Pfthb

I wrote a function to go through a list and remove list items if some conditions where met. My program crashed on it, and after a while i concluded that the outer for loop, goes through all items in the list.
While at the same routine the list of item can get shorter.

 // Lijst is a list of a struct that contains a value .scanned and .price

 for (int i = 0; i < Lijst.Count; i++)
 
 if (Lijst[i].scanned == false)
 
 // (removed deletion of list item i here)
 if (Lijst[i].price > (int)nudMinimum.Value)
 
 Totaal++;
 lblDebug.Text = Totaal.ToString();
 
 Lijst.RemoveAt(i); //<-moved to here
 
 

Now i wonder whats the correct to do this, without getting index out of range errors.

You might be looking for this

for (int i = Lijst.Count - 1 ; i >= 0 ; i--)

 if (Lijst[i].scanned == false)
 
 if (Lijst[i].price > (int)nudMinimum.Value)
 
 Totaal++;
 lblDebug.Text = Totaal.ToString();
 
 Lijst.RemoveAt(i);
 

Question in the comment:

why would the other direction for loop work ?

Because when the loop is run in from Zero to Count There is a situation arise when the index is not available to remove and the count is still left. For example:

if you have 10 items in the List the loop starts at 0 and would remove 0,1,2,3,4 and now the item left are 5 and index is also 5 it would remove that item too. After that when loop value reaches 6 and item left is 4. Then it would create a problem. and it would throw an error. i.e. index out of range

Why not direct List<T>.RemoveAll()?

https://msdn.microsoft.com/en-us/library/wdka673a(v=vs.110).aspx

In your case

 Lijst.RemoveAll(item => some condition);

E.g.

 // Count all the not scanned items each of them exceeds nudMinimum.Value
 lblDebug.Text = Lijst
 .Where(item => !item.scanned && item.price > (int)nudMinimum.Value)
 .Count()
 .ToString();

 // Remove all not scanned items
 Lijst.RemoveAll(item => !item.scanned);

here you go

// 1. Count items
lblDebug.Text = Lijst.Count(x => x.price > (int)nudMinimum.Value && !x.scanned).ToString();
//2. Remove items
Lijst.RemoveAll(x => !x.scanned);

The problems is that when you remove the element number 5, the list gets shorter and the element number 6 is now 5th, number 7 becomes 6th etc. However, if you run the loop backwards, the number is kept as expected.

for(int i = donkeys.Count - 1; i >= 0; i++)
 if(donkeys[i] == some condition here)
 donkeys.RemoveAt(i);

However, it's an like-a-boss approach. There are better ways. You've got the answer but I'd like to suggest a LINQ based approach.

int Totaal = Lijst
 .Where(item => item.scanned)
 .Where(item => item.price > (int)nudMinimum.Value)
 .Count();

Lijst = Lijst.Where(item => !item.scanned).ToList()

Also, as a side note, I wonder if you find the below more readable. Consider the following different naming (both regarding the language and the capitalization).

List<Item> items = ...;
int minimum = (int)nudMinimum.Value;

int total = items
 .Where(item => item.scanned)
 .Where(item => item.price > minimum)
 .Count();

items = items
 .Where(item => !item.scanned)
 .ToList();

First You are removing the element with index i and then using it. You need to first do your process with element having index i and then remove it. Your code will look like below:

for (int i = 0; i < Lijst.Count; i++)
 
 if (Lijst[i].scanned == false)
 

 if (Lijst[i].price > (int)nudMinimum.Value)
 
 Totaal++;
 lblDebug.Text = Totaal.ToString();
 
Lijst.RemoveAt(i);