How to skip one element in an array?

[Ls chin]

Hi all,

How to skip one element in an array and continue looping? For example, this question:

"Return the sum of the numbers in the array, returning 0 for an empty array. Except the number "13" is very unlucky, so it does not count and numbers that come immediately after a 13 also do not count".


sum13({1, 2, 13, 2, 1, 13}) → correct answer 4, but my answer is 3

sum13({13, 1, 2, 13, 2, 1, 13}) → correct answer 3, my answer is 0

public int sum13(int[] nums) {
  int sum = 0;
  for (int i=0; i<nums.length; i++) {  
    if (nums[i] == 13) {    
      break;      
    } else {
       sum = sum + nums[i];
    }
  }    
  return sum;  
}

Whether I use 'break' or 'continue', it does not work. How do I skip the number that comes after 13 and continue looping and adding to the sum?

All suggestions are appreciated. Thank you.
[ November 03, 2008: Message edited by: LS chin ]

[Campbell Ritchie]

You shouldn't post that sort of thing when I am around because I am one of those people who believe You should only use break in a switch block. It ought to give the correct result with "continue," however.

Try putting the if (nums[i] != 13) test around the line where you add. That is simpler and easier to read.

[Rob Spoor]

You should also check out the previous element (as stated in the problem description):

if (nums[i] == 13 || (i > 0 && nums[i - 1] == 13))

Please note that i > 0 check; without it the code fails if the first element is not 13.

Also, you shouldn't use break since that will abort the entire loop; any number afterwards is discarded.

[Ls chin]

Hi Rob,
Thank you! The code works.

Originally posted by Rob Prime:
You should also check out the previous element (as stated in the problem description):

if (nums[i] == 13 || (i > 0 && nums[i - 1] == 13))

Please note that i > 0 check; without it the code fails if the first element is not 13.

Also, you shouldn't use break since that will abort the entire loop; any number afterwards is discarded.

I knew it had something to do with 'nums[i-1]' or 'nums[i+1] but I didn't know where to put it. :roll: Now, I see it~.


Thank you.

[Ls chin]

Hi Campbell,

Thank you! You are right. Got to use 'continue' instead of 'break', and combined with the new test conditions, the code is now perfect.

Originally posted by Campbell Ritchie:
You shouldn't post that sort of thing when I am around because I am one of those people who believe you should only use break in a switch block. It ought to give the correct result with "continue," however.

public int sum13(int[] nums) {
  int sum = 0;
  for (int i=0; i<nums.length; i++) {  
    if (nums[i] == 13 || (i > 0 && nums[i - 1] == 13)) {    
    continue;      
    } else {
       sum = sum + nums[i];
    }
  }       
  return sum;  
}

Thank you!
[ November 03, 2008: Message edited by: LS chin ]

[Campbell Ritchie]

You're welcome. Let's see if it will work in old-fashioned "structured programming."

int sum = 0;
for (int i = 0; i < nums.length; i++)
{
   if (nums[i] != 13 && i > 0 && nums[i - 1] != 13)
   {
      sum += nums[i];
   }
}
return sum;

[Ls chin]

Hi Campbell,

I tried your code and... nope, it doesn't work. Try putting your code in this link here and you'll see the answers to some of the arrays are correct and some aren't.

Originally posted by Campbell Ritchie:
You're welcome. Let's see if it will work in old-fashioned "structured programming."

Enjoy the puzzles.

[Rob Spoor]

That's because Campbell hasn't used DeMorgan's law properly

The negation of

nums[i] == 13 || (i > 0 && nums[i - 1] == 13)

is

nums[i] != 13 && (i <= 0 || nums[i - 1] != 13)

In this case, the <= can be replaced with == since i cannot be negative.

In Campbell's code, if the first element was 13 it would never be ignored.

[Campbell Ritchie]

I never realised it was a Javabat.

It is the test; it won't recognise anything when i == 0. It works if you use de Morgan's rule.

public int sum13(int[] nums)
{
  int sum = 0;
  for (int i = 0; i < nums.length; i++)
  {
     if (!(nums[i] == 13 || i > 0 && nums[i - 1] == 13))
     {
       sum += nums[i];
     }
  }
  return sum;
}

[Campbell Ritchie]

And I didn't realise Rob was (as usual) about to post 2 minutes before I did.

[Ls chin]

Hi Rob, Campbell,

Thank you. DeMorgan's law, eh? It's pretty challenging.

Btw, can you please explain the test condition again? The question says "Return the sum of the numbers in the array, except the number "13" and the 'number that come immediately' after a 13" (i.e. only one number that comes immediately after 13 is not counted, and the rest are counted, right?).

So, why do we use nums[i-1] instead of nums[i+1]?

Originally posted by Rob Prime:
You should also check out the previous element (as stated in the problem description):

if (nums[i] == 13 || (i > 0 && nums[i - 1] == 13))

Sorry for the trouble.

Thanks again.


Edit: Wait, I think I got it!

if nums[i] == 13

nums[i - 1] == 13 // ----> is the array element that comes AFTER a 13, right?
[ November 04, 2008: Message edited by: LS chin ]

[Rob Spoor]

Originally posted by LS chin:
Thank you. DeMorgan's law, eh? It's pretty challenging.

It's not that hard, and comes in two parts.

Part 1:
!(A && B) is the same as !A || !B

Part 2:
!(A || B) is the same as !A && !B

In this case B is a bit more complex, and has DeMorgan applied again.

Edit: Wait, I think I got it!

if nums[i] == 13

nums[i - 1] == 13 // ----> is the array element that comes AFTER a 13, right?

i is the index of the element after 13 yes, so you are checking if its previous element is 13 yes. With num[i + 1] == 13 you would be checking if the next element is 13, which you should use if you should skip the number before 13.

[Campbell Ritchie]

I was taught de Morgan's law was "break the line and change the sign" which rhymes nicely but only works when you can draw _____ above the symbols.
In this version of Boolean logic, an overscore means "not" a + means "or" and a . means "and."

___    _ _
a.b =  a+b

and

___   _ _
a+b = a.b

In both cases vice versa applies.

[edit]Add code tags[/edit]
[ November 04, 2008: Message edited by: Campbell Ritchie ]

[Rob Spoor]

That's exactly what I said, but with images

[Campbell Ritchie]

Yes, it is the same law. One person will remember it one way, another the other way.

[Ls chin]

Hello Rob, Campbell,

Thank you both very much!

[James Tharakan]

public int sum13(int[] nums) { 1 2 13 2 5
int sum = 0;
for (int i=0; i<nums.length; i++) {
if (nums[i] == 13) {
i++;
continue;
} else {
sum = sum + nums[i];
}
}
return sum;
}

[Campbell Ritchie]

You're welcome

[Tom Johnson]

re: James Tharakan

Nice, reads much easier than the other solution, I think. Also, you dont need the continue, as follows:

    
public int sum13(int[] nums) {
    int sum = 0;
  
    for (int i = 0; i < nums.length; i++) {
        if (nums[i] == 13) {
            i++;
        } else {
            sum += nums[i];
        }
    }
    return sum; 
}

[ November 05, 2008: Message edited by: Tom Johnson ]

[Campbell Ritchie]

Yes, that is a nice solution, Tom Johnson.

[Rob Spoor]

It sure is.

I just don't like changing i within the loop as well, but that stems from my education. I've learned to write loops which you can prove are correct, and incrementing i within the loop would make proving correctness a lot harder.

[Tom Johnson]

Yeah I do agree its a little unusual to increment inside the loop as well, perhaps confusing. However I think

if(!(nums[i] == 13 || i > 0 && nums[i - 1] == 13)) {
...
}

is rather confusing as well, until you have a good look at it. As with most of these kind of things, its a personal choice as to which you prefer, rather than which is "better" I reckon.

[Rob Spoor]

Neither is better, they both do the job perfectly and both have its quirks.

[Tom Johnson]

I know that's what I was implying....