Some"Metadiscussion" about programming

• Critical mass
• The programming cycle in various paradigms
• "Hacking" vs. "Software engineering"
  • Q&D
  • Execute-once programs
• "Hand-execution"

Data: Variables and containers

The data used by programs exists in a variety of different forms:
• It may live in external files which are opened and processed by programs.

• It may be embedded in the program itself.

• Normally, such "embedded data" is stored in variables. Variables may be thought of as names referring to the location of the data or as containers which contain the data.

• Often data, wherever it is stored, is structured. Most programming languages have facilities for which are cognizant of the structure of certain data elements.

• Examples of structured data include:

  Description	Example
  Constants (hardly structured!)	3   "of"
  Lists	"the,quick,jumped,dog"
  Indexed elements	character i of WordForm

• In HyperCard, assignments are made using the put command.

  put line i of bg fld j of card k into Holder
  

Some container expressions in HyperCard

the fifth line of card field "Text"
line 5 of cd fld text

item 1 of line 3 of bg fld "DataList"
char 14 of item 2 of 

word 2 of the name of this card
word 2 of the short name of cd fld "My Name"
word 2 of the name of cd btn 1

Control logic

Not every statement in a program is executed when it runs, and not every statement is executed once. It is necessary to state conditions under which a statement is executed: executing one set of statements may preclude executing another set of statements. Normally, sets of statements which are all to be executed together are organized into blocks. Often the grouping is explicitly indicated by some sort of enclosing text or symbols.

statement block

do
    statement 1
    statement 2
    statement 3
    ...
    statement n
enddo

The statements in a program which alter the order of execution of statements are called control statements and the use of them in a program is called flow control or control logic. conditional execution of statements

Conditional execution: the general idea "if then else"

if condition then
    statement block
else
   statement block
end if

HyperCard example

on mouseUp
  ask "Gender?"
  if it is "boy" then
    put "male"
  else if it is "girl" then
    put "female"
  else
    put "who knows!"
  end if
end mouseUp

PERL example

if ($line =~ /$exp/) {
     print "$line matched: $& \n";
    }     else {
         print "$line (pattern not found.)\n";
    }

Iteration

Iteration (or "looping") is the process of doing the same process over and over again until some desired result is reached. You may wish to perform the same process on a number of different items (e.g. words in a file), or you may wish to perform the a process on a single item until a certain condition obtains (e.g. exchanging pairs in a list of items until the list is sorted). In TG, for example, iteration was handled via The Cycle, leading to a pernicious idea that language processing and computer processing might share important similarities.

Usually, the syntax of a loop is something like:

(DO, LOOP, REPEAT, FOR) (WHILE, UNTIL, UNLESS, OVER)

Looping: the general idea...

set loop invariant
loop while loop invariant is false
    do something
    reset loop invariant
endloop

PERL example: Read lines from a file and look for a pattern

while ($line = ) {
          chop($line);
      if ($line =~ /$exp/) {
           print "$line matched: $& \n";
          }     else {
               print "$line (pattern not found.)\n";
          }
}

Putting these all together: HyperCard examples

Writing the contents of a HyperCard field a text file

on ExportCorpus
  ask file "Name of file to write"
  if it is empty then exit ExportCorpus
  put it into fn
  open file fn
  write bg fld "Corpus" to file fn
  close file fn
end ExportCorpus

Loading a text file into a field (NB: 30000 character limit!)

on ImportCorpus
  answer file "Name of file to load" of type text
  if it is empty then
    exit ImportCorpus
  else
    put it into fn
  end if  
  put 0 into RecCount
  open file fn
  read from file fn until return
  repeat while it is not empty
    if char 1 of it = "#" then
    else
      put cleanup(it) into line RecCount of bg fld "Corpus"
    end if
    add 1 to RecCount
    read from file fn until return
  end repeat
  close file fn
  answer RecCount && "lines loaded."
end ImportCorpus

Using Functions

function cleanup str
-- remove trailing blanks. Assumes string STR has a CR at the end of it
    put empty into last char of str
    repeat while the last char of str = " "
      put empty into the last char of str
    end repeat
    return str

HyperCard example: A string searching function

Returns the line number of the first matching line in a container

function Search target,source
  put offset(return & target & return,return & source & return) into ofs
  return (the number of lines in char 1 to ofs of source)
end Search

A Palindrome Detector (NB: bug fixed!)

on mouseUp
  -- get the possible palindrome
  put line 1 of cd fld 1 into p
  -- save a copy for later use
  put p into p2
  -- delete punctuation etc. which might mess us up
  repeat with i = the number of characters in p down to 1
    if " ,;.:'" contains char i of p then
      delete char i of p
    end if
  end repeat
  -- reverse the string
  put empty into p3
  repeat with i = 1 to the number of characters in p
    put char i of p before p3
  end repeat
  -- whew! hard part is over!
  answer p && p3
  if p3 = p then
    put p2 && "is a palindrome"
  else
    put p2 && "is NOT a palindrome"
  end if
end mouseUp

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Homework 4 : Corpora (due: Mar. 13)
Homework answers

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