why the program is skipping while loop

Question

Why does my program skip the while loop?

class DigitRepeat
{
  public static void main(String args[])
  {
      int n=133,dig=3,digit,count=0;

      while(n!=0)
      {
         digit=n%10;
         if(dig==digit)
         {
            count=count++;
         }
         n=n/10;
      }

      if(count==0)
          System.out.println("digit not found");
      else
          System.out.println("digit occurs "+count+" times.");
  }
}

This would be a good time to use your debugger to work it out yourself. I suspect the while loop is being executed, and the problem is not what you think. — Peter Lawrey, Jun 05 '11 at 09:50
So what is your question? If you add the output of this program and your desired output in the question, you might get better help here. — Ozair Kafray, Jun 05 '11 at 09:50
I'm not sure if this solves your problem, but `count++;` is sufficient, you don't have to write `count=count++;`. — Martin Matysiak, Jun 05 '11 at 09:51
In fact `count = count++;` is the culprit, since count always remains 0 in this case. — spacehunt, Jun 05 '11 at 09:53
On of the most highly voted Java questions discusses about your bug : http://stackoverflow.com/questions/3831341/why-does-this-go-into-an-infinite-loop — Swaranga Sarma, Jun 05 '11 at 12:23

wuxb · Answer 1 · 2011-06-05T12:06:56.543

10

> count=count++;

should be

> count++;

explain:

> count=count++;
is
a_temp_var=count;
count=count+1;
count=a_temp_var;
equals to:
a_temp_var=count;
count=a_temp_var;
equals to do nothing.

edited Jun 05 '11 at 12:06

answered Jun 05 '11 at 09:51

wuxb

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To explain this: `count++` is a post-increment. It first assigns the value of count to the left side and then increments. On the other hand you have the pre-increment `++count` which does the inverse. SO you /could/ write `count = ++count`, but that isn't good style. – Pascal Wittmann Jun 05 '11 at 09:55
True but it's not his problem. his condition to enter the loop is false so it never goes in. (but he needs to change count++ also – Jason Rogers Jun 05 '11 at 09:56
@Jason, It does go into the loop. – Peter Lawrey Jun 05 '11 at 09:57

score 6 · Answer 2 · edited Jun 05 '11 at 10:01

6

If I look at the code in my IDE it warns

The value changed at count++ is never used.

i.e. it is warning me that the value calculated is discarded.

If I step through the code in my debugger, I can see the loop is executed but the line

count = count++;

does not change count.

You want either

count++;

count+=1;

count=count+1;

edited Jun 05 '11 at 10:01

R. Martinho Fernandes

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answered Jun 05 '11 at 09:56

Peter Lawrey

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+1 For a real explanation including the mention of compiler/IDE warnings and using the debugger. Who cares about what answer was fastest? – R. Martinho Fernandes Jun 05 '11 at 10:00
1

These things are easy once you know the answer, but when you don't know I ask myself; How was I supposed to work that out? Hopefully, the OP will know how to solve this problem and many like it. ;) – Peter Lawrey Jun 05 '11 at 10:15
what is the excellent IDE? pls let us know? I have not been using IDE for long time.. – wuxb Jun 05 '11 at 12:08
There are a number of popular IDEs; Eclipse, Netbeans and my favourite IntelliJ CE (all free) – Peter Lawrey Jun 05 '11 at 18:32

score 1 · Answer 3 · answered Jun 05 '11 at 09:53

I do not know what do you mean that the program skips something. But I think I can see your bug. It is here: count=count++;

++ operator increments count after the assumption, so the variable count remains 0 forever.

I think that you wanted to say count++;

Kevin · Answer 4 · 2011-06-05T10:23:25.497

1

You've got a little error in your code:

count = count++;

Should be change to:

count++;

Have a look over here for a running example, all I did was remove the assignment.

(Included for completeness)

  int n = 133;
  int dig = 3;
  int digit;
  int count = 0;

  while (n != 0)
  {
     digit = n % 10;
     if (dig == digit)
     {
        count++;
     }
     n = n / 10;
  }

  if(count = =0)
      System.out.println("digit not found");
  else
      System.out.println("digit occurs " + count + " times.");

edited Jun 05 '11 at 10:23

answered Jun 05 '11 at 09:53

Kevin

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Why not post the altered source in your reply? – Andrew Thompson Jun 05 '11 at 10:09
@Andrew Because it's a a pretty long piece I guess. You can edit my answer if you want to. – Kevin Jun 05 '11 at 10:11
*Long?!?* It does not even crack a scroll bar! You want to see long, check out my [File Manager questions](http://stackoverflow.com/questions/6182110) thread. ;) – Andrew Thompson Jun 05 '11 at 10:18
@Andrew: Why didn't you just create a google code project, haha. (I've also included the source code in my answer :-) – Kevin Jun 05 '11 at 10:24
Someone also suggested making a SourceForge project of it. Great minds think alike. ;) – Andrew Thompson Jun 05 '11 at 10:35

score 0 · Answer 5 · answered Jun 05 '11 at 10:10

First advice: when you think your program is skipping a loop add a print after the while it will help you narrow down the problem.

Then you have to be careful with post increments. the value countRight is assigned to countLeft then countLeft is incremented but it doesn't matter because the value of count is already set. the value of count is copied therefore when the ++ takes effect its on a different count (sorry if this isn't so clear).

you can use :

count++;

count = count +1;

count += 1;

or count = ++count;

the last one pre increment works because the value is incremented before being copied ^^

(read up on this because it always turns up in interviews ^^

score 0 · Answer 6 · answered Jun 05 '11 at 10:13

Here is kinda what you're trying to code in C#:

class Program
{
    static void Main(string[] args)
    {
        var number = 12289;
        var search = 2;

        var count = Search(number, search);
    }

    static int Search(int number, int search)
    {
        return number < 10 ?
            (number == search ? 1 : 0) :
            (number % 10 == search ? 1 : 0)
                + Search((int)Math.Floor(number / (double)10), search);
    }
}

why the program is skipping while loop

6 Answers6