Exercises for Wednesday, October 13, 2004

1. In the following program, each line marked with a "<<<" has a bug.
   Briefly describe each bug and the problem it would cause when the
   program is run.

#!/bin/csh
set line = $<
echo "<table align="center" border="1">"	    <<< 
while ( "$line" != " " )                            <<<
  printf <tr>\n                                     <<<
  foreach word ( `echo $line | sed 's/;/ ;/g'` )    <<<
    printf "<td>"                                   <<<
    if ( "$word" == " ; " ) then                    <<<
      printf "</td>\n"
    else 
      printf $word
    endif
  end
  printf "</tr>\n"                                  <<<
end                                                 <<<
echo "</table>"

2. There are multiple ways of doing just about everything.  For 
   the exercise of building an HTML table from a file containing
   semicolon-delimited database records, we suggested that you use
   'sed' to expand each record into a list of separate words and 
   then use a 'foreach' loop to build the rows of the table field 
   by field. An alternative approach would be to use 'sed' to 
   iteratively dissect each record into fields.  Describe what 
   each line of the following program does:

#!/bin/csh
set line = "Sally Ann;Smith;26;F"
printf "0 - field = [] ; line = [$line]\n"
set line_number = 1
while ( "$line" != "" )
  set field = `echo $line | sed 's/\([^;]*\);.*/\1/'`
  set line = `echo $line | sed 's/'"$field"'[;]*\(.*\)/\1/'`
  printf "$line_number - field = [$field] ; line = [$line] ; \n"
  @ line_number ++
end

3. You can also build records field by field.  Here's an example
   illustrating how you might do this in a script that interactively
   adds a record to a databate table.  This version assumes that each
   field name is a single word.  Rewrite the script to support two
   additional features: (i) add code so that the script can handle
   multi-word field names and (ii) near the end of the script add code
   to display the completed record and prompt the user for a response
   (yes or no) that determines whether or not the record is actually
   added to the data file for the corresponding table.

% ls account.*
account.fields account.data
% cat account.fields
first;last;age;gender
% cat account.data
Sally;Smith;26;F
% cat insert
#!/bin/csh

# the first and only argument specifies a table
set table = $argv

# grab the field names for the specified table
set fields = ( `cat $table.fields | sed 's/;/ /g'` )

# create an empty record 
set record = ""

# build the record field by field 
foreach field ( $fields )
  # prompt the user for the value of a field
  printf "enter $field = "
  # append the entered value to the record
  set record = "$record;$<"
end

# strip off the leading semicolon and append 
# the completed record to the table data 
echo $record | sed 's/;//' >> $table.data

% insert account
enter first = Freddy
enter last = Jones
enter age = 21
enter gender = M
% cat account.data
Sally;Smith;26;F
Freddy;Jones;21;M