Question

I am working with matrix in java. ( another story 🙂 )

I want to read a CSV file and store it in a variable. I will manipulate values then again store it in CSV file. I used STRING as data type. But if CSV file has like 500 columns. It kill my program speed :(. I think this is not good data type. Which data type I can use to temporary store LONG TEXT?

If my question is not clear please ask questions. I will explain.
Thanks

P.S: I am reading one line and storing it in variable like this

String str;

str += read line by line from CSV;

here is the loop

String reduceM="";

 for(int kk=0;kk<W2.getRowDimension();kk++){
     for(int jj=0;jj<W2.getColumnDimension();jj++){
         reduceM += Double.toString(reduceMatrix[kk][jj]);
     }
     System.out.println("\r\n");
 }

Answer 1

Use a StringBuilder (or StringBuffer if you’re using Java 1.5 or older):

StringBuilder builder = new StringBuilder();
for (int kk = 0; kk<W2.getRowDimension(); kk++) {
    for(int jj = 0; jj < W2.getColumnDimension(); jj++) {
        builder.append(reduceMatrix[kk][jj]);
    }
}

This will avoid it creating a new (and increasingly long) string for each iteration of the two loops.

However, there are no commas or line-breaks in this code – I suspect you actually want something like this:

StringBuilder builder = new StringBuilder();
for (int kk = 0; kk < W2.getRowDimension(); kk++) {
    for (int jj = 0; jj < W2.getColumnDimension(); jj++) {
        builder.append(reduceMatrix[kk][jj])
               .append(",");
    }
    builder.append("\n"); // Or whatever line terminator you want
}

Note that that will leave an extra comma at the end of each row – let me know if you want ideas of how to remove that.

See this article for why this could make a huge improvement to your running time. (Note that it’s an old article, talking about StringBuffer rather than StringBuilder – the latter is just an unsynchronized version of the former.)