pascal programming today chapter 11 1 chapter 11

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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)