CSC231m Lecture 15

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CSC-231m Lecture 15

• Files

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Todays Topics

• All about files.
• What files really look like
• How to do Homework 8.

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Data vs. Records

• An array is a collection of the same type of
  data, like an array of integers.
• A record is a heterogeneous collection of
  different types of data, like a name, price, and
  quantity for a vending machine item.
• Records can be implemented in three ways in
  MATLAB
• As a cell array.
• As a structure.
• As a simple file.

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A Simple File
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What It Looks Like

• Black ? 1.50? 10ltcrgt
• Brown ? 1.75? 10ltcrgt
• Red ? 2.00? 10ltcrgt
• Orange? 2.25? 1ltcrgt
• Yellow? 2.50? 10ltcrgt
• Green ? 2.75? 5ltcrgt
• lteofgt

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Another Simple File, And Format

• 101
• 1
• 166
• 33
• 227
• 58
• 229
• 41
• 46

• 101ltcrgt
• 1ltcrgt
• 166ltcrgt
• 33ltcrgt
• 227ltcrgt
• 58ltcrgt
• 229ltcrgt
• 41ltcrgt
• 46ltcrgt
• lteofgt

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Delimiting Fields

• Most languages support width-delimited fields,
  that is, each field is a known and fixed width.
  MATLAB does not.
• MATLAB needs fields delimited by various means
• \n so each field is on a different line.
• \t to each field is delimited with a tab.
• Etc.
• The bottom line is that you need some means of
  knowing when one field stops and the next begins.

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Delimiting atm.dat

• You could have each balance on a separate line
• 300.00
• 500.00
• 2500.00
• You could have each balance on one line with tab
  delimiting
• 300.00 500.00 2500.00
• You could have the same with only space
  delimiting
• 300.00 500.00 2500.00
• In each case, fscanf() should properly read each
  value.

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Delimiting Issues

• Function fscanf() reads until it encounters white
  space
• The end of a line.
• A tab character.
• A space.
• The important thing is that you only read in one
  value at a time.

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Opening Files

• The fopen() function returns a file ID, so it
  must be part of an expression
• fid fopen (file, access)
• The filename can be a literal in single quotes or
  a variable name.
• Access can be r for read, w for write, and
  a for append (write to end of file).
• fid fopen (sFileName, r)
• fid fopen (this.dat, w)

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Reading From Files

• Use fscanf() to read in single values or an array
  of like values (HW 8 Part C).
• Use fgetl() (get line) to read in a line of text
  from a file (HW 8 Part A).
• Use fgets() (get string) to read in a line of
  text from a file and retain the line terminator.
• Use textread() or textscan() to read in lines of
  records from a file (HW 8 Part B).

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Writing To Files

• We only need fprintf() to write to files.
• We do not assign it to a variable as we do with
  sprintf().
• fprintf() needs three things
• The open file ID.
• A string literal with placeholders.
• Variables corresponding to the placeholders.
• fprintf (fid, d d\n, iThis, iThat)

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Closing Files

• Always close an open file.
• Function fclose() only needs one thing, the open
  file ID
• fclose (fid)

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HW 8A Reading

• We can only read from a file that exists.
• To create a file, either use the editor to create
  a file, OR, use the write or append portion of
  this program.
• Once a file exists, we read in each line using
  fgetl() to get one line.
• Function fgetl() reads in the line as a string
• sOut fgetl (fid)

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HW 8A Writing

• We can write to a new file (overwriting the file
  if it already exists), or we can append to an
  existing file (add onto the end).
• In this assignment we write to create a file and
  we append to expand a file.
• In each case, we use fprintf() to write to the
  file.
• What distinguishes the operation is how we OPEN
  the file with fopen()
• fid fopen (myfile, w)
• Or fid fopen (myfile, a)

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Your Files

• As written the file looks like
• These are my fat friends
• John Vanilla 234
• Sam Chocolate 185
• Debbie Chocolate Chip 156
• Monty Python Anthrax Ripple 278
• Please come up with something better, like names,
  phone numbers, favorite drinks, age, pets,
  favorite color, etc.

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Your Data

• I used sName, sFlavor, and iWeight, and my
  sprintf function used -15s -15s d.
• Use appropriate field names and placeholders
• sWhatever is a string that uses s or -nns
• fSomething is a float that uses f or .2f
• iThis is an integer that uses d

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HW 8A Procedure

• Get my hw8a2.m file and edit it.
• Using W
• Create a file with three records.
• Using R
• Read the file.
• Using A
• Add two records to the file.
• Using R
• Read the file again.
• Done

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HW 8B Reading

• Here we are reading from a file of records.
• You need file student.txt from my directory.
• We need to use the textscan() function to get one
  line at a time.
• As we read it, we parse it into separate strings
  corresponding to placeholders.
• Then we convert elements of the cell array into
  strings, convert to numbers, compare to values of
  interest, and report results.

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HW 8B Processing

• We read in a line.
• We parse into a cell array.
• We extract items from that array.
• We convert those items to numbers.
• We see if they match our criteria.
• If they do, we print them out.
• If they dont, we discard and read the next line.
• This continues until we get to the end of the
  file.

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Finding Other People

• Numeric fields (class, gpa, units), or
  single-character fields (gender) are easy to deal
  with.
• String fields are harder, and require another
  function
• if (strcmp (sMaj, MUS) 1)
• Not
• if (sMaj MUS)

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End-Of-File lteofgt

• Function feof() tests for end of file.
• It returns 1 when the eof is reached (true) and 0
  otherwise (false).

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HW 8C Step 1

• Remember to create the atm.dat file using your
  editor. It needs to be four lines long
• 300.00ltcrgt
• 500.00ltcrgt
• 2500.00ltcrgt
• lteofgt
• Where ltcrgt does not show up, nor does the
  end-of-file marker lteofgt.

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HW 8C Reading (Originally)

• fid fopen (atm.dat, r)
• dChckBal fscanf (fid, f)
• dSaveBal fscanf (fid, f)
• dVisaBal fscanf (fid, f)
• fclose (fid)

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Fix for HW8 Part C

• fscanf() wants to read an array.
• If the file does not exist, the program fails to
  run.
• Fix
• fid fopen (atm.dat, r)
• if (fid -1) File exists
• adBal fscanf (fid, f f f)
• dChckBal adBal(1)
• dSaveBal adBal(2)
• dVisaBal adBal(3)
• fclose (fid)
• else
• dChckBal 300.00
• dSaveBal 500.00
• dVisaBal 2500.00
• end

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HW 8C Writing

• fid fopen (atm.dat, w)
• fprintf (fid, f\n, dChckBal)
• fprintf (fid, f\n, dSaveBal)
• fprintf (fid, f\n, dVisaBal)
• fclose (fid)

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Other HW8 Fixes

• Part A
• Now
• cWhatever input ()
• Fix
• cWhatever input (, s)
• Part B
• Now
• if (iCls 1 iGpa lt 200)
• fGPA str2num(char(stuff11)) / 100.0
• Fix
• if (iCls 1 iGpa lt 200)
• fGPA iGpa / 100.0

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Todays Lessons

• You should be able to open and close files.
• You should be able to read from and write to
  different types of text files.
• You should better understand Homework 8.