Java data type problem

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");
 }

there is nothing else to store text. Perhaps we can see your algorithm so that the inefficiency is identified — Bozho
– Bozho, Commented Feb 1, 2010 at 21:59
I updated my post. You can see i have CSV data in double 2d array and want to store it in STRING :( but it kill my speed — user238384
– user238384, Commented Feb 1, 2010 at 22:03
@Bozho: Incorrect. See Jon Skeet's answer. Another possibility is to write out data one row at a time from your double[][]. Re-use one StringBuffer/StringBuilder to hold the row of data before writing it to file in one chunk, and reset after each row. — BobMcGee
– BobMcGee, Commented Feb 1, 2010 at 22:10
@BobMcGee - StringBuilder is not a datatype. It's just more efficient way of manipulating character data. That's why I wanted the source-code — Bozho
– Bozho, Commented Feb 1, 2010 at 22:20
@Bozho: I'm not sure you really understand how the String and StringBuilder/StringBuffer types work. String is just a wrapper for an immutable char[], with a length and offset to the array. StringBuilder/StringBuffer allow for a modifiable buffer, which means they can modify directly. For that matter, you can roll your own implementation using a char[], but it's kind of redundant, since these containers all facilitate working with text. Or maybe there's just an language barrier at work here. — BobMcGee
– BobMcGee, Commented Feb 1, 2010 at 22:54

Jon Skeet · Accepted Answer · 2010-02-01 22:06:53Z

6

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.)

answered Feb 1, 2010 at 22:06

Jon Skeet

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1 Comment

Michael Myers Over a year ago

The most obvious improvement - I just refreshed the question to see if anyone had posted this yet.

user124626 · Accepted Answer · 2010-02-01 22:09:12Z

0

Use the 'StringBuffer' class for concatenations. It is much more efficient.

Take a look at this article for an explanation: here

EDIT - Sorry I did not see this was already answered

answered Feb 1, 2010 at 22:09

user124626

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Gabriel Ščerbák · Accepted Answer · 2010-02-01 22:11:57Z

0

Prefer StringBuilder, there is a big difference in performance compared to the string concatenation(+).

answered Feb 1, 2010 at 22:11

Gabriel Ščerbák

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Deniz Acay · Accepted Answer · 2010-02-01 22:25:17Z

0

In addition to the skeet's great answer; dont write to system.out if its not necessary, or if you want to write to console use buffers or write when loop finishes. It makes your program cumbersome because each time it is encountered in the loop System.out stream opens writes flushes and closes.

answered Feb 1, 2010 at 22:25

Deniz Acay

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