5. Implementing Software Solutions

Question 1

Define Metalanguage

Answer 1

A language used to describe the syntax of other languages

Question 2

EBNF

Answer 2

<> for variables (non-terminal elements), [] for optional, {} for 0 or more, | for or

Question 3

Railroad Diagrams

Answer 3

Rectangles for variables (non-terminal elements), circles for literal (terminal elements — written as they appear), outward semicircle for loop, inward semicircle for or

Question 4

Define Identifier

Answer 4

The name given to a variable, subroutine or function. They must commence with a letter

Question 5

How are input statements used in programming?

Answer 5

Input statements are used to obtain data and store it in some variable. The source of the data may be the keyboard, a file or some other peripheral device.

Question 6

How are output statements used in programming?

Answer 6

Output statements are used to send data to devices. This could be the monitor, printer, a file or almost any other output device.

Question 7

Define assignment

Answer 7

The process of storing the value of an expression in a variable

Question 8

How are assignment statements used in programming?

Answer 8

Assignment statements are used to set the value of variables and properties. Assignment is not the same as equivalence.

Question 9

How is sequence used in programming?

Answer 9

It is the control structure that ensures each process is executed in the correct order. In most programming languages sequence is implemented by writing statements in their correct order; each statement being separated by some character.

Question 10

How is selection used in programming?

Answer 10

By binary selection

Question 11

How is repetition used in programming?

Answer 11

There are two types of repetition – pre-test and post-test

Question 12

Syntax errors

Answer 12

Result when source code statements do not adhere to the rules of the programming language. All syntax errors will be detected as the source code is translated into object code.

Question 13

Runtime errors

Answer 13

Result when for some reason the computer is unable to continue executing instructions. They can be caused by either software or hardware faults. Exceptions are types of runtime errors. Exception handlers are executed when an exception of a particular type occurs.

Question 14

Examples of runtime errors

Answer 14

Mathematical calculations that cannot be evaluated e.g. division by zero, attempting to find the square root of negative numbers, tan 90
Inaccuracies due to non exact floating point representations e.g. 0.99999999 multiplied by 2 gives the result 2
Data that is out of the range of the identifier’s data type e.g. 16-bit integers will have a range from -32768 to 32767
Infinite calls to subroutines/recursion

Question 15

Logic errors

Answer 15

When programs do not correctly work as anticipated. The program may continue to execute, albeit incorrectly. They are the most difficult errors to correct.

Question 16

Examples of logic errors

Answer 16

A loop repeating fewer times than expected, resulting in some data not being processed.
Often logic errors are the cause of runtime errors
For example, a program calculates averages may cause a division by zero error if no data is input

Question 17

Define bug

Answer 17

An error or defect in software or hardware that causes a program to malfunction

Question 18

Stubs

Answer 18

A small routine that takes the place of a yet to be written subroutine or is substituted for an existing subroutine. No real processing takes place within the stub, its purpose is to allow testing of the calling routine or to assist the debugging process.

Question 19

Flags

Answer 19

A boolean variable used to check if a section of the code has been executed. Normally the flag is initialised to false and then set to true if the section of code has been executed. Often a flag will be used to confirm that a particular condition has been met.

Question 20

Debugging output statements

Answer 20

They can be strategically placed within the code to indicate execution has passed through the point. They can also be used to display the contents of variables at particular points in the code. These statements are removed once the source code has been tested and the bugs removed. Output statements are often used as part of a stub.

Question 21

Automated debugging tools

Answer 21

Breakpoints halt execution and allow the contents of variables at that point to be interrogated and even changed. Watch expressions can be used to display the contents of variables during expressions. They can also cause execution to halt when a certain condition becomes true.

Question 22

Data validation

Answer 22

It is common practice to obtain user input as string data. The string is then analysed according to the validation rules appropriate to the particular variable and context. For example, authentication, the process of identifying a user using known information, displays a form to get the username and password from the user, accesses usernames and passwords from a text file, stored passwords are encrypted using a one way hash algorithm such as SHA (Secure Hash Algorithm) so access to the text file does not reveal passwords, and checks if the user has successfully been authenticated.

Question 23

Define Call

Answer 23

To invoke a routine in a programming language. Calling a routine consists of specifying the routine name and, optionally, parameters.

Question 24

Define scope

Answer 24

The extent or range of operation of an identifier, where in a program, a given identifier may be accessed.

Question 25

Local variables

Answer 25

Local variables can only be accessed by the subroutines in which they are declared. In this case other subroutines are unable to directly access these variables. An instance of the local variable is created as the subroutine begins execution and is destroyed once the subroutine ends.

Question 26

Global variables

Answer 26

Variables that are available to other modules within the project. They are created as the program starts and are only destroyed when the program finally ends.

Question 27

Parameter passing

Answer 27

In most cases the variable used as a parameter is not passed, rather a copy of the data is held in the variable or the address in memory of the variable is passed. This is known as passing “by value” or passing “by reference” respectively.

Question 28

Define readability

Answer 28

The ease with which the written word can be read and understood

Question 29

Use of whitespace

Answer 29

White space should be a neutral colour that does not attract or draw the user’s attention. It breaks up the screen into sections and draws the user’s eye to important elements.

Question 30

Judicious use of colour and graphics

Answer 30

Graphics used as icons should deliver a clear message to the user as to their purpose. Effective icons communicate their purpose more quickly than the equivalent text. Coloured text should be used sparingly and only where it has some purpose or message to deliver.

Question 31

Grouping of information

Answer 31

It makes sense to logically group related items together. They can then focus on the required elements more efficiently. Grouping normally is accomplished using frames or borders, which can be used in conjunction with a label.

Question 32

Legibility of text

Answer 32

Refers to the user’s ability to make out each word and or character. How font is justified or aligned on the screen influences legibility.

Question 33

Effective prompts

Answer 33

A prompt is a reminder or a cue as to what is required. Prompts are the main method of communicating with users. Most screen elements contain or are linked to prompts.

Question 34

Unambiguous and non-threatening error messages

Answer 34

Messages are used to provide further information. Error messages are used to inform the user of some problem that has occurred. Error messages should identify the error and also provide some solution in a non-threatening way.

Question 35

Provision of feedback

Answer 35

Feedback to users is crucial and should appear to be instantaneous. The user should never be left wondering whether their actions have been successful. Feedback is provided intuitively as the processing results in some obvious change to the interface or data.

Question 36

Consistency of design

Answer 36

Consistent user interfaces are easier to learn and are therefore easier to use

Question 37

Internal documentation

Answer 37

It is designed to make the programming code readable. Internal documentation aims to help programmers understand source code; it is purely for the benefit of programmers. The use of identifier names that describe their purpose is often called intrinsic documentation.

Question 38

Online help

Answer 38

Most commercial applications provide online help. The major advantage of online help compared to printed documentation is that it can be context sensitive.