I Don't Understand How To Sort A Generic List of Objects [modified]

[That Asian Guy]

Just for testing I found this random class in an example file.

public class Product
{
public Product(string name, int quantityInStock)
{
_name = name;
_quantityInStock = quantityInStock;
}
private string _name;
public string Name
{
get { return _name; }
set { _name = value; }
}
private int _quantityInStock;
public int QuantityInStock
{
get { return _quantityInStock; }
set { _quantityInStock = value; }
}
}

Now, I created a list.

List<Product> products = new List<Product>();

And added some objects.

products.Add(new Product("A", 5));
products.Add(new Product("B", 4));
products.Add(new Product("C", 3));
products.Add(new Product("D", 2));
products.Add(new Product("E", 1));

However, when I tried using products.Sort(), I got an error. This did not surprise me since I didn't expect it to sort properly. However, I saw the other two overloads List<products>.Sort(Comparison<Product> comparison) and List<products>.Sort(IComparer<Product> comparer). I read the MSDN pages but still don't understand how to use these. Can someone provide some help? Thanks in advance. P.S. The casing is messed up in the post but don't worry about it :laugh:

modified on Sunday, October 5, 2008 5:27 PM

[Lost User]

You need to provide your own comparison method for your product class which lets you compare two products in the way you want to. Here's an example:

private static int CompareProducts(Product x, Product y)
{
if (x == null)
{
if (y == null)
{
// If x is null and y is null, they're equal.
return 0;
}
else
{
// If x is null and y is not null, y is greater.
return -1;
}
}
else
{
// If x is not null...
if (y == null) // ...and y is null, x is greater.
{
return 1;
}
else
{
// ...and y is not null, compare by whatever you want :-)
// here we compare either by name if different, otherwise by quantity in stock
if(x.Name == y.Name)
return x.QuantityInStock.CompareTo(y.QuantityInStock);
else
return x.Name.CompareTo(y.Name);
}
}

}

and then sort them:

producs.Sort(CompareProducts);

regards

[That Asian Guy]

Thanks, so return 1 means the first is greater, return 0 means they're the same, and return -1 means the second is greater?

[realJSOP]

Go here. The example is simple enough to understand. http://dotnetslackers.com/Community/blogs/simoneb/archive/2007/06/20/How-to-sort-a-generic-List_3C00_T_3E00_.aspx[^]

"Why don't you tie a kerosene-soaked rag around your ankles so the ants won't climb up and eat your candy ass..." - Dale Earnhardt, 1997
-----
"...the staggering layers of obscenity in your statement make it a work of art on so many levels." - Jason Jystad, 10/26/2001

[Guffa]

If you for example want to sort by product name, you provide a comparison that compares that property for two items: products.Sort(delegate(Product x, Product y){return string.Compare(x.Name,y.Name);}); Or if you use framework 3.5, you can use a lambda expression: products.Sort((x, y) => string.Compare(x.Name,y.Name));

Despite everything, the person most likely to be fooling you next is yourself.

[Lost User]

That's correct.

[realJSOP]

I tried both of those, and it looks like you can only do string comparisons that way. How do you do int, double, etc. without using ToString()?

"Why don't you tie a kerosene-soaked rag around your ankles so the ants won't climb up and eat your candy ass..." - Dale Earnhardt, 1997
-----
"...the staggering layers of obscenity in your statement make it a work of art on so many levels." - Jason Jystad, 10/26/2001

[leppie]

John Simmons / outlaw programmer wrote:

How do you do int, double, etc. without using ToString()?

Like normal...

list.Sort( (x,y) => x.Int.CompareTo(y.Int))

xacc.ide - now with TabsToSpaces support
IronScheme - 1.0 alpha 4a out now (29 May 2008)
((lambda (x) `((lambda (x) ,x) ',x)) '`((lambda (x) ,x) ',x))

[Guffa]

For integer values you can just subtract them: products.Sort(delegate(Product x, Product y){return x.QuantityInStock - y.QuantityInStock;}); For double values you can use the Math.Sign method to get a suitable integer value from the difference: products.Sort(delegate(Product x, Product y){return Math.Sign(x.Price - y.Price);});

Despite everything, the person most likely to be fooling you next is yourself.

[realJSOP]

After seeing that, I prefer to stick with the tried/true method of writing the various Comparison delegate functions. It's much easier to understand and maintain. When I code, I don't go "clever" nearly as often as I go "maintainable".

"Why don't you tie a kerosene-soaked rag around your ankles so the ants won't climb up and eat your candy ass..." - Dale Earnhardt, 1997
-----
"...the staggering layers of obscenity in your statement make it a work of art on so many levels." - Jason Jystad, 10/26/2001