two

1
two

• motion and changeprogramming with imperatives

2
Overview

• Review
• Imperative Programming
• More on objects
• Example
• Appendix

3
The story up until now

• Everything in your computer is data
• Including programs
• Data is divided into objects
• Objects can be inside of other objects
• Boxes inside groups
• Colors inside bitmaps
• Objects have types
• Procedures
• Numbers (1, -3.5)
• Strings (this is a string, blue)
• Bitmaps
• Picture objects (lines, groups, boxes, etc.)

4
The story up until now

• Computation is performed using expressions
• Expressions have (or return) values (i.e.
  outputs)
• Computation is performed by recursively replacing
  expressions with their values
• A computations only output is its return value
• Its return value depends on
• The expressions structure
• The other definitions in the program
• It doesnt depend on what was computed before

5
Review rules for execution

• Look at the expression
• If its a number or string
• Its its own value
• If its a name (i.e. a word)
• Look its value up in the dictionary
• (Check if its one of the special cases from the
  next slide)
• Otherwise its a procedure call
• proc-expression arg-expression1
  arg-expressionn
• Execute all the subexpressions(proc-expression
  and arg-expression1 through arg-expressionn )
• Run the value of, passing it the values of
  proc-expression , passing it the values of
  arg-expression1 through arg-expressionn as inputs
• Use its output as the value of the expression

These rules are worth memorizing
6
Special cases

• If it starts with define
• define name value-expression
• Run value-expression
• Assign its value to namein the dictionary
• If it starts with the words with or with
• with name1 value-expression1 namelast
  value-expressionlastresult-expression
• Run the value-expressions
• Assign their values to their respectivenames in
  the dictionary
• Run result-expression
• Set the dictionary back the way it was
• Return the value from result-expression

• If it has a ? inside it
• name1 namelast ? result-expression
• Make a procedure
• That names its inputsname1 namelast
• And returns the valueof result-expression(presum
  ably using those names)
• If it starts with if
• if test-expression result-expression
  alternative-expression
• Run test-expression
• If it returns true
• Run result-expression and return its value
• Otherwise,
• Run alternative-expression and return its value

7
Sussman form

• define name inputs output
• means exactly the same as
• define name inputs ? output
• Only its usually more readable
• Makes the definition look more like a call to the
  procedure

8
Overview

• Review
• Imperative Programming
• More on objects
• Example
• Appendix

9
Change and effect

• But some expressions dont really return values
• define name value Happens to print out value
  when you run it, but thats not the point.
• using package (e.g. using Examples.Stacking)
  Returns something cryptic but irrelevant what
  matters is that it causes a bunch of procedures
  to become defined
• These are examples of expressions we type
• Not because they return values
• But because they do things
• They change the computer
• These changes are called side effects, or just
  effects
• What it really means is change caused by the
  expression

10
Calling procedures for their effects

• You can write procedures that make a wide range
  of changes in the computer
• Changing a variables value (aka assignment)
• Changing a data object (aka mutation)
• Creating files
• Creating windows
• Performing input or output
• As with everything else in this class,
• Complex effects
• Are ultimately built up from a few kinds of
  simple effects
• And methods for combining them

11
Assignment statements

• The simplest change primitive is assignment (?)
• name ? new-value
• After execution, the variable name is changed to
  have the value new-value
• Variable must have already been created using
  define or with
• You type ? as lt -
• Why is this different from define?
• You use define to make new global variables
• You shouldnt use it inside of a procedure
• You use ? to change a variables value, be it a
  local or global variable, while the program is
  running
• Safe to use inside a procedure

12
Changing a global variable

• define count 0
• define increment! ? count ?
  count 1
• define clear! ? count ? 0

• gt count
• 0
• gt increment!
• ltNoValuegt
• gt count
• 1
• gt clear!
• ltNoValuegt
• gt count
• 0
• gt

13
In Sussman form

• define count 0
• define increment!count ? count 1
• define clear!count ? 0

• gt count
• 0
• gt increment!
• ltNoValuegt
• gt count
• 1
• gt clear!
• ltNoValuegt
• gt count
• 0
• gt

Sussman form is a more readable shorthand for
defining procedures
14
Sequencing

• Changes are most useful when we can chain them
  together
• That means we need some way of specifying that
• We want to do several things in a row
• And we want them done in a specific order

15
Sequencing with procedures

• Procedures can specify a series of expressions to
  run
• args ? expression expression
• define name args expression
  expression
• The expressions are run in order, first to last
• The value of the last expression is returned as
  the value of the procedure
• The values of the other expressions are ignored
  (although the expressions are still executed)

16
Changing a global variable

• define count 0
• define increment!count ? count 1count
• define clear! count ? 0 count

• gt count
• 0
• gt increment!
• 1
• gt count
• 1
• gt clear!
• 0
• gt count
• 0
• gt

17
Iteration (aka looping) so far

• So far, when weve wanted to do something
  repeatedly, weve
• Written the something as a procedure
• Call another procedure that iterates and passed
  our procedure to it as an argument
• So forms of iteration are represented by
  specialized procedures

• iterated-group n ? box n 2
  n 2 10
• filter beatles? list
• map album-title filter beatles? list
• fold list

18
Looping as a sequencing primitive

• Most imperative languages have special constructs
  for iteration
• The most basic is the while loop
• Like an if that keeps running

• while test expressions
• Means
• Run test
• If its true, run expressions
• And run test again, etc,
• Keep going until test is false

19
Folding procedures over lists

• Remember fold?
• It uses a procedure to squish all the elements
  of a list together
• Folding with adds all the elements
• Folding with multiplies them

• fold proc list
• Calls proc on the first two elements of the list
• Then calls it again on the result and the next
  element
• And again, and so on
• fold list 1 2 3returns 6

20
Fold in imperative form

• define fold proc list Make some variables
  with answer first list position
  1 Loop over all the elements of the list
  while lt position length
  list Fold in one more element
  answer ? proc answer item list position
  position ? position 1
• Return the accumulated answer
  answer

21
Example fold list 1 2 3 4
Var Value proc list 1 2 3 4

• define fold proc list Make some variables
  with answer first list position
  1 Loop over all the elements of the list
  while lt position length
  list Fold in one more element
  answer ? proc answer item list position
  position ? position 1
• Return the accumulated answer
  answer

22
Example fold list 1 2 3 4
Var Value proc list 1 2 3 4 answer 1 position
1

• define fold proc list Make some variables
  with answer first list position
  1 Loop over all the elements of the list
  while lt position length
  list Fold in one more element
  answer ? proc answer item list position
  position ? position 1
• Return the accumulated answer
  answer

23
Example fold list 1 2 3 4
Var Value proc list 1 2 3 4 answer 1 position
1

• define fold proc list Make some variables
  with answer first list position
  1 Loop over all the elements of the list
  while lt position length
  list Fold in one more element
  answer ? proc answer item list position
  position ? position 1
• Return the accumulated answer
  answer

24
Example fold list 1 2 3 4
Var Value proc list 1 2 3 4 answer 3 position
1

• define fold proc list Make some variables
  with answer first list position
  1 Loop over all the elements of the list
  while lt position length
  list Fold in one more element
  answer ? proc answer item list position
  position ? position 1
• Return the accumulated answer
  answer

25
Example fold list 1 2 3 4
Var Value proc list 1 2 3 4 answer 3 position
2

• define fold proc list Make some variables
  with answer first list position
  1 Loop over all the elements of the list
  while lt position length
  list Fold in one more element
  answer ? proc answer item list position
  position ? position 1
• Return the accumulated answer
  answer

26
Example fold list 1 2 3 4
Var Value proc list 1 2 3 4 answer 3 position
2

• define fold proc list Make some variables
  with answer first list position
  1 Loop over all the elements of the list
  while lt position length
  list Fold in one more element
  answer ? proc answer item list position
  position ? position 1
• Return the accumulated answer
  answer

27
Example fold list 1 2 3 4
Var Value proc list 1 2 3 4 answer 6 position
2

• define fold proc list Make some variables
  with answer first list position
  1 Loop over all the elements of the list
  while lt position length
  list Fold in one more element
  answer ? proc answer item list position
  position ? position 1
• Return the accumulated answer
  answer

28
Example fold list 1 2 3 4
Var Value proc list 1 2 3 4 answer 6 position
3

• define fold proc list Make some variables
  with answer first list position
  1 Loop over all the elements of the list
  while lt position length
  list Fold in one more element
  answer ? proc answer item list position
  position ? position 1
• Return the accumulated answer
  answer

29
Example fold list 1 2 3 4
Var Value proc list 1 2 3 4 answer 6 position
3

• define fold proc list Make some variables
  with answer first list position
  1 Loop over all the elements of the list
  while lt position length
  list Fold in one more element
  answer ? proc answer item list position
  position ? position 1
• Return the accumulated answer
  answer

30
Example fold list 1 2 3 4
Var Value proc list 1 2 3 4 answer 10 position
3

• define fold proc list Make some variables
  with answer first list position
  1 Loop over all the elements of the list
  while lt position length
  list Fold in one more element
  answer ? proc answer item list position
  position ? position 1
• Return the accumulated answer
  answer

31
Example fold list 1 2 3 4
Var Value proc list 1 2 3 4 answer 10 position
4

• define fold proc list Make some variables
  with answer first list position
  1 Loop over all the elements of the list
  while lt position length
  list Fold in one more element
  answer ? proc answer item list position
  position ? position 1
• Return the accumulated answer
  answer

32
Example fold list 1 2 3 4
Var Value proc list 1 2 3 4 answer 10 position
4

• define fold proc list Make some variables
  with answer first list position
  1 Loop over all the elements of the list
  while lt position length
  list Fold in one more element
  answer ? proc answer item list position
  position ? position 1
• Return the accumulated answer
  answer

33
Example fold list 1 2 3 4
Var Value proc list 1 2 3 4 answer 10 position
4

• define fold proc list Make some variables
  with answer first list position
  1 Loop over all the elements of the list
  while lt position length
  list Fold in one more element
  answer ? proc answer item list position
  position ? position 1
• Return the accumulated answer
  answer

34
Programming with effects can be tricky

• ? define fold proc list Make some
  variables with answer first list
  position 1 Loop over all the
  elements of the list while lt position
  length list Fold in
  one more element position ?
  position 1 answer ? proc answer
  item list position
• Return the accumulated answer
  answer
• ltProcedure foldgt
• ? fold list 1 2 3 4
• Error Index was outside the bounds of the array.
• ?

35
What happened?
Var Value proc list 1 2 3 4 answer 8 position
4

• ? define fold proc list Make some
  variables with answer first list
  position 1 Loop over all the
  elements of the list while lt position
  length list Fold in
  one more element position ?
  position 1 answer ? proc answer
  item list position
• Return the accumulated answer
  answer
• ltProcedure foldgt
• ? fold list 1 2 3 4
• Error Index was outside the bounds of the array.
• ?

36
What happened?
Var Value proc list 1 2 3 4 answer 8 position
4

• ? define fold proc list Make some
  variables with answer first list
  position 1 Loop over all the
  elements of the list while lt position
  length list Fold in
  one more element position ?
  position 1 answer ? proc answer
  item list position
• Return the accumulated answer
  answer
• ltProcedure foldgt
• ? fold list 1 2 3 4
• Error Index was outside the bounds of the array.
• ?

37
Functional programmingversus imperative
programming

• Functional programming is programming without
  effects and sequencing
• The only effects (output) of an expression are
  its return value
• Value of a procedure call is determined entirely
  by the procedures arguments
• Dont need to worry about what order arguments
  are executed in
• Imperative programming
• Expressions can have many different effects
  return value, changing variables, deleting files,
  etc.
• Those effects can influence the behavior of other
  expressions
• Even if they seem unrelated

38
Functional programmingversus imperative
programming

• Functional programming is programming without
  effects and sequencing
• Only output from a procedure is its return value
• Procedures behave like clauses in English (or
  functions in math)
• Computation is achieved by nesting procedure
  calls
• We think about execution in terms of call and
  response, transformation, and the other metaphors
  we discussed last quarter
• Imperative programming
• Output of a procedure is its effect on the
  computer
• Computation is achieved by sequencing effects
• We think about execution in terms of changes and
  causality

39
Fold in functional an imperative form

• Functional version
• define fold proc list if length list
  1 first list proc first list
  fold proc rest list
• Notes
• The functional version uses recursion(remember
  recursion?)
• And it uses the rest procedure, which returns all
  but the first element of a list
• Also, these two versions dont do exactly the
  same thing
• They process the elements of the list in opposite
  orders
• But theyre easier to understand this way

• Imperative version
• define fold proc listwith answer first
  list position 1 while lt
  position length list
  answer ? proc answer item list
  position position ? position 1
  answer

40
Fold in functional an imperative form

• Functional version
• define fold proc list if length list
  1 first list proc first list
  fold proc rest list
• More focused on what to compute(at least some
  people think so)

• Imperative version
• define fold proc listwith answer first
  list position 1 while lt
  position length list
  answer ? proc answer item list
  position position ? position 1
  answer
• More focused on how to compute it

41
Advantages of imperative programming

• Imperative programs can be more efficient than
  functional programs
• Sometimes it really is simpler
• Simulations
• What youre computing just is a series of changes
• The changes the simulated system would have made
• Imperative style is much more natural
• Directly expresses change
• Example video games
• Using random numbers
• If your random number procedure always returns
  the same value, it isnt very useful
• Other applications where the task definition
  involves change

42
Overview

• Review
• Imperative Programming
• More on objects
• Example
• Appendix

43
Looking inside data objects
Ellipse Width 15 Height 10

• Data objects are like forms
• They have fields (aka members)
• Filled in by values
• The fields
• Have names (Width, Height)
• The fields are filled in by other data objects
• The objects type (Box, Color) determines what
  fields it has

Box Width 10 Height 10
Number Value 10
Procedure Name iterated-group Arguments proc
count Body apply group
up-to count proc
Color R 240 G 220 B 0
44
Member notation
Ellipse Width 15 Height 10

• You can ask for a field of a data object using
  the . notation object.memberName
• myColor.R
• pixel myBitmap 0 0.R
• iterated-group.Name
• iterated-group.Arguments
• mybox.Width
• Note to simplify the presentation, Ive lied
  here about what the actual fields of boxes,
  procedures, etc. are.
• You can find out what fields are really in an
  object using the inspector.

Box Width 10 Height 10
Number Value 10
Procedure Name iterated-group Arguments proc
count Body apply group
up-to count proc
Color R 240 G 220 B 0
45
Generalized assignment (aka mutation)

• You can also change fields of an object using ?
• object.member-name ? new-value
• After execution, the field member-name of object
  is changed to have the value new-value
• Object may be any expression (not just a
  variable)
• Examples
• myColor.R ? 7
• myPen.Brush.Color.R ? 7
• form.Text ? This is a the title of this window

46
Member procedures (aka methods)

• A lot of procedures are stored inside of
  objects
• You access them like any other members, except
  theyre procedures so you can call them
• object.member arg arg

• Examples
• someobject.ToStringConverts someobject into a
  string that (hopefully) is descriptive of the
  object.
• Directory.GetFiles c/Returns a list of all
  the files in the specified directory (C\, in
  this case)

47
Namespaces

• Namespaces are objects that are used to organize
  other objects into a directory-like structure
• This is sort of a lie, but its close enough for
  now
• For example, the type System.Windows.Forms.But
  ton implements buttons you can place in windows

• System
• Holds most of the objects that make up .NET
• System.Drawing
• Holds the objects that implement 2D graphics
• System.Windows.Forms
• Holds the objects that implement windows,
  buttons, etc.
• System.IO
• Holds objects that implement reading and writing
  to file and directories (folders)
• Meta
• Holds the objects that make up Meta
• Packages, Examples
• Contains other namespaces that

48
The using directive

• Typing System.Windows.Forms.Buttonover and
  over again is tedious and error prone
• We can tell the system that were using a
  namespace
• So it understands to look in the namespace for
  names it doesnt otherwise recognize

• using namespaces
• Tells the system to search the specified
  namespaces for variables whose names it doesnt
  recognize
• Example
• ? System.Windows.Forms.Button
• ltButtongt
• ? Button
• Error Undefined variable
• ? using System.Windows.Forms
• ? Button
• ltButtongt

49
Overview

• Review
• Imperative Programming
• More on objects
• Example
• Appendix

50
File system operations

• The File and Directory objects contain procedures
  for working with files and directories.
• They are in the System.IO namespace

• using System.IO
• File.Delete C/junk.txt
• Directory.GetFiles C/

51
Specifying files using pathnames

• An (absolute) pathname is a string describing
  exactly where a file is on disk
• On windows, it starts with the drive letter
• Then has all the directory names it takes to get
  to the file (in order)
• And finally, the files name and extension

• C/documents and settings/ian/desktop
• The true name of my desktop folder
• c/windows/system32/virus.exe
• A file called virus.exe in my windows/system32
  folder
• d/foo/bar/baz.txt
• A text file on my D drive

52
Delimiters in pathnames

• The first hierarchical file system was MULTICS,
  which used the gt character to separate directory
  names, as in
• gtfoogtbargtbaz.txt
• ATT did a simplified version of MULTICS called
  Unix. It used slashes instead of gt
• /foo/bar/baz.txt
• DOS used Unix-style pathnames (with drive letters
  for backward compatibility with an earlier
  operating system called CP/M)
• C/foo/bar/baz.txt

53
Delimiters in pathnames

• Then ATT sued the (then) small company Microsoft
  for infringing their patent on using / as a
  folder separator
• Microsoft changed the / to a \
• C\foo\bar\baz.txt
• Unfortunately, in almost every programming
  language, \ means something special in a string
  and so you have to type it as two \s
• C\\foo\\bar\\baz.txt
• This is a pain, so windows lets you use / too

54
Some file operations

• File.Copy fromPath toPath overwrite?
• Copies the data in the file specified by fromPath
  to toPath
• If overwrite? is true, any existing file named
  toPath will be overwritten (destroyed)
• If overwrite? is false and the file toPath
  already exists, then this will generate an error
• File.Exists path
• Returns true if theres a file named path,
  otherwise false
• File.Delete path
• Nukes the specified file
• File.GetLastWriteTime path
• Returns an object of type DataTime indicating
  when the file was last changed
• File.Move fromPath toPath
• Changes the name and/or directory (folder) of the
  specified file

55
Some directory operations

• Directory.CreateDirectory path
• Creates a new directory with the specified path
• Parent directory must already exist
• Directory.Delete path recursive?
• Nukes a directory and any files within it
• If recursive? is true, it first deletes all files
  and subdirectories in the directory. If its
  false, then the directory had better already be
  empty.
• Directory.Exists path
• Returns true if there is a directory by that name
• Directory.GetDirectories path
• Returns a list of the names of all subdirectories
  of the directory path
• Directory.GetFiles path
• Returns a list of the names of all the files in
  the directory path.
• Directory.Move fromPath toPath
• Moves directories, but more finicky than
  File.Move
• Doesnt work if theres already a directory
  called toPath
• Doesnt work if the directories are on different
  drives

56
Example

• ? Directory.GetDirectories c/
• "c/cygwin" "c/Documents and Settings"
  "c/Program Files" "c/RECYCLER" "c/RioDrivers"
  "c/RioKarma" "c/SCHEME-UTILS""c/Schemebot"
  "c/sonysys" "c/System Volume Information
  "c/WINDOWS"
• ?

57
Finding all the files in a subtree

• Directory.GetFiles gives us all the files inside
  a directory
• How do we find all the files in the directory and
  its subdirectories?
• Use recursion!

58
Finding all files in a subtree

• define all-files directoryapply append
  Directory.GetFiles directory
  map all-files
  Directory.GetDirectories directory

59
Copying all the files in a directory

• How do we copy all the files in a directory (but
  not the subdirectories)?

60
Procedure for working with Path names

• Path.GetFileName path
• Returns just the filename (no directories etc.)
  from the path
• Path.Combine directory filename
• Forms a new path by combining the specified
  directory and filename
• (The actual behavior of Combine is more
  complicated than this, but this will do for now)

61
Copying all the files in a directory

• define copy-files from towith files
  Directory.GetFileNames from position
  0 while lt position length files
  with file item files position
  File.Copy file
  Path.Combine to
  
  Path.GetFileName
  file position ? position
  1

62
Copying all the files in a directory

• define copy-files from towith files
  Directory.GetFileNames from position
  0 while lt position length files
  with file item files position
  File.Copy file
  Path.Combine to
  
  Path.GetFileName
  file position ? position
  1

Get the files in the directory
63
Copying all the files in a directory

• define copy-files from towith files
  Directory.GetFileNames from position
  0 while lt position length files
  with file item files position
  File.Copy file
  Path.Combine to
  
  Path.GetFileName
  file position ? position
  1

Start with the first file
64
Copying all the files in a directory

• define copy-files from towith files
  Directory.GetFileNames from position
  0 while lt position length files
  with file item files position
  File.Copy file
  Path.Combine to
  
  Path.GetFileName
  file position ? position
  1

Repeat until were done with all the files
65
Copying all the files in a directory

• define copy-files from towith files
  Directory.GetFileNames from position
  0 while lt position length files
  with file item files position
  File.Copy file
  Path.Combine to
  
  Path.GetFileName
  file position ? position
  1

Get the current file
66
Copying all the files in a directory

• define copy-files from towith files
  Directory.GetFileNames from position
  0 while lt position length files
  with file item files position
  File.Copy file
  Path.Combine to
  
  Path.GetFileName
  file position ? position
  1

Figure out the name that file would have in the
new directory
67
Copying all the files in a directory

• define copy-files from towith files
  Directory.GetFileNames from position
  0 while lt position length files
  with file item files position
  File.Copy file
  Path.Combine to
  
  Path.GetFileName
  file position ? position
  1

Copy the file
68
Copying all the files in a directory

• define copy-files from towith files
  Directory.GetFileNames from position
  0 while lt position length files
  with file item files position
  File.Copy file
  Path.Combine to
  
  Path.GetFileName
  file position ? position
  1

Move to the next file
69
Copying all the files in a directory

• define copy-files from towith files
  Directory.GetFileNames from position
  0 while lt position length files
  with file item files position
  File.Copy file
  Path.Combine to
  
  Path.GetFileName
  file position ? position
  1

Repeat until weve done all the files
70
A cleaner way

• define copy-files from tofor-each file ?
  File.Copy file
  Path.Combine to
  Path.GetFileName file
  Directory.GetFiles from
• For-each is like map
• It calls the procedure on each element of the
  list
• But it doesnt return anything

71
A simpler way

• define copy-files from tofor-each file ?
  File.Copy file
  Path.Combine to
  Path.GetFileName file
  Directory.GetFiles from
• Repeatedly run

72
A simpler way

• define copy-files from tofor-each file ?
  File.Copy file
  Path.Combine to
  Path.GetFileName file
  Directory.GetFiles from
• This procedure

73
A simpler way

• define copy-files from tofor-each file ?
  File.Copy file
  Path.Combine to
  Path.GetFileName file
  Directory.GetFiles from
• On every element of this list

74
A simpler way

• define copy-files from tofor-each file ?
  File.Copy file
  Path.Combine to
  Path.GetFileName file
  Directory.GetFiles from
• Which is to say, on every file in the directory

75
Copying a whole subtree

• Okay, now how do we change it to copy all the
  subdirectories too?

76
Copying a subtree

• define copy-subtree from toDirectory.CreateDi
  rectory tofor-each file ?
  File.Copy file
  Path.Combine to
  Path.GetFileName
  file Directory.GetFiles
  fromfor-each subdir ?
  copy-directory subdir
  Path.Combine to
  
  Path.GetFileName subdir
• Directory.GetDirectories
  from

77
Copying a subtree

• define copy-subtree from toDirectory.CreateDi
  rectory tofor-each file ?
  File.Copy file
  Path.Combine to
  Path.GetFileName
  file Directory.GetFiles
  fromfor-each subdir ?
  copy-directory subdir
  Path.Combine to
  
  Path.GetFileName subdir
• Directory.GetDirectories
  from

Make the target directory if it doesnt already
exist
78
Copying a subtree

• define copy-subtree from toDirectory.CreateDi
  rectory tofor-each file ?
  File.Copy file
  Path.Combine to
  Path.GetFileName
  file Directory.GetFiles
  fromfor-each subdir ?
  copy-directory subdir
  Path.Combine to
  
  Path.GetFileName subdir
• Directory.GetDirectories
  from

Copy every file
79
Copying a subtree

• define copy-subtree from toDirectory.CreateDi
  rectory tofor-each file ?
  File.Copy file
  Path.Combine to
  Path.GetFileName
  file Directory.GetFiles
  fromfor-each subdir ?
  copy-directory subdir
  Path.Combine to
  
  Path.GetFileName subdir
• Directory.GetDirectories
  from

Copy every subdirectory
80
Overview

• Review
• Imperative Programming
• More on objects
• Appendix

81
begin expressions

• Another way of sequencing effects
• begin expression
• Execute each expression, in order
• Again, all return values are ignored
• Except for the last
• Which is returned as the value of the begin
  expression

82
Conditional imperatives

• You can omit the last expression from an if
• Then it will run the expression if the test is
  true
• But it wont run anything (but the test) if the
  test is false
• You can also use when to run a series of
  expressions when a test is true
• Equivalent toif test begin expressions
  
• Or use unless to run a series of expressions when
  the test is false
• Equivalent toif not test begin
  expressions

• if test expression
• when test expressions
• unless test expressions

83
Cond expressions

• Like a hybrid of if and begin
• Lets you specify several things to run if a
  condition is true
• Lets you specify several conditions to check

• cond test1 expressions
  test2 expressions else
  expressions