Develop Reference

"I i can traverse a list using indexing as i do in an array.Why can't i add an item in a list as i do in an array using indexing."

Why do I need to use the Add() to add elements to a List. Why can't I use indexing and do it. When I traverse the elements through the List I do it using the help of indexes.
int head = -1;
List<char> arr = new List<char>();
public void push(char s)
{
++head;
arr[head] = s;//throws runtime error.
arr.Add(s);
}
It doesn't throw any error during compile time. But throws an error at runtime stating IndexOutOfRangeException.

++head;
arr[head] = s;
This attempts to set element 1 of the list to s, but there is no element 1 yet because you've not added anything, or set the length of the list.
When you create an array, you define a length, so each item has a memory address that can be assigned to.
Lists are useful when you don't know how many items you're going to have, or what their index is going to be.

Arrays are fixed sizes. Once you allocate them, you can not add or remove "slots" from it. So if you need it to be bigger, you need to:
Detect that you need a bigger array.
Allocate a new, bigger array
copy all existing values to teh new, bigger array
start using the bigger array from now on everywhere
All that Lists do is automate that precise process. It will automatically detect that it needs to increase during Add() and then do step 2-4 automagically. It is even responsible to pick the initial size and by how much to grow it (to avoid having to grow to often.
They could in theory jsut react to List[11000] by growing the size to 11000. But chances are very big, that this value is a huge mistake. And preventing the Progarmmer from doing huge mistakes is what half the classes and compiler rules (like strong typisation) are there for. So they force you to use Add() so such a mistake can not happen.

Actually calling myArray[2] does not add the element, but just assigns the object to the specified index within the array. If the array´s size is less you´d get an IndexOutOfBoundsException, as in a list<T> also. So also in case of an array using the indexer assumes you actually have that many elements:
var array = new int[3];
array[5] = 4; // bang
This is because arrays have a fixed size which you can´t change. If you assign an object to an index greater the arrays size you get the exat same exception as for a List<T> also, there´s no difference here.
The only real difference here is that when using new array[3] you have an array of size 3 with indices up to 2 and you can call array[2]. However this would just return the default-value - in case of int this is zero. When using new List<int>(3) in contrast you don´t have actually three elements. In fact the list has no items at all and calling list[2] throws the exception. The parameter to a list is just the capacity, which is a parameter for the runtime to indicate when the underlying array of a list should be resized - an ability your array does not even have.

A list is an array wrapper, where the internal array size is managed by its methods. The constructor that takes a capacity simply creates an array of that size internally, but the count property (which reflects the count elements that has been added) will be zero. So in essence, zero slots in the array has been assigned a value.
The size of an array is managed by you the programmer. That is why you have to call static methods like System.Array.Resize (notice that the array argument is ref), if you want to change an array yourself. That method allocates a new chunk of memory for the new size.
So to sum up, the list essentially manages an array for you, and as such, the tradeoff is that you can only access as many array-like slots as has been added to it.

Passing by reference to n-th element in C#

In C, if we have an array, we can pass it by reference to a function. We can also use simple addition of (n-1) to pass the reference starting from n-th element of the array like this:
char *strArr[5];
char *str1 = "I want that!\n";
char *str2 = "I want this!\n";
char *str3 = "I want those!\n";
char *str4 = "I want these!\n";
char *str5 = "I want them!\n";
strArr[0] = str1;
strArr[1] = str2;
strArr[2] = str3;
strArr[3] = str4;
strArr[4] = str5;
printPartially(strArr + 1, 4); //we can pass like this to start printing from 2nd element
....
void printPartially(char** strArrPart, char size){
int i;
for (i = 0; i < size; ++i)
printf(strArrPart[i]);
}
Resulting in these:
I want this!
I want those!
I want these!
I want them!
Process returned 0 (0x0) execution time : 0.006 s
Press any key to continue.
In C#, we can also pass reference to an object by ref (or, out). The object includes array, which is the whole array (or at least, this is how I suppose it works). But how are we to pass by reference to the n-th element of the array such that internal to the function, there is only string[] whose elements are one less than the original string[] without the need to create new array?
Must we use unsafe? I am looking for a solution (if possible) without unsafe
Edit:
I understand that we could pass Array in C# without ref keyword. Perhaps my question sounds quite misleading by mentioning ref when we talk about Array. The point why I put ref there, I should rather put it this way: is the ref keyword can be used, say, to pass the reference to n-th element of the array as much as C does other than passing reference to any object (without mentioning the n-th element or something alike)? My apology for any misunderstanding occurs by my question's phrasing.

The "safe" approach would be to pass an ArraySegment struct instead.
You can of course pass a pointer to a character using unsafe c#, but then you need to worry about buffer overruns.
Incidentally, an Array in C# is (usually) allocated on the heap, so passing it normally (without ref) doesn't mean copying the array- it's still a reference that is passed (just a new one).

Edit:
You won't be able to do it as you do in C in safe code.
A C# array (i.e. string[]) is derived from abstract type Array.
It is not only a simple memory block as it is in C.
So you can't send one of it's element's reference and start iterate from there.
But there are some solutions which will give you the same taste of course (without unsafe):
Like:
As #Chris mentioned you can use ArraySegment<T>.
As Array is also an IEnumerable<T> you can use .Skip and send the returned value. (but this will give you an IEnumerable<T> instead of an Array). But it will allow you iterate.
etc...

If the method should only read from the array, you can use linq:
string[] strings = {"str1", "str2", "str3", ...."str10"};
print(strings.Skip(1).Take(4).ToArray());

Your confusion is a very common one. The essential point is realizing that "reference types" and "passing by reference" (ref keyboard) are totally independent. In this specific case, since string[] is a reference type (as are all arrays), it means the object is not copied when you pass it around, hence you are always referring to the same object.
Modified Version of C# Code:
string[] strArr = new string[5];
strArr[0] = "I want that!\n";
strArr[1] = "I want this!\n";
strArr[2] = "I want those!\n";
strArr[3] = "I want these!\n";
strArr[4] = "I want them!\n";
printPartially(strArr.Skip(1).Take(4).ToArray());
void printPartially(string[] strArr)
{
foreach (string str in strArr)
{
Console.WriteLine(str);
}
}

Question is old, but maybe answer will be useful for someone.
As of C# 7.2 there are much more types to use in that case, ex. Span or Memory.
They allow exactly for the thing you mentioned in your question (and much more).
Here's great article about them
Currently, if you want to use them, remeber to add <LangVersion>7.2</LangVersion> in .csproj file of your project to use C# 7.2 features

Can't do arithmetical operations on array values in c#

I'm writing a program for homework, but I have stumbled upon a very hard problem for me.
Now, I'm pretty new to C#, so please bear with me. This may be really easy and obvious.
On-topic:
C# doesn't allow me to perform arithmetical operations on multidimensional array values:
if(map[0,1] - map[0,0] == 10)
This statement doesn't return a value, but instead throws me an error:
Object reference not set to an instance of an object.

You need to first declare the array. Example:
var map = new int[2,2];
creates a two-dimensional array with four integer elements.

the error sounds like you didn't initiate the values of the array
also don't forget that you took [,] arrays
int[,] example = new int[,] { {11,5}, {1,10} };//initiate the array
if (example[0,0]-example[1,0] == 10)
{
}

Translating C pointer statements to their C# equivalent

I have some C code I am trying to translate into C# code and I'm running into pointers which I am not familiar with so I don't know the C# equivalent. Get I get some help?
Case 1: Given these three lines in C, how do I declare p in C#?
double snorm[169];
double *p = snorm;
*p = 1.0;
Case 2: I have no idea what the pointers are actualy doing so I don't know how to change this line to C#.
*(snorm+n) = *(snorm+n-1) * (double)(2*n-1) / (double)n;

First:
double[] snorm = new double[169];
snorm[0] = 1.0;
Than just use snorm instead of p.
Second:
snorm[n] = snorm[n-1] * (double)(2*n-1)/(double)n;
Basically *p means that you take the value at the address of memory, referenced by p. Incrementing and adding to the pointer are moving the pointer in memory, so p++, as well as (p+1) just refers to the next item in memory (how far it really moves in memory depends on the data type the pointer points to). And, *(p+n) is just a value of the n-th item in the array (if p points to an array)
Anyway, you should get yourself familiar with pointers.

That code is basically using pointers as an alternative to array access. So your first snippet is equivalent to:
double[] snorm = new double[169];
snorm[0] = 1.0;
The next bit is equivalent to:
snorm[n] = snorm[n-1] * (double)(2*n-1) / (double)n;
(I'd use more spaces, but obviously that's a matter of taste.)
The only tricky bit is going to be if something increments a pointer - at that point you'll need to remember that you've basically got an extra offset to add to any future array indexes.

Array of an unknown length in C#

I've just started learning C# and in the introduction to arrays they showed how to establish a variable as an array but is seems that one must specify the length of the array at assignment, so what if I don't know the length of the array?

Arrays must be assigned a length. To allow for any number of elements, use the List class.
For example:
List<int> myInts = new List<int>();
myInts.Add(5);
myInts.Add(10);
myInts.Add(11);
myInts.Count // = 3

Use List<> to build up an 'array' of unknown length.
Use List<>.ToArray() to return a real array, and not a List.
var list = new List<int>();
list.Add(1);
list.Add(2);
list.Add(3);
var array = list.ToArray();

A little background information:
As said, if you want to have a dynamic collection of things, use a List<T>. Internally, a List uses an array for storage too. That array has a fixed size just like any other array. Once an array is declared as having a size, it doesn't change. When you add an item to a List, it's added to the array. Initially, the List starts out with an array that I believe has a length of 16. When you try to add the 17th item to the List, what happens is that a new array is allocated, that's (I think) twice the size of the old one, so 32 items. Then the content of the old array is copied into the new array. So while a List may appear dynamic to the outside observer, internally it has to comply to the rules as well.
And as you might have guessed, the copying and allocation of the arrays isn't free so one should aim to have as few of those as possible and to do that you can specify (in the constructor of List) an initial size of the array, which in a perfect scenario is just big enough to hold everything you want. However, this is micro-optimization and it's unlikely it will ever matter to you, but it's always nice to know what you're actually doing.

You can create an array with the size set to a variable, i.e.
int size = 50;
string[] words = new string[size]; // contains 50 strings
However, that size can't change later on, if you decide you need 100 words. If you need the size to be really dynamic, you'll need to use a different sort of data structure. Try List.

Use an ArrayList if in .NET 1.x, or a List<yourtype> if in .NET 2.0 or 3.x.
Search for them in System.Collections and System.Collections.Generics.

You might also want to look into Dictionarys if your data is unique, This will give you two columns to work with.
User name , Total bill
it gives you a lot of built in tools to search and update just the value.

var yummy = new List<string>();
while(person.FeelsHappy()) {
yummy.Add(person.GetNewFavoriteFood());
}
Console.WriteLine("Sweet! I have a list of size {0}.", list.Count);
Console.WriteLine("I didn't even need to know how big to make it " +
"until I finished making it!");

try a generic list instead of array

In a nutshell, please use Collections and Generics.
It's a must for any C# developer, it's worth spending time to learn :)

As detailed above, the generic List<> is the best way of doing it.
If you're stuck in .NET 1.*, then you will have to use the ArrayList class instead. This does not have compile-time type checking and you also have to add casting - messy.
Successive versions have also implemented various variations - including thread safe variants.

If you really need to use an array instead of a list, then you can create an array whose size is calculated at run time like so...
e.g i want a two dimensional array of size n by n. n will be gotten at run time from the user
int n = 0;
bool isInteger = int.TryParse(Console.ReadLine(), out n);
var x = new int[n,n];