Pascal Programming Today Chapter 11 1 Chapter 11.
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Transcript of Pascal Programming Today Chapter 11 1 Chapter 11.
Pascal Programming Today Chapter 11
1
Chapter 11
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» Data are usually stored in text files.
» Text files save data one by one in the format of a sequence of characters.
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» Each line in a text file is ended with an end-of-line marker.
» An end-of-file marker, indicates the end of the file, is placed at the end of the file.
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» Procedures involved in writing data to files:–assign
–rewrite
–write
–close
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» Procedures involved in reading data from files:–assign
–reset
–read
–close
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» Variables of type text are used to identify files.
Syntax of declaring text files
var
<File variable> : text;
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» Procedure assign associates file variables with files in secondary storage.
Syntax of the procedure assign
assign(<File variable>, <File name>)
It is a user-defined identifier declared as type text.
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» Procedure assign associates file variables with files in secondary storage.
Syntax of the procedure assign
assign(<File variable>, <File name>)
May be a string constant of the file name or a string variable holding the file name.
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» Procedure rewrite prepares files for writing data to them.
» Original data will be lost when procedure rewrite is applied to the files.
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Syntax of the procedure rewrite
rewrite(<File variable>)
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» Procedure reset prepares files for reading data from them.
Syntax of the procedure reset
reset(<File variable>)
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Differences between procedures rewrite and reset
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» Procedure close is used to close a file.
Syntax of the procedure close
close(<File variable>)
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Summary of procedures of writing data to and reading data from a file
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» Procedures writeln and write can save data to files.
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Syntax of the procedures writeln and write for writing data to files
writeln(<File variable>, <Output item 1>, <Output item 2>, ...)
or
write(<File variable>, <Output item 1>, <Output item 2>, ...)
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» File variables must be supplied in front of the output lists, otherwise data will be displayed on screen only.
» Data written to files can be formatted in similar way as data displayed on screen.
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» Procedure writeln will add an end-of-line marker after a list of output items to files while procedure write will not.
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» Procedures readln and read can obtain data from files.
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Syntax of the procedures readln and read for reading data from files
readln(<File variable>, <Variable 1>, <Variable 2>, ...)
or
read(<File variable>, <Variable 1>, <Variable 2>, ...)
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» File variables must be supplied in front of the variable lists, otherwise data will be read from keyboard.
» Result of reading depends on the maximum width and the types of the variables.
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Results of reading variables of different widths and types
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» Procedure readln will advance the pointer after the end-of-line marker after reading while procedure read will advance to pass the read data only.
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Assume the contents of a file ‘Exampl1_6.txt’ is
210220230240250260270.5
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var S : string[5]; T, U : string[2]; C, D : char; M, N : integer; R : real; FileVariable : text;
Output
After the execution of the above program segment, the contents of the variables are as shown below.
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begin read(FileVariable, S, T); readln(FileVariable, C); read(FileVariable, M); readln(FileVariable,U);
Output
After the execution of the above program segment, the contents of the variables are as shown below.
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begin read(FileVariable, S, T); readln(FileVariable, C); read(FileVariable, M); readln(FileVariable,U);
Output
After the execution of the above program segment, the contents of the variables are as shown below.
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begin read(FileVariable, S, T); readln(FileVariable, C); read(FileVariable, M); readln(FileVariable,U);
Output
After the execution of the above program segment, the contents of the variables are as shown below.
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begin read(FileVariable, S, T); readln(FileVariable, C); read(FileVariable, M); readln(FileVariable,U);
Output
After the execution of the above program segment, the contents of the variables are as shown below.
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begin read(FileVariable, S, T); readln(FileVariable, C); read(FileVariable, M); readln(FileVariable,U);
Output
After the execution of the above program segment, the contents of the variables are as shown below.
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begin readln(FileVariable, C); read(FileVariable, M); readln(FileVariable,U); readln(FileVariable, D);
Output
After the execution of the above program segment, the contents of the variables are as shown below.
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begin read(FileVariable, M); readln(FileVariable,U); readln(FileVariable, D); read(FileVariable, N);
Output
After the execution of the above program segment, the contents of the variables are as shown below.
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begin readln(FileVariable,U); readln(FileVariable, D); read(FileVariable, N); readln(FileVariable, R);
Output
After the execution of the above program segment, the contents of the variables are as shown below.
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» The eof function returns the value TRUE if the pointer is currently pointing at the end-of-file marker; otherwise FASLE.
Syntax of the eof function
eof(<File variable>)
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» To update a sequential file means to – append data to the file
(i.e. add data to the end of the file)
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– insert data to the file(i.e. add data to the proper position of the file so that the order of the data can be maintained)
– delete data from the file(i.e. remove data from the file)
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– edit data of the file(i.e. modify the data)