RemoveAt() breaks for loop

[Markus]

Ok, so after some research I found that you have to iterate backwards through an array if you're removing elements. But the reason as to why has eluded me.

Can someone explain why this doesn't work and always deletes 1 less than the amount I have selected:

for (int i = 0; i < IPArray.Count; ++i)
            {
                if (dataGridView1.Rows[i].Cells[2].Value != null)
                    if ((bool)dataGridView1.Rows[i].Cells[2].Value == true)
                    {
                        MessageBox.Show(dataGridView1.Rows[i].Cells[2].Value.ToString());
                        IPArray.RemoveAt(i);
                    }
            }
 

While this works perfectly:

for (int i = IPArray.Count - 1; i >= 0; --i)
            {
                if (dataGridView1.Rows[i].Cells[2].Value != null)
                    if ((bool)dataGridView1.Rows[i].Cells[2].Value == true)
                    {
                        MessageBox.Show(dataGridView1.Rows[i].Cells[2].Value.ToString());
                        IPArray.RemoveAt(i);
                    }
            }

Cheers.

[joedeene]

Try using the following line of code in your first line of your first code:

for (int i = 0; i <= IPArray.Count; ++i)

Notice you just had the less than sign(<) which means whatever the selected items are it has to be less. So, if you selected 8, it can't be 8 because that's equal to 8. But 7 is equal to 8.

So I just replaced the less than sign with a less than OR equal to sign(<=). This should work since you said it was working for all selected minus one. Does this help?

joedeene

[Markus]

No, that doesn't solve it. It's something to do with the way the elements are removed from the array, I think.

Anyway, got to run.

[Plater]

Well your loop condition is based on the number of items in the array.
If there are 10 items in the array lets say, your loop will go from index 0 to 9 (correctly)
If you remove the item at index [2], now your loop will only go from 0 to 8.
...Well now I don't know where I was going with this.
BUT, if you tried to remove an item while using a foreach loop, it throws exceptions (due to changes in itterators or something)

What you could do is mark the indexes that you want to delete somewhere, and then loop through THAT collection to delete the indexes in a different collection.

[Curtis Rutland]

No, no, Plater, you were on the right track. If you're looping through an array, and deleting elements, you need to decriment your counter variable when you remove an entry.

So add an i--; after you remove an element.

[tlhintoq]

Picture the array as a vertical stack of Jenga game pieces, with 0 being the bottom piece, and 10 being the top piece.

If you increment from 0 to 10, moving item zero...
Item 1 'drops down' to fill the void where item zero used to be, and your stack is now 9 items tall.

But then in your loop, you are incrementing your counter, so now you remove item 1, forgetting that you still have a new item zero that used to be item 1.

Then increment to item 2... and so on,

So you remove...
Item 0 from an array of 10
Item 1 from an array of 9
Item 2 from an array of 8
Item 3 from an array of 7
Item 4 from an array of 6
Item 5 from an array of 5 - Breaks here since it is zero indexed

If you are going to increment your counter, then you would have to go from zero to 10, always removing item zero, not item "indexed counter variable"

[Plater]

But then in your loop, you are incrementing your counter, so now you remove item 1, forgetting that you still have a new item zero that used to be item 1.

That's the part I was missing. I *knew* I had learned it before but couldn't remember.