Adding an if statement before a while loop gives wrong results

Question

Why does adding an if statement before the while loop result in the first match being removed?

import java.util.Scanner;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class testRegex {
    public static void main(String[]args) {
        Pattern p = Pattern.compile("[a-zA-Z0-9]+");
        Matcher m = p.matcher("Name, 245 Street, City, Country, 101010");
        
        System.out.println(m.find());
        
        if(m.find()) {
            while(m.find()) {
                System.out.println(m.group());
            }
        } else {
            System.out.println("Nothing found");
        }
     }
}

The output without the if is correct.

Name
245
Street
City
Country
101010

Adding the if or anything before the while loop for that matter results in this:

245
Street
City
Country
101010

What am I doing wrong and how can I correct this? I'm doing this on Eclipse IDE.

Based on documentation, find() "attempts to find the next subsequence of the input sequence that matches the pattern", so when you have println or if before while, they "consume" one or several find() iterations. — Boris Krassi
– Boris Krassi, Commented Aug 23, 2022 at 6:43
Ok. I need the if find to print something like "Nothing found" if nothing matched. — Norman
– Norman, Commented Aug 23, 2022 at 6:45
Very strange reason to close this question. How is this not reproducible or was caused by typos? — anubhava
– anubhava, Commented Aug 24, 2022 at 6:17

anubhava · Accepted Answer · 2022-08-23 07:52:59Z

4

Here is an alternative approach using Java 8 streams:

final Pattern p = Pattern.compile("[a-zA-Z0-9]+");
Matcher m = p.matcher("Name, 245 Street, City, Country, 101010");
    
List<String> res = m.results().map(MatchResult::group)
                  .collect(Collectors.toList());
if (res.isEmpty()) {
    System.out.println("Nothing found");
} else {
    System.out.println(res);
}

Output:

[Name, 245, Street, City, Country, 101010]

answered Aug 23, 2022 at 7:52

anubhava

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Comments

Joop Eggen · Accepted Answer · 2022-08-23 07:13:22Z

2

m.find() changes state, goes to the next match if present.

    //NO: System.out.println(m.find());
   
    if (m.find()) {
        System.out.println("Found:");
        do {
            System.out.println(m.group());
        } while (m.find());
    } else {
        System.out.println("Nothing found");
    }

edited Aug 23, 2022 at 7:13

answered Aug 23, 2022 at 6:51

Joop Eggen

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2 Comments

Andy Turner Over a year ago

Of course, if you want to print something, you can assign the result of m.find() to a variable, print it and check it in the if condition.

Joop Eggen Over a year ago

Added alternative println for when something was found. When not found there already is a println.

mahieyin-rahmun · Accepted Answer · 2022-08-23 07:38:35Z

2

Why not use matcher.reset()?

import java.util.Scanner;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class testRegex {
    public static void main(String[]args) {
        Pattern p = Pattern.compile("[a-zA-Z0-9]+");
        Matcher m = p.matcher("Name, 245 Street, City, Country, 101010");
        
        System.out.println(m.find());
        m.reset();

        if(m.find()) {
            m.reset();
            while(m.find()) {
                System.out.println(m.group());
            }
        } else {
            System.out.println("Nothing found");
        }
     }
}

edited Aug 23, 2022 at 7:38

answered Aug 23, 2022 at 7:26

mahieyin-rahmun

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Comments

Mariano · Accepted Answer · 2022-08-23 07:21:56Z

1

Each call of "m.find" will move to the next match in within the search String. So you have to remember the Finding of the first call of m.find() or you use the while loop without the if statement. With a boolean flag you could keep track whether you found something or not to decide whether you want to print "Nothing found" afterwards.

Something like:

      boolean foundSomething = false;
           
      while(m.find()) {
        foundSomething = true;
        System.out.println(m.group());
      }
      if(!foundSomething){        
        System.out.println("Nothing found");
      }

answered Aug 23, 2022 at 7:21

Mariano

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Comments

Eng Fares · Accepted Answer · 2022-08-23 06:58:24Z

0

Don't use while alone because it is a fulfillment of the condition if it doesn't find what you are looking for it moves to the next use for or do-while

answered Aug 23, 2022 at 6:58

Eng Fares

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1 Comment

Norman Over a year ago

Ok. How would you do it with a for loop? I'm been trying for sometime.

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