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sum of multiples of 3 or 5 below 1,000

This is a discussion on sum of multiples of 3 or 5 below 1,000 within the C Programming forums, part of the General Programming Boards category; I wrote a program which should compute the sum of all of the multiples of 3 or 5 below 1,000. ...

  1. #1
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    sum of multiples of 3 or 5 below 1,000

    I wrote a program which should compute the sum of all of the multiples of 3 or 5 below 1,000.
    Code:
    int main()
    {
    	int sum = 0;
    	int i;
    
    
    	for (i=0; i < 1000; i++)
    	{
    		if (i % 3 == 0)
    		sum += i;
    		
    
    		if (i % 5 == 0)
    		sum += i;
    	}
    
    	printf(" \n\n%i\n\n", sum);
    
    	return 0;
    }
    This program produces 266,333. According to the internet the correct answer is 233,168 which I verified in another program in ruby. Any thoughts as to why I am getting this incorrect result?

  2. #2
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    Your if statements are not mutually exclusive, so you count numbers that are multiples of both 3 and 5 twice, e.g. 15, 30, ....

  3. #3
    C++ Witch laserlight's Avatar
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    You are double counting. Try i = 15, for example.
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

  4. #4
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    Like this...

    Code:
    int main( void )
    {
       int sum = 0;
       int i;
    
       for (i=0; i < 1000; i++)
         {
            if (i % 3 == 0)
              sum += i;
            else if (i % 5 == 0)
              sum += i;
         }
       printf(" \n\n%i\n\n", sum);
       return 0;
    }

  5. #5
    C++ Witch laserlight's Avatar
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    Personally, I would write that part as:
    Code:
    if (i % 3 == 0 || i % 5 == 0)
    {
        sum += i;
    }
    deadrabbit likes this.
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

  6. #6
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    Ahh, I see. Thank you all.

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    My pleasure...

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