Java For-Loop Error

Question

I'm trying to print out all odd numbers that aren't multiples of 7 or 9. It works by seeing if the remainder is first not 0 when divided by two, giving the odd numbers.

But when I've put it to show the numbers if they are not multiples of 7 it just displays ALL the odd numbers, have I made a mistake?

public class NoMultiples7and9 {

    public static void main(String[] args) {

        for (int i = 1; i <= 30; i++) {

            if (i % 2 != 0) {

                if (i % 7 != 0 || i % 9 != 0) {

                    System.out.println(i);
                }
            }
        }
    }
}

This looks like homework, so I'll just give a hint. What does i % 7 !=0 || i % 9 != 0 evaluate if i is a multiple of only 7 (e.g. 21)? What should it evaluate to? Get that working correctly and the rest should fall out. — Michael Barker
– Michael Barker, Commented Nov 20, 2011 at 15:33
It would print it because although 21 is divisible by 7, it's not divisible by 9 so I'd need to set it to when it's not divisible by 7 and not divisible by 9. — mino
– mino, Commented Nov 20, 2011 at 19:32

Aurelio De Rosa · Accepted Answer · 2011-11-20 15:32:15Z

3

Change your code with:

for (int i = 1; i <= 30; i = i + 2) {
   if (i % 7 != 0 && i % 9 != 0) {
      System.out.println(i);
   }
}

Please note the use of the && (AND) instead of the || (OR) and the useless of the i % 2 because you can loop only on the odd numbers by changing a little bit the for loop.

answered Nov 20, 2011 at 15:32

Aurelio De Rosa

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Comments

esaj · Accepted Answer · 2011-11-20 15:30:31Z

2

You need to use AND instead of OR in your comparison. In the comparison i % 7 != 0 || i % 9 != 0, even if i mod 7 is 0, i mod 9 may not be and vice versa.

answered Nov 20, 2011 at 15:30

esaj

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1 Comment

vidstige Over a year ago

on homeworkquestions prefer learn-how-to-fish type answers rather than giving away free fish.

Eric Rosenberg · Accepted Answer · 2011-11-20 15:31:32Z

1

your inner if statement is wrong, it will cause all odd numbers that aren't divisible by by both 7 and 9 to be printed. I bet if you change your loop to go to 63 it won't print 63. The initial % 2 check is also not needed.

public class NoMultiples7and9 {

    public static void main(String[] args) {

        for (int i = 1; i <= 30; i++) {

            if (i % 7 != 0 && i % 9 != 0) {

                System.out.println(i);

            }
        }
    }
}

answered Nov 20, 2011 at 15:31

Eric Rosenberg

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1 Comment

vidstige Over a year ago

on homeworkquestions prefer learn-how-to-fish type answers rather than giving away free fish.

check123 · Accepted Answer · 2011-11-20 15:35:32Z

0

for (i = 1; i <= 30; i++) {  
        if (i % 2 != 0) {
            if(i % 7 != 0) {
                if(i % 9 != 0)
                    System.out.println(i);
            }

        }
 }

answered Nov 20, 2011 at 15:35

check123

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3 Comments

Aurelio De Rosa Over a year ago

Isn't this solution a little bit ugly (I mean too unnecessary nested if)?

Aurelio De Rosa Over a year ago

True but you're teaching very ugly pratice. To do 5 + 3 you can also use a = 5 for(i = 1; i <= 3;i++) a++; It works but you know you should not do this kind of stuff.

vidstige Over a year ago

this is a homework question and this is pedagogically troublesome.

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