CMP 131 Introduction to Computer Programming Violetta Cavalli-Sforza Week 11, Lecture 2 (Wed)

CMP 131Introduction to Computer

Programming

Violetta Cavalli-Sforza

Week 11, Lecture 2 (Wed)

THIS WEEK

• Today – Programming with text files– Quiz results & going over ..

• Thursday: LAB 01– More work on homework #5

NEXT WEEK

• Quiz #4– Wednesday May 30th

• No make-up session

• I don’t feel a need for more class time but… I can possibly start classes 15 minutes early and end 15 minutes late (since you have ½ hour breaks).– Let me know if you want me to do that!

Topics

• Definitions• Advantages of data files• Text file operations

– Declare– Initialize– Use– Close

• <eof> & <eoln> characters• eof & eoln functions• Reading/Writing characters• Reading/Writing numeric data

Definitions

• Interactive program:– Reads all input data from the keyboard and displays

output on the screen (terminal I/O)

• Batch processing:– Input and output are from data files

• Pure batch processing:– Input and output are strictly from data files, no

interaction with user• Examples are the pipe and filter processing in Unix, Linux and

MSDOS

• Mixed interactive and batch processing:– Have both terminal I/O and file I/O

Definitions [cont.]

• Data file (input file):– A general definition– A file containing input data for a program– In Pascal, it could be a binary file (not readable by a

human) or a text file.• Output file:

– A file containing program results• Text file:

– A disk file containing a collection of characters– Text files can be viewed or created using a text editor– Can be used for input or output

Advantages of Using Data Files• Entering data interactively

– Is ok, if you only have a few data items – If you have lots of data, you want to do it ONCE and

then store the data– It is easy to make errors

• If you do, it’s hard to fix them• You need to start all over again … lots of extra work

• Entering data with a data file– We can run the program as many times a you wish

without reentering the data.• If program output is written to a file, a permanent

copy will be available.• The output file generated by one program can

be used as input data to another file.

Advantages of Using Text Files

• Created with a text editor or similar tool that can write text files

• Easy to check & edit.

• You can establish your own format, e.g.– Have several fields per line– Fields are separated by tab characters,

blanks, comma, or other special character

A File, Conceptually Is

dataitem separator dataitem … dataitem separator dataitem <eoln>



…



<eof>

Text Files In General• A collection of characters stored under some name in

secondary memory.• Consist of multiple lines of character type• Each character occupies one byte, including the <eoln>

& <eof>• Have no fixed size• <eof> marks an end-of-file• The last <eoln> character is followed by the <eof>

character in the file• Both the keyboard & screen are considered as text files• Some systems represent the <eof> on the keyboard by

ctrl/Z

Special File Characters

• <eoln>– Indicates the end-of-line– Inserted each time the keyboard's Enter key is

pressed – Similar to end-of-line in terminal input stream

• <eof>– Indicates the end-of-file – Inserted automatically after the last character of a text

file when the file is saved– Ctrl-D (Unix and related) or Ctrl-Z (MS DOS) are

equivalents for terminal streams

Reading Text Files• readln:

– Read data and skip the rest of input line and go to the start of the next data line.

– Any additional characters remaining on the current data line are not processed.

– Example: If the file contains on each line: number1 number2 number3 … numberN <eoln>and you read with the statement: readln(Value1, Value2); number3 … numberN will be thrown away

• read:– Read data from the same data line during successive

calls.– When you get to the end of the line, use readln

without arguments to go to the next line

Writing Text Files

• Use writeln, and write

• These work like when you are writing to standard output

• You can use formatting directives

Reading a Line of Characters

• Input data will be stored in the variables specified in the input list.

• The order of the data must correspond to the order of the variables in the input list.

• One or more blank character must be inserted between numeric data.

• No blanks required between character data items.

• readln without an input list will skip over any characters on the current data line.

• All of this is the same as reading from the keyboard (input)

Testing for End-of-Line

• eoln function– Pascal defined function (standard function)– Returns true when the next data character is the <eoln>

symbol; otherwise false– Usually used as a sentinel– Form:

eoln(filename) Input is a filename Output is a boolean

– If filename is omitted, keyboard is assumed (input)

Processing A Text File Line• Example: Counting nonblank characters in a line

{Process each input character up to eoln} Count := 0;

WHILE NOT eoln(infile) DOBEGIN

read ( infile, Next ); {Get next character.}

IF Next <> Blank THENCount := Count + 1;

END;readln(infile); {Skip the <eoln>.}

• The last readln statement skips the <eoln>, not the character at the start of the next data line.

• An attempt to read beyond the <eof> character results in run-time error.

Safer to use WHILE than REPEAT

Testing for End-of-File

• eof function– Pascal defined function (standard function)– Returns true if the next character is the <eof>

character– Form:

eof(filename) Input is a filename Output is a boolean

– If filename is omitted, screen is assumed (input)

Using Text Files

• To use Text files in your program, you must:1. Create a text file variable2. Associate the variable with a physical disk file3. Open the file for reading or writing4. Read and write to the file5. Close the file

Declaring Text Files

• Pascal's predefined data type text is used to declare text files.

• Syntax:VAR file-name : text;

• Example:VAR InData,OutData : text;

Text File Variables, Logical Files, and Physical Files

• It is possible to read data from more than one input file or to write result to more than one output file in a single program.

• A file variable represents a logical file• A logical file is explicitly associated with a physical file by

stating the drive, the path, and the physical file name (next slide)

• The same file variable (logical file) can be associated with different physical files in the same program but not at the same time.– If you have to keep accessing several different input and/or

output files at the same time, use different file variables

Opening/Preparing Input Files: assign, reset

• First, associate a file variable (logical file) with a physical file:

assign(input file logical name, physical file name);

• Then, prepare the file for reading:

reset ( input file logical name );

• Example:

VAR InFile: text;

assign(InFile, 'InData.Dat' );reset (InFile);

Opening/Preparing Input Files [cont.]

• reset:– Moves the file position pointer to the beginning of the file– The file position pointer points to the file buffer and selects the

next character to be processed in the file– Must be done before any characters are read from the input file– An error is generated if the input file was not previously saved on

disk

dataitem dataitem … dataitem <eoln>dataitem dataitem … dataitem <eoln>dataitem dataitem … dataitem <eoln>…<eof>

file position pointer

Opening/Preparing Output Files: assign, rewrite

• First, associate a file variable (logical file) with a physical file:

assign(output file logical name, physical file name);

• Then, prepare the file for writing:

rewrite (output file logical name );

• Example:

VAR OutFile: text;

assign(OutFile, ‘OutData.dat' );rewrite (OutFile);

Opening/Preparing Output Files [cont.]

• rewrite:– Prepares the file for output– If the file doesn’t exist, an empty file is created– If the file exists, file position pointer is put to the

beginning of the file. – All old data are lost!

file position pointer

Closing Input & Output Files

• When you are done processing the file, close it: close (input/output logical file name)

• Example:

VAR MyFile: text;

assign(MyFile, ‘MyData.txt' );… {process data/file}close(MyFile);

• Closing essentially makes sure the entire file is saved correctly.

Self-Check 1

• What is wrong with this program?

assign(‘MyData.txt’, InFile);

rewrite(Infile); /* ?? */

reset(Infile);

WHILE NOT eoln DO /* ?? */

BEGIN

read(InFile, Next);

write(Next)

END; {while}

Example: Copying a filePROGRAM CopyFile; {Copies InData file to OutData file} VAR InData, {input file} OutData : text; {output file}

PROCEDURE CopyLine (VAR InData, OutData : text);{Copies a line of file InData to file OutData.BEGIN . . . END { see next slide }

BEGIN {CopyFile}

assign (InData, ‘C:\TP\TEST\InFile.dat’); assign (OutData, ‘C:\TP\TEST\OutFile.dat’); reset (InData); rewrite (OutData); WHILE NOT eof(InData) DO

CopyLine (InData, OutData); writeln (OutPut, 'Input file copied to output file.'); close (InData); close (OutData)

end. {CopyFile}

Example: Copying a file [cont.]PROCEDURE CopyLine (VAR InData, OutData : text);{Copies a line of file InData to file OutData.} VAR Ch : char;

BEGIN

WHILE NOT eoln(InData) DO BEGIN read(InData,Ch); write(OutData,Ch); END; readln(InData); writeln(OutData);

END; {CopyLine}

Example: Copying a file [alternative]

BEGIN {CopyFile}

assign (InData, ‘C:\TP\TEST\InFile.dat’); assign (OutData, ‘C:\TP\TEST\OutFile.dat’); reset (InData); rewrite (OutData); WHILE NOT eof(InData) DO BEGIN WHILE NOT eoln(InData) DO BEGIN read(InData,Ch); write(OutData,Ch); END; readln(InData); writeln(OutData); END; writeln (OutPut, 'Input file copied to output file.'); close (InData); close (OutData)

END. {CopyFile}

Need to add Ch: char; to VAR declarations

Reading/Writing Numeric Data

• When reading numeric data, the computer skips over any leading blanks or <eoln> character. until it encounters a sign or digit

• If several variables are listed in the parameter list for Read or ReadLn, the computer reads data into each variable in the order in which the variables appear

• The file position pointer advances after each value is entered

• Writing a numeric value to a file is similar to writing to the screen

Reading/Writing Numeric Data• Example: If the following variables are declared:

X : real; N : Integer; C: Char;

• For the sequence:

1234.56 789 A345.67<eoln> W<eoln><eof>

Variables valuesStatement X N C1.Read (InData, X, N, C) 1234.56 789 ‘ ‘2.Read (InData, X, N, C, C) 1234.56 789 ‘A’3.ReadLn (InData, X, N, C) 1234.56 789 ‘ ‘4.Read (InData, X, C, N) 1234.56 789 ‘ ‘5.ReadLn (InData, X, C, N) 1234.56 789 ‘ ‘6.Read (InData, C, X, N) 234.56 789 ‘1‘7.ReadLn (InData, C, X, N) 234.56 789 ‘1‘8.ReadLn(InData,X,N); Read(InData,C) 1234.56 789 ‘W‘ 9.ReadLn (InData,X) ReadLn(InData,C) 1234.56 --- ‘W‘

Reading/Writing with Text Files

• Syntax:read (infile, input-list)

readln (infile, input-list)

write (outfile, output-list)

writeLn (outfile, output-list)

• If infile or outfile are omitted, the standard default input (keyboard) or output (Screen) will be assumed

Recursion

Looping/Iteration vs. Recursion

• Looping/iteration is repetition of the same piece of code with slightly different values of – the index variable (for FOR loops)– the control variable/expression (for WHILE and

REPEAT loops)

• With each repetition, the index variable approaches the maximum (FOR .-. TO…) or minimum limit (FOR … DOWNTO …)

• During some repetition, the control variable will cause the loop entrance condition to become false (WHILE loop) or true (REPEAT loop).

Iteration vs. Recursion

• Recursion is an alternative to iteration

• The idea is to– Do a little bit of work towards a problem so

that you can then– Perform a similar operation on a “smaller”

version of the same problem

• Requires the use of procedures or functions

• A recursive function / procedure calls itself

Conceptual Example

• You have a list of ordered (sorted) numbers:1 3 7 15 22 23 28 30 31 45 60 71 72 83 90

• You want to know whether it contains a specific number: 45

• You can search through the list starting at the beginning or at the end

• You can “bisect” the list recursively

1. Look at the middle number:1 3 7 15 22 23 28 30 31 45 60 71 72 83 9045 > 30, so look above 30:2. Look at the middle number: 31 45 60 71 72 83 9045 < 71 so look below 71:3. Look at the middle number: 31 45 60Success!(If you had wanted 47, you would have continued looking at 60 and decided that it wasn’t there).

NOTE: Each time we are looking at a smaller set of numbers!!!

Example: Exponentiation

• Exponentiation = powers of a number

• There is no standard Pascal function for it

• So we make one: Power– A program with iteration– A function with iteration– A function with recursion

A Program that Computes Powers

PROGRAM Power;VAR Num, NumExp: real; I, Pow :integer;BEGIN write('Enter Number (real) and Power (integer): '); readln(Num,Pow); NumExp := 1; FOR I := 1 TO Pow DO NumExp := NumExp * Num; writeln(Num:5:2, ' raised to the power ', Pow,

' is ', NumExp:10:2); readlnEND.

An Iterative Function that Computes Powers

FUNCTION Exponentiate (Num : real; Pow : integer) : real;

VAR NumExp: real;

I : integer;

BEGIN

NumExp := 1;

FOR I := 1 TO Pow DO

NumExp := NumExp * Num;

Exponentiate := NumExp

END; {Exponentiate}

How the Function is UsedPROGRAM Power;VAR TheNumber, NumToPower: real; ThePower :integer;

FUNCTION Exponentiate (Num : real; Pow : integer) : real; … END; {Exponentiate}BEGIN write('Enter Number (real) and Power (integer): '); readln(TheNumber, ThePower); NumToPower := Exponentiate(TheNumber, ThePower); writeln(TheNumber:5:2, ' raised to the power ', ThePower, ' is ', NumToPower:10:2); readln

END.

A Recursive Function that Computes Powers


BEGIN

IF Pow = 0 THEN

Exponentiate := 1

ELSE

Exponentiate := Num * Exponentiate(Num, Pow -1)

END; {Exponentiate}

How Does it Work?


VAR Result : real;BEGIN IF Pow = 0 THEN Result := 1 ELSE Result := Num * Exponentiate(Num, Pow -1) writeln('Num is ', Num:5:2, ' Pow is ', Pow:5,

' Result is ', Result:10:2); Exponentiate := Result;END; {Exponentiate}

Enter Number (real) and Power (integer): 3.00 4

Num is 3.00, Pow is 0, Result is 1.00





Why Recursion?

• Some problems lend themselves recursion

• Mathematically very elegant

• Computationally:– More expensive– But can be efficient in special cases

THE END!

CMP 131 Introduction to Computer Programming Violetta Cavalli-Sforza Week 11, Lecture 2 (Wed)

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