We Keep Coding

Is there a simple way of creating a list of identical objects without using a loop?

I currently have something like this:
public class Person
{
public string Firstname { get; set; }
public string Lastname { get; set; }
}
public List<Person> CreatePersons(int numberOfPersons)
{
var persons = new List<Person>();
for (int i = 0; i < numberOfPersons; i++)
{
persons.Add(new Person())
}
return persons;
}
I know that if I were to use an array then I could just do this:
var persons = new Person[numberOfPersons];
My question is:
Is there a simple / direct way of creating a list of objects without using a loop?
I guess that an array probably does the exact same thing internally as using a loop to create this array but with a different notation. But that's not a concern for now, for this question I'm just interested in a simpler notation.

How about,
var persons = Enumerable.Range(0, numberOfPersons).Select(i => new Person()).ToList();
You could also do the the more obtuse,
var persons = Enumerable
.Repeat<Func<Person>>(() => new Person(), numberOfPersons)
.Select(f => f())
.ToList();
In the end, I find the for loop easier to understand and it performs more quickly.
var persons = new List<Person>(numberOfPersons);
for (int i = 0; i < numberOfPersons; i++)
{
persons.Add(new Person())
}

You already have a one-liner to construct an array, how about converting that into the list you want?
new Person[numberOfPersons].ToList();

Or you could use System.Array.ConvertAll, it still use for loop but you don' see it :)...
var persons = Array.ConvertAll(new Person[30], _ => new Person());
n.b. this approach will allocate the array twice as it will allocate the resulting array and copy elements from input using the for loop...

How to quickly match names (fname, lname) in different order with full name c#

I have this linq query I am trying to optimize. I want to replace this query with a fast constant (preferably) retrieval of the value. I thought about a twin key dictionary but I have no idea which order the fname or lname will come first. I wanted to ask here if there is a fast way to do this.
I wanted to take a list of names, search through it for fname-lname the - is the delimeter and return all that match the full name that is searched. The list of people could be moderately large.
var nameList = from p in listOfPeople
where ((p.lname+"-"+p.fname == fullName)
|| (p.fname+"-"+p.lname == fullname))
select p;
Edit: listOfPeople can be any datatype, not necessarily a list.

Here's how you can create your dictionary.
var nameLookup = new Dictionary<Tuple<string,string>, List<Person>>();
foreach(var person in listOfPeople)
{
List<Person> people = null;
var firstLast = Tuple.Create(person.fname, person.lname);
if(nameLookup.TryGetValue(firstLast, out people))
{
people.Add(person);
}
else
{
nameLookup.Add(firstLast, new List<Person> { person });
}
// If the person's first and last name are the same we don't want to add them twice.
if(person.fname == person.lname)
{
continue;
}
var lastFirst = Tuple.Create(person.lname, person.fname);
if(nameLookup.TryGetValue(lastFirst, out people))
{
people.Add(person);
}
else
{
nameLookup.Add(lastFirst, new List<Person> { person });
}
}
Then your lookup would be
// split by the delimiter to get these if needed
var fullName = Tuple.Create(firstName, lastName);
List<Person> nameList = null;
if(!nameLookup.TryGetValue(fullName, out nameList))
{
nameList = new List<Person>();
}
It's important to keep the first and last names separate or you have to pick a delimiter that will not show up the the first or last name. Hyphen "-" could be part of a first or last name. If the delimiter is guaranteed to not be part of the first or last name you can just substitute the use of the Tuple.Create(x,y) with x + delimiter + y and change the dictionary to Dictionary<string, List<Person>>.
Additionally the reason for having a List<Person> as the value of the dictionary is to handle cases like "Gary William" and "William Gary" being two different people.

In your "P" definition, which I guess it's a "People" type, I would add a "FullName" property, which will be your comparator:
public string FullName {get {return fname + "-" + lname;}}
And modify your LINQ with:
Where string.Equals(p.FullName, fullName) .
If you REALLY want to use with ANY datatype, which would include just string or even DataTable, i really don't see any better way than the way you did...

I tested with Stopwatch and this appears to be a little more effective
var nameList = from n in(
from p in listOfPeople
select new{FullName = p.fname +"-"+ p.lname}
)
where n.FullName==fullName
select n;

C# Linq to CSV Dynamic Object runtime column name

I'm new to using Dynamic Objects in C#. I am reading a CSV file very similarly to the code found here: http://my.safaribooksonline.com/book/programming/csharp/9780321637208/csharp-4dot0-features/ch08lev1sec3
I can reference the data I need with a static name, however I can not find the correct syntax to reference using a dynamic name at run time.
For example I have:
var records = from r in myDynamicClass.Records select r;
foreach(dynamic rec in records)
{
Console.WriteLine(rec.SomeColumn);
}
And this works fine if you know the "SomeColumn" name. I would prefer to have a column name a a string and be able to make the same type refrence at run time.

Since one has to create the class which inherits from DynamicObject, simply add an indexer to the class to achieve one's result via strings.
The following example uses the same properties found in the book example, the properties which holds the individual line data that has the column names. Below is the indexer on that class to achieve the result:
public class myDynamicClassDataLine : System.Dynamic.DynamicObject
{
string[] _lineContent; // Actual line data
List<string> _headers; // Associated headers (properties)
public string this[string indexer]
{
get
{
string result = string.Empty;
int index = _headers.IndexOf(indexer);
if (index >= 0 && index < _lineContent.Length)
result = _lineContent[index];
return result;
}
}
}
Then access the data such as
var csv =
#",,SomeColumn,,,
ab,cd,ef,,,"; // Ef is the "SomeColumn"
var data = new myDynamicClass(csv); // This holds multiple myDynamicClassDataLine items
Console.WriteLine (data.OfType<dynamic>().First()["SomeColumn"]); // "ef" is the output.

You will need to use reflection. To get the names you would use:
List<string> columnNames = new List<string>(records.GetType().GetProperties().Select(i => i.Name));
You can then loop through your results and output the values for each column like so:
foreach(dynamic rec in records)
{
foreach (string prop in columnNames)
Console.Write(rec.GetType().GetProperty (prop).GetValue (rec, null));
}

Try this
string column = "SomeColumn";
var result = rec.GetType().GetProperty (column).GetValue (rec, null);

Using Contains() list method to evaluate list contents

I have a list that contains 3 items, two of type_1, and one of type_2. I want to return a second list that contains the type and number of that type that exists. When stepping through the breakpoints set at the foreach loop, the IF statement is never true. I assume there is something wrong with my attempt to use Contains() method.
The output should be something like:
type_1 2
type_2 1
Instead, it evaluates as:
type_1 1
type_1 1
type_2 1
Is my use of Contains() not correct?
public List<item_count> QueryGraphListingsNewAccountReport()
List<item> result = new List<items>();
var type_item1 = new item { account_type = "Type_1" };
var type_item2 = new item { account_type = "Type_1" };
var type_item3 = new item { account_type = "Type_2" };
result.Add(type_item1);
result.Add(type_item2);
result.Add(type_item3);
//Create a empty list that will hold the account_type AND a count of how many of that type exists:
List<item_count> result_count = new List<item_count>();
foreach (var item in result)
{
if (result_count.Contains(new item_count { account_type = item.account_type, count = 1 } ) == true)
{
var result_item = result_count.Find(x => x.account_type == item.account_type);
result_item.count += 1;
result_count.Add(result_item);
}
else
{
var result_item = new item_count { account_type = item.account_type, count = 1 };
result_count.Add(result_item);
}
}
return result_count;
}
public class item
{
public string account_type { get; set; }
}
public class item_count
{
public int count {get; set;}
public string account_type { get; set; }
}

I think your problem is that you don't want to use contains at all. You are creating a new object in your contains statement and, obviously, it isn't contained in your list already because you only just created it. The comparison is comparing references, not values.
Why not just use the find statement that you do in the next line instead? If it returns null, then you know there isn't an item already with that type.
So you could do something like this:
var result_item = result_count.Find(x => x.account_type == item.account_type);
if (result_item != null)
{
result_item.count++;
// note here you don't need to add it back to the list!
}
else
{
// create your new result_item here and add it to your list.
}
Note: Find is o(n), so this might not scale well if you have a really large set of types. In that case, you might be better off with Saeed's suggestion of grouping.

You can do:
myList.GroupBy(x=>x.type).Select(x=>new {x.Key, x.Count()});
If you want use for loop, it's better to use linq Count function to achieve this, If you want use Contains you should implement equal operator as the way you used.

Enumerated int List<GradeRange>

I really have no clue about enumerated list, but after some research I found that this list may help solve my problem. So I have a string in my settings called strGrades, and it is a range of strings that I manually update. The range is 0155-0160, 0271-0388, 0455-0503, 0588-687. What I basically want to do is find the values that are not in this grade list (for example 0161,0389, 0504-0587...)
So I came up with a function that will allow me to get each match in the grade range:
public static List<GradeRange> GetValidGrades()
{
MatchCollection matches= Regex.Matches(Settings.Default.productRange,
Settings.Default.srGradeRange);
List<GradeRange> ranges= new List<GradeRange();
if(matches.Count >0)
{
foreach (Match match in matches)
{
ranges.Add(new GradeRange() 23 {
Start= int.Parse(match.Groups["Start"].Value),
Stop= int.Parse(match.Groups["Stop"].Value)
});
}
}
return ranges;
}
here is the grade range class
public class GrandRange
{
public int Start{get; set;)
public int Stop {get; set; )
}
So the function above caputures my Start and End values, can anyone please help me get this into a list where I can find the values that fall outside of the range values, I just need a starting point. Thanks so much!

You could use a custom extension method that creates .Between along with a Where
var myFilteredList = list.Where(x=>!myValue.Between(x.Start, x.Stop, true));
This isnt the most performant answer, but if you need a list of all the numbers that are not between certain ranges, then you could do something like this:
var missingNumbers = new List<int>();
var minStop = list.OrderBy(x=>x.Stop).Min().Stop;
var maxStart = list.OrderBy(x=>x.Start).Max().Start;
Enumerable.Range(minStop, maxStart).ToList()
.ForEach(x=>
{
if(!x.Between(x.Start, x.Stop, true))
missingNumbers.Add(x);
}
);

Here this should get you started
var strings = "0155-0160, 0271-0388, 0455-0503, 0588-687";
var splitStrings = strings.Split(char.Parse(","));
var grads = new List<GrandRange>();
foreach (var item in splitStrings) {
var splitAgain = item.Split(char.Parse("-"));
var grand = new GrandRange
{
Start = int.Parse(splitAgain[0]),
Stop = int.Parse(splitAgain[1])
};
grads.Add(grand);
}
}