after the
execution of a program, then every condition that logically implies the
pre-condition will also ensure that the post-condition holds after
execution of the program.

Coding 75
Rule of Iteration
 Let us consider while loop of the form while B do S
 Let P be an assertion that will be true when the loop
terminates
 P will hold true after every execution of statement S,
and will be true before every execution of S, because
the condition that holds true after an execution of S
will be the condition for the next execution of S
P ∩ B{S}P
______________________
P {while B do S}P∩ ~B
 Furthermore, we know that the condition B is false when the loop
terminates and is true whenever S is executed

Coding 76
An Example
(* Remainder of x/y )
Begin
q:=0;
r:=x;
While r≥y do
Begin
r:=r - y;
q:=q+1;
end
end
Pre-condition: P={x ≥ 0 ∩ y>0}
Post-condition: Q={x=qy+r ∩ 0≤r<y}

Coding 77
Example…
 First statement before the end of the program is the loop
 Removing ~B from Q, factor Q into a form like I ∩~B, Choose I as an
Invariant
 ~B={r<y}, Q={x=qy+r ∩ 0≤r<y}, Invariant I is {x=qy+r ∩ 0≤r}
 Using the assignment axiom and the precondition for statement 7
x=(q+1)y+r ∩ 0 ≤r{q:=q+1}I
 Using the assignment axiom for statement 6, we get pre-condition for
statement 6 as
x=(q+1)y+(r - y) ∩ 0 ≤(r-y) which is the same as x=qy+r ∩y ≤r
 Using the rule of composition (for statements 6 and 7)
x=qy+r ∩ y ≤r{r:=r-y;q:q+1}I
 Because x=qy+r ∩ y ≤r  I ∩ B, by rule of consequence and the rule for the
while loop, we have
I{while loop in program}I ∩ ~(r≥y)

Coding 78
Example…
 For the statements before the loop (i.e., statement 2 and 3)
Post condition for these statements is I
 Using the axiom of assignment, we first replace r with x
and then replace q with 0 to get (x=x ∩0 ≤x) (0 ≤x)
 Composing these statements with the while statement, we
get
0 ≤x{the entire program} I ∩~B
 Because (I ∩~B) is the post condition Q of the program
and 0 ≤x is the pre-condition, the program is proved to be
correct

9-Coding.ppt

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9-Coding.ppt