Introduction To Files In Pascal

Transcription

1 Why other With Files? Introduction To Files In Pscl Too much informtion to input ll t once The informtion must be persistent (RAM is voltile) Etc. In this section of notes you will lern how to red from nd write to files in Pscl. Wht You Know About Input And Output Comes from the user or is displyed to the user Person to progrm (red / redln) Wht You Need In Order To Red Informtion From A File 1. Declre file vrible 2. Open the file 3. A commnd to red the informtion Person to progrm (write / writeln) Wht You Will Lern Input And Output Using Files Informtion is retrieved from nd written out to file (typiclly on disk) File (on disk) File to progrm (red / redln) Progrm to file (write / writeln) 1. Declring File Vribles Allows the progrm ccess to file nme of file vrible text; lettergrdes text;

2 2. Opening Files Prepres the file for reding A. Links the file vrible with the physicl file (references to the file vrible re references to the physicl file). Positions the file pointer reset (nme of file vrible, loction nd nme of file); reset (lettergrdes, lettergrdes.txt ); 3. Reding Informtion From Files Performed with red or redln red (nme of file vrible, vrible to store the informtion); redln (nme of file vrible, vrible to store the informtion); redln(lettergrdes, letter); A. Linking The File Vrible With The Physicl File Your progrm lettergrdes.txt (text file) File vrible (lettergrdes) 3. Reding Informtion From Files (2) Typiclly reding is done within the body of loop while NOT EOF (nme of file vrible) do red (nme of file vrible, vrible to store the informtion); OR redln (nme of file vrible, vrible to store the informtion); end; (* Done reding from input file *) while NOT EOF (lettergrdes) do redln(lettergrdes, letter); writeln(letter); end; (* Loop to red letter grdes file *). Positioning The File Pointer lettergrdes.txt A C An Alterntive Approch To Reding Files Employ sentinel in the file Keep reding from the file until the sentinel vlue is encountered vr inputfile text; vr num integer; redln (inputfile, num); while NOT (num = -1) do writeln(num); redln(inputfile, num); end; (* Done reding input file *)

3 Reding From Files Putting It All Together A complete version of this progrm cn be found in Unix under /home/231/exmples/storge/grdes.p progrm grdes (output); vr lettergrdes text; vr letter chr; Wht You Need To Write Informtion To A File 1. Declre file vrible 2. Open the file 3. A commnd to write the informtion (* Open file for reding, confirm file with user. *) reset(lettergrdes, 'lettergrdes.txt'); writeln('opening file "lettergrdes" for reding.'); while NOT EOF (lettergrdes) do redln(lettergrdes, letter); writeln(letter); end; (* Loop to red letter grdes file *) Reding From Files Putting It All Together (2) close(lettergrdes); writeln('completed reding of file "lettergrdes"'); end. 1. Declring An Output File Vrible No difference in the declrtion of file vrible when writing to file from the cse of reding from file. nme of file vrible text; lettergrdes text; grdepoints text; View Of Files In Unix \ home 231 exmples storge 2. Opening The File Two methods 1) Rewriting erses the old contents of the file (rewrites over wht ws lredy there). 2) Appending retin the old contents of the file (ppends the new informtion t the end). Formt (rewriting / ppending) rewrite (nme of file vrible, loction nd nme of physicl file); ppend (nme of file vrible, loction nd nme of physicl file); grdes.p grdes* lettergrdes.txt Exmple (rewriting / ppending) rewrite(grdepoints, grdepoints.txt ); ppend(grdepoints, grdepoints.txt );

4 3. Writing To A File write (nme of file vribles, vrible(s) nd/or strings to write); writeln (nme of file vribles, vrible(s) nd/or strings to write); writeln(grdepoints, gp); Writing To A File Putting It All Together (3) writeln(grdepoints, gp); end; (* Loop to red letter grdes file *) writeln('finished reding nd writing to files.'); close(lettergrdes); close(grdepoints); end. Writing To A File Putting It All Together A complete version of this progrm cn be found in Unix under /home/231/exmples/storge/grdes2.p progrm grdes (output); vr lettergrdes, grdepoints text; vr letter chr; vr gp integer; reset(lettergrdes, 'lettergrdes.txt'); rewrite(grdepoints, 'grdepoints.txt'); Detils Of Write And Writeln For Files Intuitive View Progrm sttement Effect on file rewrite(dt, dt.txt ); write (dt, 'x'); write(dt, 'y'); write(dt, 'z'); x xy "dt.txt" nd pointer t strt) writeln('opening file "lettergrdes" for reding.'); writeln('opening file "grdepoints" for writing.'); writeln(dt); _ write(dt,''); Writing To A File Putting It All Together (2) while NOT EOF (lettergrdes) do redln(lettergrdes, letter); cse (letter) of 'A' gp = 4; '' gp = 3; Detils Of Write And Writeln For Files Actul View Progrm sttement Effect on file rewrite(dt, dt.txt ); "dt.txt" nd pointer t strt) 'C' gp = 2; write (dt, 'x'); x 'D' gp = 1; 'F' gp = 0; else gp = -1; end; (* cse *) write(dt, 'y'); write(dt, 'z'); xy

5 Detils Of Red And Redln For Files Intuitive View 1 Progrm sttement Effect on file Effect in progrm reset (dt, dt.txt ); "dt.txt" nd pointer t strt) red(dt, ch); Vlue of ch 'x' redln(dt, ch); Vlue of ch Y Pssing File Vribles As Prmeters Must be pssed s vrible prmeters only. procedure nmeprocedure (vr nmefile text); procedure fileinputouput (vr lettergrdes text; vr grdepoints text); red(dt, ch); Vlue of ch '' 1 Assume tht the code on the previous slide hs creted the file clled "dt.txt" Detils Of Red And Redln For Files Actul View 1 Progrm sttement Effect on file Effect in progrm reset (dt, dt.txt ); <EOL> "dt.txt" nd pointer t strt) red(dt, ch); <EOL> Vlue of ch 'x' redln(dt, ch); <EOL> Vlue of ch 'y red(dt, ch); <EOL> Vlue of ch'' red(dt,ch); <EOL> You Should Now Know How to declre file vrible How to open file for reding How to open file file for writing (rewrite nd ppend mode) How to red (red/redln) from nd write (write/writeln) to file The detils of informtion is red from nd written to file How to close file nd why it is good prctice to do this explicitly How to pss file vribles s prmeters 1 Assume tht the code on the previous slide hs creted the file clled "dt.txt" Detils Of Red And Redln For Files Actul View 1 Progrm sttement Effect on file Effect in progrm reset (dt, dt.txt ); <EOL> "dt.txt" nd pointer t strt) red(dt, ch); <EOL> Vlue of ch 'x' redln(dt, ch); <EOL> Vlue of ch 'y red(dt, ch); <EOL> Vlue of ch'' red(dt,ch); <EOL> Error red pst end of file 1 Assume tht the code on the previous slide hs creted the file clled "dt.txt"