Lecture 3 - Functions, Scoping and Software Development

Question 1

Advantages of creating functions

Answer 1

Constructing - does only one thing
Testing - does only one thing
Debugging - does only one thing
Maintenance - less duplication, modificiations easy to make
Expansion/enhancement - improving a function makes improvement available to all code using function
Re-use - general functions can built into warehouse of tools

Question 2

Disadvantages of creating functions

Answer 2

More planning required before coding

Slight execution time cost

Question 3

What are environments?

Answer 3

Structures that organise objects in an R-programme

Question 4

What do environments do?

Answer 4

Associates a set of names to a set of values, perhaps pointing to same object address

Question 5

What are the two types of environments?

Answer 5

Global and current environments

Question 6

What happens to the environment when packages are loaded?

Answer 6

They are added as parents of the global environment

Question 7

What are the scoping rules?

Answer 7

Name making
Function making
Fresh start
Where to look for values associated with objects

Question 8

What happens if a name isn’t defined in an environment?

Answer 8

R looks up one level in the environment structure

Question 9

What happens to a functions environment for each new use?

Answer 9

It is wiped clean

Question 10

When does a lookup of names happen?

Answer 10

At the time the code is executed, not created

Question 11

Why should you test code?

Answer 11

Nobody writes perfect code
Testing can demonstrate sections of code working properly
Reduces possible suspects for further debugging

Question 12

What are three different debugging methods?

Answer 12

Compile time
Batch debugging
Interactive debugging

Question 13

Compile time debugging method

Answer 13

Only errors found here are syntax or for some languages, whether variables have been initialised for use.

Question 14

What is batch debugging?

Answer 14

Peppering code with print( ) statements.
Run code, examine output to find unexpected results. Clumsy approach as code needs to be sanitised of print( ) after errors found)

Question 15

What is interactive debugging?

Answer 15

Set breakpoints, step through code, examine and alter variables while code pauses.

Question 16

What are the stages of statistical software

Answer 16

specification
definition
implementation
verification
delivery and usage
maintenance

Question 17

What are two different types of approach to software development?

Answer 17

Plan drive

Agile

Question 18

What is the waterfall model for software development?

Answer 18

specification/definition/implementation done in unwavering order.
Agreement between user and programmers from the beginning
Documentation and verification of each stage of the process

Question 19

Advantages of waterfall model

Answer 19

End result agreed at beginning
Lots of documentation
Works well for large projects and large organisations

Question 20

Disadvantages of waterfall model

Answer 20

Inflexible

Struggles when project requirements change

Question 21

What is the iterative model for software development?

Answer 21

Planning a project is incremental
Do a bit of specification, bit of coding, bit of verification
Start simple, modify design, enhance capabilities.

Question 22

Advantages of iterative model

Answer 22

Can respond to changing circumstances

Less time agreeing and documenting decisions

Question 23

Disadvantages of iterative model

Answer 23

End result of code is undefined until the end

Less documentation of decisions

Question 24

What to do if you use code written by someone else?

Answer 24

Acknowledge them
Test it actually performs for your purpose
Take time to understand and learn from it
R code in packages might not be examples of good practice or easy to comprehend.

Question 25

Programming style

Answer 25

Implement principle of least privilege
Working code preferable to fast, broken codes
seek out redundant code and convert to module

Question 26

Coding practice within modules

Answer 26

Initialise object used in module at beginning
Include error checks - arguments
Resist temptation to make code terribly dense
Do not nest conditional/looping structures beyond 3 levels deep.

Question 27

Coding conventions

Answer 27

Rationale - decrease coding errors, increase readability, enhance possible reuse.
All functions begin with comments proving information on purpose, input/output, implementation details.
Use indentation for control structures
Meaningful variable names
Consistency of spacing