data structures / collections

Week 9

Data structures / collections

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Week 9

Data structures

(informally:)

By size:– Static (e.g. arrays)

– Dynamic (e.g. vectors)

By ordering:– Last In First Out (LIFO)

– First In First Out (FIFO)

– Priority


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Week 9

Data Structures

Types of dynamic generic data structures are:

Stacks Queues Lists Trees


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Week 9

What is a Stack?

A stack is similar in concept to a pile of plates, books, blocks, boxes, etc.– The first item put on the stack is on the bottom of the

stack.

– All items added to the stack are placed on top of the previous item.

– The last item put on the stack is on the top of the stack.


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What is a Queue?

A queue is similar to waiting in line for a service, e.g., at the bank, at the bathroom– The first item put on the queue is at the front of the

queue.

– All items added to the queue are placed behind of the previous items.

– The last item put on the queue is at the back of the queue.


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Week 9

Queues

Queues are called First-in First-out (FIFO) data structures.– The first person to enter the queue is the first person to

be served, i.e., leave the queue.

– The last person to enter the queue is the last one to be removed from it.


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Queues

Characteristics of queues:– Data can only be placed at the back of the queue.

– Data can only be removed from the front of the queue.

– Data can only be removed from the rear of the queue if there is only one item on the queue.

– Data can not be removed from the middle of the queue without first removing all items in front of it.


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Queue Behaviors

The behavior of putting an item in the queue is called enqueue( ). – Enqueue 4 onto the queue.

The behavior of removing and item from the queue is called dequeue( ).– Dequeue 4 from the stack. (Remember this only works

if 4 is in the front of the queue.)


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Example

What queue exists after executing the following commands?– enqueue(3)– enqueue(6)– enqueue(8)– enqueue(1)– dequeue()– dequeue()– enqueue(14)

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Week 9

Queue Implementation

The ability to use a queue is not built into Java like arrays; nor is it available in util– You can implement a queue in many different ways,

e. g. using a list or an array.

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A queue class can be written using arrays that will simulate a queue in your programs.– A restriction of using arrays to implement a queue is

that the total size of the queue is limited.» Why?

» Can you define a method that will resize the array to hold a queue of any size?

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Can you write a program to implement the Queue class that holds data of any data type?

How do we modify our queue implementation so that the queue will grow?

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Week 9

Palindrome Example

A program that determines if a word is a palindrome using a stack and a queue.– A palindrome is a word that is spelled the same both

forwards and backwards.

Week 9

Testing and debugging

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Why testing and debugging?

Software development has a “black eye”– Schedules and time-to-market pressure

– Reliability and warranty issues

– Rising costs

– New technology and market demands

Increased focus on security and safety– Poor results

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Week 9

What we know

We must shift the focus from detection to prevention. Learn to look for the root cause

Pareto Principle - 80% of the defects will cluster in 20% of the components or causes

The more rigorously tested at the front end, the more reliable at the back end

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Fixing defects is more error-prone than the original development. One in 4 fixes introduces another defect elsewhere

Finding and fixing defects– accounts for most of maintenance costs

– accounts for 50% or more of typical development costs

– introduces a new risk. Fixing a defect late in the cycle introduces a significant risk

Fixing errors

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Week 9

Cannot rely on compiler to find ALL errors!

The number of errors at the first compile may be indicative of the number of defects still in code (Myers)

What can’t the compiler find?– Logical errors

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Week 9

What is Testing?

Testing is the process of finding errors in the system implementation.

– The intent of testing is to find problems with the system.

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What is Debugging?

Debugging is the process of finding the source of errors and fixing such errors.

– Testing is done before debugging.

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Week 9

Why Test?

The purpose of testing is to identify implementation errors before the product is shipped.

The errors may be:– Actual bugs in the code.– Incorrect implementation of the requirements or

functional specifications.» Misunderstandings» Incomplete requirements or functional specifications.

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Week 9

What Testing is not

Testing is not a random process.– There is a rhyme and reason to the process.

Testing is not debugging.– Testing identifies the problems.

– Debugging finds the location of a problem and fixes the problem.

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Who is responsible for Testing?

Multiple people are responsible for testing a system.– Initially the programmers are responsible for testing

their implementation but this is not systems testing.

– Usually a testing team will perform the majority of the tests, particularly at the system level.

– The customer will also test the entire system.» Alpha and Beta testing.

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Who is responsible for Debugging?

There is also a number of people who are responsible for debugging.– If there is a testing team responsible for testing the

system, this team will also attempt to precisely identify the problem and report it to the appropriate programmer.

– The programmer is responsible for determining the actual problem and repairing it.

– The customer should not debug a system.

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Week 9

When does Testing begin?

Testing begins during the implementation phase. – The programmer is responsible for testing their unit to

ensure the code meets the design and functional specifications.

– As multiple units become available and can be combined, system testing can begin by the testing team.

» It is not unusual for implementation and testing phases to overlap. This is particularly true with today’s shorter development cycles.

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What is tested?

The system is tested by:– Units of the system.

» In object oriented programming the classes would be tested.

– Related units of the system.

– The entire system.

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Week 9

How are Tests Passed?

A system passes the tests if it produces results that are consistent with the functional specification and requirements.– The program does what it is supposed to do.

– The program does not do anything it is not supposed to.

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Week 9

How are Tests Passed?

If any single unit test fails, then the entire system is not correct.

If all unit tests pass, then there is a good probability that the entire system will work together.

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Week 9

Types of Testing

Formal verification is a process that uses mathematical and logical assertions to prove that the program is correct.

– Formal verification is difficult to do.

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Week 9

Types of Testing

Empirical testing is the process of generating test cases and running the tests to show that errors exist.

Empirical testing involves observing the results of using the system.– Empirical testing can only prove that an error exists. It

can not prove that there are no errors.

There are two types of Empirical Testing: white box and black box testing.

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Empirical Testing

– White box testing:» Requires access to the actual implementation code.

» Requires the development of test cases that will exercise each unit of the system, and possible “flows” through the system based upon the actual implementation.

All statements, all decisions, all conditions, and all inputs.

» This type of testing is not very practical but sometimes it is required.

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Empirical Testing

– Methodologies that are used for White Box testing are:» Statement coverage

» Decision coverage

» Condition coverage

» Decision/condition coverage

» Multiple-condition coverage

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Week 9

Empirical Testing

– Black Box Testing:» Typically a testing team develops use cases based upon the

requirements and functional specification without looking at the actual implementation.

» Tests valid and invalid inputs but can not possibly test all inputs.

Must determine what subset of inputs will sufficiently cover all inputs.

» You want to break the system, it is your job with Black Box Testing.

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Empirical Testing

– Methodologies for Black Box Testing:» Equivalence partitioning:

A set of inputs that are processed identically by the program

– Legal input values

– Numeric/non-numeric values

» Boundary Testing

» Error Guessing

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Week 9

TestingTesting

Formal verificationFormal verification Empirical testingEmpirical testing

White Box testingWhite Box testing Black Box testingBlack Box testing

•Statement coverage•Decision coverage•Condition coverage•Decision/condition coverage•Multiple-condition coverage

•Statement coverage•Decision coverage•Condition coverage•Decision/condition coverage•Multiple-condition coverage

•Equivalence partitioning

•Boundary Testing

•Error Guessing

•Equivalence partitioning

•Boundary Testing

•Error Guessing

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Week 9

Testing Statement Coverage

Statement coverage tests that each statement in the system is executed at least once by the test data.– Testing the statement coverage is necessary but is not

sufficient.

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Week 9

Testing Statement Coverage

What problems can you find?

Assume that a = 2, b = 2, and that x is properly defined and initialized.

if (a > 1) && (b == 2){x = x / a;

} if (a == 2) || ( x > 1) {

x++;}

if (a > 1) && (b == 2){x = x / a;

} if (a == 2) || ( x > 1) {

x++;}

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Week 9

Testing Decision Coverage

Testing for Decision Coverage requires testing every decision for both a true and false outcome.

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Week 9


a) Assume: a = 2, b = 2, x > 1

b) Assume: a = 1, b = 2, x = 0

if (a > 1) && (b == 2){x = x / a;

} if (a == 2) || ( x > 1) {

x++;}

if (a > 1) && (b == 2){x = x / a;

} if (a == 2) || ( x > 1) {

x++;}

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Week 9


Testing for Decision Coverage also tests for statement coverage in modern languages.– This is not true of languages that have multiple entry

points, contain self-modifying code, etc.

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Week 9

Testing Condition Coverage Testing Condition Coverage requires testing

each possible outcome for every condition within a decision at least once. – The Decision Coverage testing we did only covered

half of the cases in the previous example. » The cases are:

a > 1 (1) b == 2 (2) a == 2 (3) x > 1 (4)

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Week 9

Testing Condition Coverage

a) Assume: a = 2, b = 2, x = 4

b) Assume a = 1, b = 3, x = 1

if (a > 1) && (b == 2){x = x / a;

} if (a == 2) || ( x > 1) {

x++;}

if (a > 1) && (b == 2){x = x / a;

} if (a == 2) || ( x > 1) {

x++;}

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Week 9

Multiple-Condition Coverage

Testing for Multiple-condition coverage requires test cases that test all possible combinations of condition outcomes for every decision tested.– This type of testing will generate many test cases.

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Week 9

Debugging

Debugging should be a formal process of attempting to narrow down the location of the problem and then identifying the problem.– Debugging does not mean simply changing code until

the problem goes away.

– Debugging requires thinking about what might be the problem.

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Week 9

Debugging

Methods of determining the location of a bug:– Use extra output statements in the program to trace the

program execution.

– Use a debugger to trace the program execution.

– Possibly write special test code to exercise parts of the program in special ways that will allow you to better understand the error.

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Debugging

– Potentially test a certain range of values to see which ones fail.

– Attempt to eliminate parts of the program as the problem, thus narrowing your search.

– Check that the data is valid.

– Many times, the location where you see the first instance of the bug is not the source of the bug.

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Fixing Bugs

Steps for fixing bugs:– Fix only one bug at a time and then rerun the same

exact tests.» Changing multiple things makes id difficult to identify which

change caused the behavior change.

– If the problem appears to be fixed, still run a full test suite to ensure the “fix” did not break something else.

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General Rules to follow

Test your code as your write it:– Test the code boundaries.

– Test pre- and post- conditions.» The necessary or expected properties before and after the code

is executed.

– Use assertions (if you are programming in C or C++)

– Program defensively by adding code to handle the “can not happen” cases.

– Check error returns

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General Rules to Follow

Steps for Systematic Testing– Test incrementally by writing part of the system, test it,

then write some more code, test that code, etc.

– Test the simple parts of the system first.

– Know what output you are expecting.

– Compare independent implementations of a library or program provide the same answers.

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General Rules

– Ensure that testing covers every statement of the program.

» Every line of the program should be exercised by at least one test.

data structures / collections

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