Java read file line by line and replace nth column

Question

Say the following is my file content (hundreds of same pattern lines)

    1979,2013-08-07 19:03:35,abc,12345,310012
    1980,2013-08-07 19:05:03,fds,12345,310160
.
.

I want to read the file and scan the file line by line and replace the 4th column of the line (12345 which repeats on all the lines) to another value as well as adding a new value at the end of each line.

Finally I need to regenerate the file with the updated values as an output.

here is what I do so far:

URL path = ClassLoader.getSystemResource("test.txt");

        File file = new File(path.toURI());

        try
        {

            Scanner scanner = new Scanner(file);

            while (scanner.hasNextLine())
            {
                String line = scanner.nextLine();

                // Here I need to do the replacement codes
            }
            scanner.close();
        }
        catch (FileNotFoundException e)
        {
            e.printStackTrace();
        }

Is it a good idea to split each line to an array and do the processes like that or is there any better solution for this?

I am also not sure how to create an output with the edited content. Any help would be appreciated.

values to be edited or added are taken from some other db queries. so can be anything depends on an element on each line (one to many) — ComeRun, Aug 28 '13 at 03:34
You need to replace the 12345 value and add a new value at the end of the line or the same position — newuser, Aug 28 '13 at 03:36
edit 12345 with one value and add different value at the end — ComeRun, Aug 28 '13 at 03:39

score 2 · Accepted Answer · answered Aug 28 '13 at 03:43

You could try something like this:

public void replace(final int column, final String replacement, final File file, final String... appends) throws IOException {
    assert column >= 0 : "column < 0";
    final List<String> lines = new LinkedList<String>();
    final Scanner reader = new Scanner(file, "UTF-8");
    while(reader.hasNextLine()){
        final String line = reader.nextLine().trim();
        if(line.isEmpty())
            continue;
        final String[] tokens = line.split(",");
        assert column < tokens.length-1 : "column > tokens.length-1";
        tokens[column] = replacement;
        final StringBuilder builder = new StringBuilder();
        for(final String token : tokens)
            builder.append(token + ",");
        for(final String append : appends)
            builder.append(append + ",");
        builder.deleteCharAt(builder.length()-1);
        lines.add(builder.toString());
    }
    reader.close();
    final BufferedWriter writer = new BufferedWriter(new FileWriter(file, false));
    for(final String line : lines){
        writer.write(line);
        writer.newLine();
    }
    writer.flush();
    writer.close();
}

Or if you are using Java 8, you could try something like this:

public void replace(final int column, final String replacement, final File file, final String... appends) throws IOException {
    assert column >= 0 : "column < 0";
    final List<String> lines = new LinkedList<>();
    final Scanner reader = new Scanner(file, "UTF-8");
    while(reader.hasNextLine()){
        final String line = reader.nextLine().trim();
        if(line.isEmpty())
            continue;
        final String[] tokens = line.split(",");
        assert column < tokens.length-1 : "column > tokens.length-1";
        tokens[column] = replacement;
        final List<String> temp = new LinkedList<>();
        temp.addAll(Arrays.asList(tokens));
        temp.addAll(Arrays.asList(appends));
        lines.add(temp.stream().collect(Collectors.joining(",")));
    }
    reader.close();
    final BufferedWriter writer = new BufferedWriter(new FileWriter(file, false));
    lines.forEach(
            l -> {
                try{
                    writer.write(l);
                    writer.newLine();
                }catch(IOException ex){
                    ex.printStackTrace();
                }
            }
    );
    writer.flush();
    writer.close();
}

To call this method, you could do something like this:

replace(3, "string to replace 12345", file, strings_you_want_to_append_to_the_end);

Thanks for the answer, can u let me know how to actually get the modified file out and use it? — ComeRun, Aug 28 '13 at 04:02
Well, I was under the impression that you wanted to re-write the file with the necessary modifications? — Josh M, Aug 28 '13 at 04:03
Yes; after it finishes reading the file (and making the necessary changes to each line) it will re-write the file with all of the modified lines. — Josh M, Aug 28 '13 at 04:08

score 1 · Answer 2 · edited May 23 '17 at 11:57

I'd do this by putting each line into an arraylist, using split(",") on each line of the array, and replacing the 4th index in each array. To rejoin the array back into a string, you'd have to write your own function (Java doesn't have one built-in). There is another stack overflow question that addresses this problem, though.

try
    {
        Scanner scanner = new Scanner(file);

        // initialize the arraylist, which will hold the split strings
        ArrayList<String[]> data = new ArrayList<String[]>();

        while (scanner.hasNextLine())
        {
            // filling the arraylist
            String line = scanner.nextLine();
            String[] split = line.split(",");
            data.add(split);
        }
        scanner.close();

        // replacing the values
        for (String[] split : data) {
            split[3] = "new value";
        }

        // sloppily glueing everything back together
        String output = "";
        for (String[] split : data) {
            for (int i = 0; i < 5; i++) {
                if (i < 4) {output += datum + ",";}
                else {output += datum;}
            }
            output += "\n";
        }
        return output;
    }
catch (FileNotFoundException e)
    {
        e.printStackTrace();
    }

Probably really sloppy and inefficient, but it's pretty logical.

@Josh-M Yep, if you're counting from zero, but since that's English, not Java, I figured I'd say 4th. :D — ajiang, Aug 28 '13 at 03:41

score 0 · Answer 3 · answered Aug 28 '13 at 03:43

You can do that with a regular expression:

(?<=^[^,]+,[^,]+,[^,]+,)[^,]+

e.g.:

private static final Pattern REGEX_PATTERN = 
        Pattern.compile("(?<=^[^,]+,[^,]+,[^,]+,)[^,]+");

public static void main(String[] args) {

    // Open input file

    // Open output file                

    // Read input line

    String inputLine = "1979,2013-08-07 19:03:35,abc,12345,310012";
    String outputLine = REGEX_PATTERN.matcher(input)
                                     .replaceFirst("YOUR_REPLACEMENT");

    // Print (only for debug)
    System.out.println(
        outputLine 
    );  // prints "1979,2013-08-07 19:03:35,abc,YOUR_REPLACEMENT,310012"

    // Write output line

    // Close both files

}

score 0 · Answer 4 · answered Aug 28 '13 at 03:57

Try the following

stringBuilder = new StringBuilder();
String[] resultArray = sCurrentval.split(",");

resultArray[3] = EDIT_VALVE; //your new value to replace

for (String s : resultArray)
   {
       stringBuilder.append(s);
       stringBuilder.append(",");
   }

sCurrentval = stringBuilder.append(LAST_VALUE).toString(); // add your last value

Java read file line by line and replace nth column

4 Answers4

Linked