How to scan through file and replace certain phrases?

Question

Please I would like your help with the following issue:

This is a sample code customized solely for clarifying my question:

    File accounts_File = new File("Sample_Folder\\example.txt");
    FileInputStream fis = null;
    BufferedInputStream bis = null;
    DataInputStream dis = null;

    String str_String = ""; //For storing the input from the file.

    fis = new FileInputStream(accounts_File);
    bis = new BufferedInputStream(fis);
    dis = new DataInputStream(bis);

    while (dis.available() != 0)
    {
        str_String = dis.readLine(); //Reading the line from the file.

        StringTokenizer st = new StringTokenizer(str_String); 

        while (st.hasMoreTokens())
        {
               //I need some help here
        }

    }

    fis.close();
    bis.close();
    dis.close();

The Input File (example.txt) looks as follows: James>Hector>AUS>25

The Objective is simply scanning and replacing "Hector" with "Anderson". The problem is that the name "Hector" is not known; the only thing known is its location (after the first ">").

Note: I can change the format of the input file if that makes it easier. As an example I can change the format to something like:

FirstName: James
LastName: Hector
Country: AUS
Age: 25

*Where the program now will scan for the keyword "LastName:" and replace everything following it with "Anderson".

Thanks in advance for your help!

Any Suggestions are greatly appreciated.

Andreas Dolk · Accepted Answer · 2011-01-05T09:06:18.853

Keep the format, it looks like a standard csv file (comma separated values, where the "comma" is the ">" char) and csv files are easy to read and write!

As you can't replace file content "on the disk", you'll have to read the file into memory, rewrite it in memory with the changes needed and rewrite it to the disk.

A simple model for the file is a two dimensional string array, each line of the file is a row, each value a column, which makes 4 columns. The last name (the string, you want to replace) now is always at matrix[lineNumber][1]. Change the value on the second column for each row and your done.

For production code, I'd code a class to represent a record. Then, your code could look like this:

public void replace(File inputFile, String newName) {
  List<Person> persons = new ArrayList<Person>();

  // read the file
  List<String> lines = readFile(inputFile);

  // create a person instance for each line
  for (String line:lines) {
    String[] values = line.split(">");
    persons.add(new Person(values));
  }

  // change the last name
  for (Person person:persons) {
    person.setLastName(newName);
  }

  // store all persons to file
  writeFile(inputFile, persons);
}

Note, that the actual work (file IO, changing names) is done in different methods and classes. This keeps the main algorithm inside the replace method readable.

This is a good approach assuming more would be done to the data than simply replacing the second item in the character separated string. If you just want to replace the second item I would suggest using Pattern and Matcher in conjunction with regex and capture groups. The regex pattern would be similar to (.*[>])(.*)([>].*) where you would write to file group 1 followed by the replacement text then group 3 and group 2 would contain the value of the second item if you need it. This regex is untested, but should be close if not correct. — LINEMAN78, Jan 05 '11 at 08:53
@LINEMAN78 - as noted - this is a solution for production code, a quick hack for one-time-usage can be a lot simpler. — Andreas Dolk, Jan 05 '11 at 09:01
@LINEMAN +1 for filling my regex gap. Turn your comment into an answer and you have my vote. — Filburt, Jan 05 '11 at 09:25

Filburt · Answer 2 · 2011-01-05T08:33:41.817

2

I'd suggest using a regex to replace the match between the first and the second ">". I confess I'm neither an expert in Java nor an expert with regular expressions so I can't provide "the codes" right away.

Your question suggests that you could structure the source file to your desire - so why not make that an Xml document? I that case of course you wouldn't do text parsing.

edited Jan 05 '11 at 08:33

answered Jan 05 '11 at 08:25

Filburt

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Thanks Filburt! Your answer is reasonable. Using a combination of (pattern.compile), (pattern.matcher), (matcher.replaceAll) serves the purpose if we are dealing with a known set of phrases. As if (Hector) was known to exist. However if i changed my input file to the (second suggestion, written above; using firstname, lastname..etc), there will be a way around it. Using XML is also possible though am not able to imagine the code at all. I am not a Java programmer either. – CompilingCyborg Jan 05 '11 at 08:46
@CompilingCyborg The best answer is the one from @Andreas_D because it's even simpler to split your lines with the known delimiter. You'd only use regex if you suspect that names could contain `">"` and wanted to check that with a more complex regex. – Filburt Jan 05 '11 at 08:51
thx for your vote ;) Just a small note, the argument `">"` of the split method is in fact a RegExp expression - a fairly simple one, of course. But `split` takes a regular expression to calculate the split points. – Andreas Dolk Jan 05 '11 at 09:39

score 1 · Answer 3 · answered Jan 05 '11 at 08:23

Try the following...

File accounts_File = new File("Sample_Folder\\example.txt");
FileInputStream fis = null;
BufferedInputStream bis = null;
DataInputStream dis = null;

String str_String = ""; //For storing the input from the file.

fis = new FileInputStream(accounts_File);
bis = new BufferedInputStream(fis);
dis = new DataInputStream(bis);

while (dis.available() != 0)
{
    str_String = dis.readLine(); //Reading the line from the file.

    final int firstIndex = str_String.indexOf('>');

        if(firstIndex != -1 && firstIndex != str_String.length() - 1) {
            final int secondIndex = str_String.indexOf('>', firstIndex + 1);

            if(secondIndex != -1) {
               final StringBuilder builder = new StringBuilder();
               builder.append(str_String.substring(0, firstIndex + 1));
               builder.append("Anderson");
               builder.append(str_String.substring(secondIndex));
               System.out.println(builder.toString()); 
            }
        }

}

dis.close();

How to scan through file and replace certain phrases?

3 Answers3