Reading a file of lines

Example

We have seen an example program that copies one file to another. It does so in complete lines, a line at a time.

Its heart is the following code:

        with TEXT_IO; use TEXT_IO;

        procedure copy_file is

            inf         : TEXT_IO.FILE_TYPE;   -- input file
            outf        : TEXT_IO.FILE_TYPE;   -- output file
            line        : string (1..200);     -- one line of a file
            line_length : NATURAL;             -- line length

	        ... 

        begin

            ...

            while not END_OF_FILE (inf) loop
                GET_LINE (inf,  line, line_length);
                PUT_LINE (outf, line (1..line_length);
            end loop;

            ...

        end copy_file;

Notes:

• it tests for END_OF_FILE before reading each line, so that it knows when to stop reading.
• GET_LINE is used to read a complete line in one single operation.

Template

The following pattern is common to all code that reads a file one line at a time:

Using the template

You can use the above template to write your file processing code:

1. Decide what (if anything) needs to be done before any of the file is read.
   • Write statements for it, and plug them in at point 1 in the template.
2. Decide what needs to be done to process a line that has been read in.
   • Write statements for it, and plug them in at point 2 in the template.
3. Decide what final processing needs to be done (if anything) once the end of data is reached.
   • Write statements for it, and plug them in at point 3 in the template.

The previous example can be understood in this way.

They don't have to be lines of arbitrary characters. Another example shows that the code still fits into this pattern if lines are treated as containing integers. The important thing is that the whole line is treated as a unit.