String parsing in Java with delimiter tab "\t" using split

I'm processing a string which is tab delimited. I'm accomplishing this using the split function, and it works in most situations. The problem occurs when a field is missing, so instead of getting null in that field I get the next value. I'm storing the parsed values in a string array.

String[] columnDetail = new String[11];
columnDetail = column.split("\t");

Any help would be appreciated. If possible I'd like to store the parsed strings into a string开发者_运维问答 array so that I can easily access the parsed data.

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String.split uses Regular Expressions, also you don't need to allocate an extra array for your split.

The split-method will give you a list., the problem is that you try to pre-define how many occurrences you have of a tab, but how would you Really know that? Try using the Scanner or StringTokenizer and just learn how splitting strings work.

Let me explain Why \t does not work and why you need \\\\ to escape \\.

Okay, so when you use Split, it actually takes a regex ( Regular Expression ) and in regular expression you want to define what Character to split by, and if you write \t that actually doesn't mean \t and what you WANT to split by is \t, right? So, by just writing \t you tell your regex-processor that "Hey split by the character that is escaped t" NOT "Hey split by all characters looking like \t". Notice the difference? Using \ means to escape something. And \ in regex means something Totally different than what you think.

So this is why you need to use this Solution:

\\t

To tell the regex processor to look for \t. Okay, so why would you need two of em? Well, the first \ escapes the second, which means it will look like this: \t when you are processing the text!

Now let's say that you are looking to split \

Well then you would be left with \\ but see, that doesn't Work! because \ will try to escape the previous char! That is why you want the Output to be \\ and therefore you need to have \\\\.

I really hope the examples above helps you understand why your solution doesn't work and how to conquer other ones!

Now, I've given you this answer before, maybe you should start looking at them now.

OTHER METHODS

StringTokenizer

You should look into the StringTokenizer, it's a very handy tool for this type of work.

Example

 StringTokenizer st = new StringTokenizer("this is a test");
 while (st.hasMoreTokens()) {
     System.out.println(st.nextToken());
 }

This will output

 this
 is
 a
 test

You use the Second Constructor for StringTokenizer to set the delimiter:

StringTokenizer(String str, String delim)

Scanner

You could also use a Scanner as one of the commentators said this could look somewhat like this

Example

 String input = "1 fish 2 fish red fish blue fish";

 Scanner s = new Scanner(input).useDelimiter("\\s*fish\\s*");

 System.out.println(s.nextInt());
 System.out.println(s.nextInt());
 System.out.println(s.next());
 System.out.println(s.next());

 s.close(); 

The output would be

 1
 2
 red
 blue 

Meaning that it will cut out the word "fish" and give you the rest, using "fish" as the delimiter.

examples taken from the Java API

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Try this:

String[] columnDetail = column.split("\t", -1);

Read the Javadoc on String.split(java.lang.String, int) for an explanation about the limit parameter of split function:

split

public String[] split(String regex, int limit)
Splits this string around matches of the given regular expression.
The array returned by this method contains each substring of this string that is terminated by another substring that matches the given expression or is terminated by the end of the string. The substrings in the array are in the order in which they occur in this string. If the expression does not match any part of the input then the resulting array has just one element, namely this string.

The limit parameter controls the number of times the pattern is applied and therefore affects the length of the resulting array. If the limit n is greater than zero then the pattern will be applied at most n - 1 times, the array's length will be no greater than n, and the array's last entry will contain all input beyond the last matched delimiter. If n is non-positive then the pattern will be applied as many times as possible and the array can have any length. If n is zero then the pattern will be applied as many times as possible, the array can have any length, and trailing empty strings will be discarded.

The string "boo:and:foo", for example, yields the following results with these parameters:

Regex   Limit   Result
:   2   { "boo", "and:foo" }
:   5   { "boo", "and", "foo" }
:   -2  { "boo", "and", "foo" }
o   5   { "b", "", ":and:f", "", "" }
o   -2  { "b", "", ":and:f", "", "" }
o   0   { "b", "", ":and:f" }

When the last few fields (I guest that's your situation) are missing, you will get the column like this:

field1\tfield2\tfield3\t\t

If no limit is set to split(), the limit is 0, which will lead to that "trailing empty strings will be discarded". So you can just get just 3 fields, {"field1", "field2", "field3"}.

When limit is set to -1, a non-positive value, trailing empty strings will not be discarded. So you can get 5 fields with the last two being empty string, {"field1", "field2", "field3", "", ""}.

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Well nobody answered - which is in part the fault of the question : the input string contains eleven fields (this much can be inferred) but how many tabs ? Most possibly exactly 10. Then the answer is

String s = "\t2\t\t4\t5\t6\t\t8\t\t10\t";
String[] fields = s.split("\t", -1);  // in your case s.split("\t", 11) might also do
for (int i = 0; i < fields.length; ++i) {
    if ("".equals(fields[i])) fields[i] = null;
}
System.out.println(Arrays.asList(fields));
// [null, 2, null, 4, 5, 6, null, 8, null, 10, null]
// with s.split("\t") : [null, 2, null, 4, 5, 6, null, 8, null, 10]

If the fields happen to contain tabs this won't work as expected, of course.
The -1 means : apply the pattern as many times as needed - so trailing fields (the 11th) will be preserved (as empty strings ("") if absent, which need to be turned to null explicitly).

If on the other hand there are no tabs for the missing fields - so "5\t6" is a valid input string containing the fields 5,6 only - there is no way to get the fields[] via split.

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String.split implementations will have serious limitations if the data in a tab-delimited field itself contains newline, tab and possibly " characters.

TAB-delimited formats have been around for donkey's years, but format is not standardised and varies. Many implementations don't escape characters (newlines and tabs) appearing within a field. Rather, they follow CSV conventions and wrap any non-trivial fields in "double quotes". Then they escape only double-quotes. So a "line" could extend over multiple lines.

Reading around I heard "just reuse apache tools", which sounds like good advice.

In the end I personally chose opencsv. I found it light-weight, and since it provides options for escape and quote characters it should cover most popular comma- and tab- delimited data formats.

Example:

CSVReader tabFormatReader = new CSVReader(new FileReader("yourfile.tsv"), '\t');

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You can use yourstring.split("\x09"); I tested it, and it works.

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I just had the same question and noticed the answer in some kind of tutorial. In general you need to use the second form of the split method, using the

split(regex, limit)

Here is the full tutorial http://www.rgagnon.com/javadetails/java-0438.html

If you set some negative number for the limit parameter you will get empty strings in the array where the actual values are missing. To use this your initial string should have two copies of the delimiter i.e. you should have \t\t where the values are missing.

Hope this helps :)

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String[] columnDetail = new String[11];
columnDetail = column.split("\t", -1); // unlimited
OR
columnDetail = column.split("\t", 11); // if you are sure about limit.

 * The {@code limit} parameter controls the number of times the
 * pattern is applied and therefore affects the length of the resulting
 * array.  If the limit <i>n</i> is greater than zero then the pattern
 * will be applied at most <i>n</i>&nbsp;-&nbsp;1 times, the array's
 * length will be no greater than <i>n</i>, and the array's last entry
 * will contain all input beyond the last matched delimiter.  If <i>n</i>
 * is non-positive then the pattern will be applied as many times as
 * possible and the array can have any length.  If <i>n</i> is zero then
 * the pattern will be applied as many times as possible, the array can
 * have any length, and trailing empty strings will be discarded.