Reading from a file instead of the console (keyboard) can be done
by:
read (file_variable,
argument_list);
write (file_variable,
argument_list);
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Similarly with readln and writeln. file_variable
is declared as follows:
var
...
filein, fileout
: text;
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The text data type indicates that the file is just plain text.
After declaring a variable for the file, and before reading from or
writing to it, we need to associate the variable with the filename on the
disk and open the file. This can be done in one of two ways. Typically:
reset (file_variable,
'filename.extension');
rewrite (file_variable,
'filename.extension');
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reset opens a file for reading, and
rewrite opens a file
for writing. A file opened with
reset can only be used with
read
and
readln. A file opened with
rewrite can only be used with
write and
writeln.
Turbo Pascal introduced the assign notation. First you assign a
filename to a variable, then you call reset or rewrite using
only the variable.
assign (file_variable,
'filename.extension');
reset (file_variable)
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The method of representing the path differs depending on your operating
system. Windows uses backslashes and drive letters due to its DOS heritage
(e.g. c:\directory\name.pas), while MacOS X and Linux use forward slashes
due to their UNIX heritage.
After you're done with the file, you can close it with:
Here's an example of a program that uses files. This program was written
for Turbo Pascal and DOS, and will create file2.txt with the first
character from file1.txt:
program CopyOneByteFile;
var
mychar : char;
filein, fileout : text;
begin
assign (filein,
'c:\file1.txt');
reset (filein);
assign (fileout,
'c:\file2.txt');
rewrite (fileout);
read (filein, mychar);
write (fileout, mychar);
close(filein);
close(fileout);
end.
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