Develop Reference

Adding a number to variable names in C#

My question is sort of like the one found here:
How do I name variables dynamically in C#?
However its a bit different so I'm wondering if its possible.
I'm trying to read in a bunch of strings from a .settings file.
I have them all named Time1, Time2,Time3 etc...
I want the User to be able to add more Times to the file so there could be Time100 or more.
I have a Size in the settings file that will keep track of the amount of Time Variables.
I want to write something that will read in all of the Time strings. I think it would be silly to pre-fill the .settings file with 100 Time Variables so I know they are there and then manually read in each one.
So I'm wondering if there is a way where I can read in Timei or Time+i where I is an integer that I can put in a loop that will find them all.
(note that the data is string not time, its just going to be converted to days of the week)
Such as: (Days is from ApplicationSettingsBase [aka file add new Settings1.settings]
public static int AvDaysIndex = Days.Default.Size; //This holds the number of items in Days
public static DayOfWeek[] AvailableDays = new DayOfWeek[AvDaysIndex]; //This is where I wants to read in all the variables Aka Time1 Time2 Times3
public ReadInDays() //Reads in from the .settings File
{
if(AvDaysIndex>0) // Makes sure there is something in Days to read
{
int I=0;
//I can Manually do the following
AvailableDays[I++] = Days.Default.Time1;
AvailableDays[I++] = Days.Default.Time2;
AvailableDays[I++] = Days.Default.Time3; //etc...
//Is there a way to do something like this
for (int i = 0; i < AvDaysIndex; i++) //reads in each time
{
AvailableDays[i] = Days.Default.Time +i;//where I would be added to the variable name to find it?
//Or something like
AvailableDays[i] = Days.Default.Time(I.tostring())
}
}
}
Hopefully all that at least makes it clear what I'm trying to do.
Edit - I'm starting to think my issue is actually with the .settings file. and that if I just read values in from another file type where the values don't have names I can easily read them in even though there is a variable number of elements in the file.
Solution -
for (int i = 0; i < Index; i++)
{
AvailableDays[i] = getFromFile(_Days.Default.Properties["Time" + (i+1).ToString()].DefaultValue.ToString());
AvailableTimes[i] = Convert.ToDateTime(_Times.Default.Properties["Time" + (i + 1).ToString()].DefaultValue);
}
It was all in figuring out how to read in from the .settings file and instead of reading it in directly aka Days.Default.Time1; I had to to do a generic lookup from Days.Default.Properties and then I could create a dynamic name and find it. You guys probably were trying to tell me how to do this, I just didn't understand.
Thanks again to all those that helped.

I would use a hashtable/dictionary to store the Days.Default.TimeX variations
hashtable["Time1"]...hashtable["TimeN"]

As already mentioned a hashtable or a dictionary would probably serve you best. If you go the dictionary route you can create a string/int indexer on the class and you would be able to alter your code slightly:
http://msdn.microsoft.com/en-us/library/2549tw02%28v=vs.80%29.aspx - Example of creating indexer on a class:
Example Indexer Class:
public class Default
{
private Dictionary<int, DayOfWeek> _values = new Dictionary<int,DayOfWeek>();
public DayOfWeek this[int index]
{
get
{
if (_values.ContainsKey(index))
return _values[index];
else
return null;
}
set
{
_values[index] = value;
}
}
}
Original:
AvailableDays[i] = Days.Default.Time(I.tostring())
Would become:
AvailableDays[i] = Days.Default.Time[I];
Reflection is always an option too and i have an example below that is in a Windows Console Application:
public class Default
{
public int Time1 { get; set; }
public int Time2 { get; set; }
public int Time3 { get; set; }
}
class Program
{
static void Main(string[] args)
{
Default d = new Default();
Type t = d.GetType();
foreach (var info in t.GetProperties())
{
//SET VALUE
info.SetValue(d, 1);
}
foreach (var info in t.GetProperties())
{
//GET VALUE
Console.WriteLine("Property: {0}", info.Name);
Console.WriteLine("Value: {0}", info.GetValue(d));
}
//OR JUST ONE PROPERTY
Console.WriteLine("Time1 Property Value: {0}", t.GetProperty("Time1").GetValue(d));
Console.ReadLine();//PAUSE THE CONSOLE AFTER PROCESSING
}
}
In your example using reflection:
Days.Default.GetType().GetProperty("Time" + I.ToString()).GetValue(Days.Default) as DayOfWeek;

Another option could be to use Reflection. And getting the values from the enum on the fly.
See the link: How to Get Enum Values with Reflection in C#
However, using a Dictionary<string, DayOfWeek> will give you better performance and more readable code.

I think you could better resolve your problem by implementing your configuration using a
ConfigurationElementCollection. Then the 'names' of the configuration elements are irrelevant. You enumerate a collection of values and use those directly.
See here for an example; How to implement a ConfigurationSection with a ConfigurationElementCollection.

Why can't I edit a List member directly in C#

I have a class declared as public class DatumSet : List<datum>, where
public struct datum {
public UInt32[] chan;
public UInt64 sample_number;
public float time;
public UInt32 source_sector;
}
I want to iterate through the List and make some changes. Why does this NOT work
for (int i = 0; i < this.Count; i++) {
this[i].sample_number = startSample;
this[i].time = (float)startSample / _sample_rate;
startSample++;
}
but this DOES work
for (int i = 0; i < this.Count; i++) {
datum d = this[i];
d.sample_number = sampleNumber;
d.time = (float)sampleNumber / _sample_rate;
sampleNumber++;
}
I get the error:
Cannot modify the return value of 'System.Collections.Generic.List.this[int]' because it is not a variable

This is what you get for using a mutable value type :)
Think of an indexer as just like a method call. So this:
this[i].sample_number = startSample;
is like:
GetValue(i).sample_number = startSample;
But because datum is a value type (a struct), the method is returning a copy of the value in the list - so modifying it would do you no good at all.
The compiler is stopping you from making that mistake.
You claim that this works:
for (int i = 0; i < this.Count; i++) {
datum d = this[i];
d.sample_number = sampleNumber;
d.time = (float)sampleNumber / _sample_rate;
sampleNumber++;
}
... but in reality, it does nothing useful. It's equivalent to:
sampleNumber += this.Count;
That's all. It compiles, but that's not the same as it working.
I would suggest that you make all value types immutable; it helps to prevent you from getting into this mess. So either you can keep datum as a value type, and replace the value in the list on each iteration, or you can change it to be a class, and modify the objects via the reference stored in the list.
(Either way, I'd strongly suggest that you start using properties instead of public fields, and start following .NET naming conventions too.)

You're having problems because you are using a struct rather than a class.
When you retrieve a struct from a collection, a copy is made. Your first set of code gives you an error because it detects you're doing something you may not mean to do. You'd actually be editing a copy of the struct rather than the copy in the collection.
The second doesn't produce an error because you explicitly pull the copy out of the collection before editing. This code may compile, but won't modify any of the structs in the collection and thus won't give you the results that you're expecting.

C#: Dynamically Constructing Variables

I get from an input a group of double variables named: weight0, weight1...weight49.
I want to dynamically insert them into a double Array for easier manipulation.
But instead of calling each one like: Weights[0] = weight0...Weights[49] = weight49 I want to do it with a single loop.
Is there a way to do it?

No, basically - unless you mean at the same time that you create the array:
var weights = new[] {weight0, weight1, weight2, ... , weight48, weight49};
Personally, I'd be tempted to get rid of the 50 variables, and use the array from the outset, but that may not be possible in all cases.

you could use reflection to determine the index of the array from the variable names but this is far from efficient. See this post for details.

I would try to do it with a KeyValuePair- Listobject
// sample data
var weight = 1.00;
// create a list
var tmp = new List<KeyValuePair<string,object>>();
// Here you can add your variables
tmp.Add(new KeyValuePair<string,object>("weights" + tmp.Count.ToString()
, weight));
// If needed convert to array
var weights = tmp.ToArray();
// get the information out of the array
var weightValue = weights[0].Value;
var weightKey = weights[0].Key;
I think this will give you all the options, you might need for the array. Give it a try.

I'm putting this up because you can do it - so long as these variables are actually fields/properties. Whether you should is another matter - this solution, while reusable, is slow (needs delegate caching) and I have to say I agree with Marc Gravell - consider using an array throughout if you can.
If the variables are properties then it needs changing. Also if you need to write the array back to the variables in one shot (because this solution generates an array with copies of all the doubles, I wouldn't consider creating an object array with boxed doubles), that requires another method...
So here goes. First a holy wall of code/extension method:
//paste this as a direct child of a namespace (not a nested class)
public static class SO8877853Extensions
{
public static TArray[] FieldsToArray<TObj, TArray>(this TObj o,string fieldPrefix)
{
if(string.IsNullOrWhiteSpace(fieldPrefix))
throw new ArgumentException("fieldPrefix must not null/empty/whitespace",
"fieldPrefix");
//I've done this slightly more expanded than it really needs to be...
var fields = typeof(TObj).GetFields(System.Reflection.BindingFlags.Instance
| System.Reflection.BindingFlags.Public
| System.Reflection.BindingFlags.NonPublic)
.Where(f =>f.Name.StartsWith(fieldPrefix) && f.FieldType.Equals(typeof(TArray)))
.Select(f =>new{ Field = f, OrdinalStr = f.Name.Substring(fieldPrefix.Length)})
.Where(f => { int unused; return int.TryParse(f.OrdinalStr, out unused);})
.Select(f => new { Field = f.Field, Ordinal = int.Parse(f.OrdinalStr) })
.OrderBy(f => f.Ordinal).ToArray();
//doesn't handle ordinal gaps e.g. 0,1,2,7
if(fields.Length == 0)
throw new ArgumentException(
string.Format("No fields found with the prefix {0}",
fieldPrefix),
"fieldPrefix");
//could instead bake the 'o' reference as a constant - but if
//you are caching the delegate, it makes it non-reusable.
ParameterExpression pThis = Expression.Parameter(o.GetType());
//generates a dynamic new double[] { var0, var1 ... } expression
var lambda = Expression.Lambda<Func<TObj, TArray[]>>(
Expression.NewArrayInit(typeof(TArray),
fields.Select(f => Expression.Field(pThis, f.Field))), pThis);
//you could cache this delegate here by typeof(TObj),
//fieldPrefix and typeof(TArray) in a Dictionary/ConcurrentDictionary
return lambda.Compile()(o);
}
}
The extension method above will work on any type. It's generic over both the instance type and desired array type to simplify the creation of the lambda in code - it doesn't have to be generic though.
You pass in the name prefix for a group of fields - in your case "weight" - it then searches all the public and private instance fields for those with that prefix that also have a suffix which can be parsed into an integer. It then orders those fields based on that ordinal. It does not check for gaps in the ordinal list - so a type with weight0 and weight2 would work, but would only create a two-element array.
Then it bakes a dynamic piece of code using Expression trees, compiles it (at this point, as mentioned in the code, it would be good to cache the delegate against TObj and TArray for future use) and then executes it, returning the result.
Now add this to a test class in a standard unit test project:
private class SO8877853
{
private double field0 = 1.0;
private double field1 = -5.0;
private double field2 = 10.0;
public double[] AsArray()
{
//it would be nice not to have to pass both type names here - that
//can be achieved by making the extension method pass out the array
//via an 'out TArray[]' instead.
return this.FieldsToArray<SO8877853, double>("field");
}
}
[TestMethod]
public void TestThatItWorks()
{
var asArray = new SO8877853().AsArray();
Assert.IsTrue(new[] { 1.0, -5.0, 10.0 }.SequenceEqual(asArray));
}
Like I say - I'm not condoning doing this, nor am I expecting any +1s for it - but I'm a sucker for a challenge :)

What is the need Indexers in C#

Today I've gone through what indexers are, but I am bit confused. Is there really a need for indexers? What are the advantages of using an indexer..... thanks in advance

I guess the simplest answer is to look at how you'd use (say) List<T> otherwise. Would you rather write:
string foo = list[10];
or
string foo = list.Get(10);
Likewise for dictionaries, would you rather use:
map["foo"] = "bar";
or
map.Put("foo", "bar");
?
Just like properties, there's no real need for them compared with just named methods following a convention... but they make code easier to understand, in my view - and that's one of the most important things a feature can do.

Indexers let you get a reference to an object in a collection without having to traverse the whole collections.
Say you have several thousands of objects, and you need the one before last. Instead of iterating over all of the items in the collection, you simply use the index of the object you want.
Indexers do no have to be integers, so you can use a string, for example, (though you can use any object, so long as the collection supports it) as an indexer - this lets you "name" objects in a collection for later retrieval, also quite useful.

I think zedo got closest to the real reason IMHO that they have added this feature. It's for convenience in the same way that we have properties.
The code is easer to type and easier to read, with a simple abstraction to help you understand.
For instance:
string[] array;
string value = array[0];
List<string> list;
string value = list[0]; //Abstracts the list lookup to a call similar to array.
Dictionary<string, int> map;
int value = map["KeyName"]; //Overloaded with string lookup.

Indexers allow you to reference your class in the same way as an array which is useful when creating a collection class, but giving a class array-like behavior can be useful in other situations as well, such as when dealing with a large file or abstracting a set of finite resources.

yes , they are very use of
you can use indexers to get the indexed object.
Taken from MSDN
Indexers are most frequently implemented in types whose primary purpose is to encapsulate an internal collection or array.
Full Story

for some reason, use indexer can let you create meaningful index to store or map your data. then you can get it from other side by the meaningful index.

using System;
/* Here is a simple program. I think this will help you to understand */
namespace Indexers
{
class Demo
{
int[] a = new int[10];
public int Lengths
{
get
{
return a.Length;
}
}
public int this[int index]
{
get
{
return a[index];
}
set
{
a[index] = value;
}
}
}
class Program
{
static void Main(string[] args)
{
Demo d = new Demo(); // Notice here, this is a simple object
//but you can use this like an array
for (int i = 0; i < d.Lengths; i++)
{
d[i] = i;
}
for (int i = 0; i < d.Lengths; i++)
{
Console.WriteLine(d[i]);
}
Console.ReadKey();
}
}
}
/*Output:
0
1
2
3
4
5
6
7
8
9
*/

how to create multiple objects and enumerate them in c#

my problem is as follows:
Im building a console application which asks the user for the numbers of objects it should create and 4 variables that have to be assigned for every object.
The new objects name should contain a counting number starting from 1.
How would you solve this?
Im thinking about a class but im unsure about how to create the objects in runtime from userinput. Is a loop the best way to go?
What kind of class, struct, list, array .... would you recommend. The variables in the object are always the same type but i need to name them properly so I can effectivly write methods to perform operations on them in a later phase of the program.
Im just learning the language and I would be very thankful for a advice on how to approach my problem.

If I understand your problem correctly:
class MyClass
{
public int ObjectNumber { get; set; }
public string SomeVariable { get; set; }
public string AnotherVariable { get; set; }
}
// You should use keyboard input value for this
int objectsToCreate = 10;
// Create an array to hold all your objects
MyClass[] myObjects = new MyClass[objectsToCreate];
for (int i = 0; i < objectsToCreate; i++)
{
// Instantiate a new object, set it's number and
// some other properties
myObjects[i] = new MyClass()
{
ObjectNumber = i + 1,
SomeVariable = "SomeValue",
AnotherVariable = "AnotherValue"
};
}
This doesn't quite do what you described. Add in keyboard input and stuff :) Most of this code needs to be in some kind of Main method to actually run, etc.
In this case, I've chosen a class to hold your 4 variables. I have only implemented 3 though, and I've implemented them as properties, rather than fields. I'm not sure this is necessary for your assignment, but it is generally a good habit to not have publically accessible fields, and I don't want to be the one to teach you bad habits. See auto-implemented properties.
You mentioned a struct, which would be an option as well, depending on what you want to store in it. Generally though, a class would be a safer bet.
A loop would indeed be the way to go to initialize your objects. In this case, a for loop is most practical. It starts counting at 0, because we're putting the objects in an array, and array indexes in C# always start at 0. This means you have to use i + 1 to assign to the object number, or the objects would be numbered 0 - 9, just like their indexes in the array.
I'm initializing the objects using object initializer syntax, which is new in C# 3.0.
The old fashioned way would be to assign them one by one:
myObjects[i] = new MyClass();
myObjects[i].ObjectNumber = i + 1;
myObjects[i].SomeVariable = "SomeValue";
Alternatively, you could define a constructor for MyClass that takes 3 parameters.
One last thing: some people here posted answers which use a generic List (List<MyClass>) instead of an array. This will work fine, but in my example I chose to use the most basic form you could use. A List does not have a fixed size, unlike an array (notice how I initialized the array). Lists are great if you want to add more items later, or if you have no idea beforehand how many items you will need to store. However, in this case, we have the keyboard input, so we know exactly how many items we'll have. Thus: array. It will implicitly tell whoever is reading your code, that you do not intend to add more items later.
I hope this answered some questions, and raised some new ones. See just how deep the rabbit hole goes :P

Use a list or an array. List example:
int numberOfObjects = 3;
List<YourType> listOfObjects = new List<YourType>();
for(int i = 0 ; i < numberOfObjects ; i++ )
{
// Get input and create object ....
// Then add to your list
listOfObjects.Add(element);
}
Here, listOfObjects is a Generic list that can contain a variable number of objects of the type YourType. The list will automatically resize so it can hold the number of objects you add to it. Hope this helps.

If I understood what you are asking you could probably do something like this:
class Foo
{
private static int count;
public string name;
public Foo(...){
name = ++count + "";
}
}

I'm guessing what you're trying to do here, but this is a stab in the dark. The problem I'm having is dealing with the whole "the new objects name should contain a counting number starting from 1" thing. Anyway, here's my attempt:
public class UserInstantiatedClass
{
public int UserSetField1;
public int UserSetField2;
public int UserSetField3;
public int UserSetField4;
public string UserSpecifiedClassName;
}
public static class MyProgram
{
public static void Main(string [] args)
{
// gather user input, place into variables named
// numInstances, className, field1, field2, field3, field4
List<UserInstantiatedClass> instances = new List< UserInstantiatedClass>();
UserInstantiatedClass current = null;
for(int i=1; i<=numInstances; i++)
{
current = new UserInstantiatedClass();
current.UserSpecifiedClassName = className + i.ToString(); // adds the number 1, 2, 3, etc. to the class name specified
current.UserSetField1 = field1;
current.UserSetField2 = field2;
current.UserSetField3 = field3;
current.UserSetField4 = field4;
instances.Add(current);
}
// after this loop, the instances list contains the number of instances of the class UserInstantiatedClass specified by the numInstances variable.
}
}