How does the below loop break?

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Could you please tell me how second loop or the if statement breaks here?

C#

int outer;
int inner;

for (outer = 2; outer < 100; outer++)
{
    for (inner = 2; inner < 100; inner++)
        if ((outer % inner) == 0)  break;
    if (inner > (outer / inner))
        Console.WriteLine("{0} is prime", outer);
}

What I have tried:

I changed the code as below to understand the second loop but then it only prints "break".

C#

for (inner = 2; inner < 100; inner++)
     if ((outer % inner) == 0) Console.WriteLine("Breaks Here!"); break;

Thank you.

Posted 18-Sep-18 20:58pm

A. J. Bozdar

Updated 4-Mar-20 21:49pm

v2

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2 solutions

Solution 1

Quote:
I changed the code as below to understand the second loop but then it only prints "break".

That's not the loop that does it - that's the if. Let's just reformat your code so it's easier to see:

C#

for (inner = 2; inner < 100; inner++)
     if ((outer % inner) == 0) Console.WriteLine("Breaks Here!"); break;

Becomes:

C#

for (inner = 2; inner < 100; inner++)
   if ((outer % inner) == 0) 
      Console.WriteLine("Breaks Here!"); 
   break;

And now it's obvious: the break statement is executed regardless of the condition.
If you want to print and break, you need curly brackets:

C#

for (inner = 2; inner < 100; inner++)
   if ((outer % inner) == 0) 
      {
      Console.WriteLine("Breaks Here!"); 
      break;
      }

In fact, as a beginner, I'd strongly recommend that you use curly brackets all the time, even if you don;t need to - it makes problems like this a lot less likely to happen:

C#

for (int outer = 2; outer < 100; outer++)
   {
   for (inner = 2; inner < 100; inner++)
      {
      if ((outer % inner) == 0)  
         {
         break;
         }
      }
   if (inner > (outer / inner))
      {
      Console.WriteLine("{0} is prime", outer);
      }
   }

Posted 18-Sep-18 21:23pm

OriginalGriff

Comments

Richard Deeming 20-Sep-18 10:20am

In the reformatted code block, the break is indented one level too far. :)

Since there are no braces, it should be:

for (inner = 2; inner < 100; inner++)
   if ((outer % inner) == 0) 
      Console.WriteLine("Breaks Here!"); 

break;

OriginalGriff 20-Sep-18 10:28am

Good point!

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This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

Patrice T · Accepted Answer · 2018-09-18T22:39:00

Quote:
I would highly appreciate if someone can point me to right direction to understand this "prime numbers" logic.

This prime checking logic is extremely slow.
Think about it, when you check if 97 is a prime, by hand (with just a calculator), once you know that 2 is not a factor, will you check 4, 6, 8, 10, 12 ... ?
When will you stop ? Why ? will you check 95 ? 93 ? 91 ? 89 ? ...
Advice: make a function 'isprime', it will simplify code.

Quote:
Could you please tell me how second loop or the if statement breaks here?

How about watching the code as it performs ?

Your code do not behave the way you expect, or you don't understand why !

There is an almost universal solution: Run your code on debugger step by step, inspect variables.
The debugger is here to show you what your code is doing and your task is to compare with what it should do.
There is no magic in the debugger, it don't know what your cpde is supposed to do, it don't find bugs, it just help you to by showing you what is going on. When the code don't do what is expected, you are close to a bug.
To see what your code is doing: Just set a breakpoint and see your code performing, the debugger allow you to execute lines 1 by 1 and to inspect variables as it execute.
Debugger - Wikipedia, the free encyclopedia[^]

Mastering Debugging in Visual Studio 2010 - A Beginner's Guide[^]
Basic Debugging with Visual Studio 2010 - YouTube[^]
Debugging C# Code in Visual Studio - YouTube[^]
The debugger is here to only show you what your code is doing and your task is to compare with what it should do.

How does the below loop break?

2 solutions

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Solution 2

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How does the below loop break?

2 solutions

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