Getting User Input and Adding It to an Array At C#

How to Fill an array from user input C#?

string []answer = new string[10];
for(int i = 0;i<answer.length;i++)
{
    answer[i]= Console.ReadLine();
}

Getting User Input and Adding it to an array at C#

I would use a while loop combined with int.TryParse to check if the user input is a number. Also it doesn't make any sense to put average = marks.Average(); inside a for loop, because LINQ Average calculates the average of a collection (in your case the marks array).

static void Main()
{
    int[] marks = new int[5];

    int a = 0;

    Console.WriteLine("Enter 5 elements:");

    while (a < 5)
    {
        if (int.TryParse(Console.ReadLine(), out marks[a]))
            a++;
        else
            Console.WriteLine("You didn't enter a number! Please enter again!");
    }

    double average = marks.Average();

    if (average > 0)
        Console.WriteLine("Positive");
    else
        Console.WriteLine("Negative");
}

DEMO HERE

How to Make User Input into Array and Pass It to a Method in C#?

you can ask user for input of comma delimited numbers. Or, you can repeat input until user enters something like 0. I that case, you can change your program into this:

using System;
using System.Collections.Generic;
using System.Linq;

namespace Averages
{
    class Program
    {
        static void Main(string[] args)
        {
            //list to hold your numbers, way more flexible than array
            List<double> enteredNubers = new List<double>();

            //message user
            Console.WriteLine("Enter number(s) or 0 to end: ");

            //run this block of code indefinitely
            while (true)
            {
                //take user input
                string userinput = Console.ReadLine().Trim();
                //if user enters 0, exit this loop
                if (userinput == "0")
                    break;


                double num;
                //try to convert text to number
                if (double.TryParse(userinput, out num))
                {
                    //if it is successful, add number to list
                    enteredNubers.Add(num);
                }
                else //else message user with error
                    Console.WriteLine("Wrong input. Please enter number or 0 to end");
            }

            //when loop is exited (when user entered 0), call method that calculates average
            Average(enteredNubers);
            Console.ReadKey();
        }
        static void Average(List<double> numbers)
        {
            double sum = 0;
            //go through list and add each number to sum
            foreach (double num in numbers)
            {
                sum += num;
            }
            //or, you can sum it using linq like this:
            //sum = numbers.Sum();
            //or you can even calculate average by calling Average method on list, like numbers. Average();

            //show message  - all the entered numbers, separated by comma
            Console.WriteLine("You have entered: " + string.Join(", ", numbers.ToArray()));
            //write average
            Console.WriteLine("The average is: " + sum/numbers.Count);
        }
    }
}

how to add user input to 2d array in C#

If you want to get a vector from user, you can try asking user to provide its components separated by some delimiter(s), e.g.

  private static int[] ReadVector(string title) {
    while (true) { // keep asking user until valid input is provided
      if (!string.IsNullOrEmpty(title))
        Console.WriteLine(title);

      string[] items = Console
        .ReadLine()
        .Split(new char[] { ' ', '\t', ';', ',' }, 
               StringSplitOptions.RemoveEmptyEntries);

      if (items.Length <= 0) {
        Console.WriteLine("You should provide at least one component");

        continue; 
      } 

      bool failed = false; 
      int[] result = new int[items.Length];

      for (int i = 0; i < items.Length; ++i) {
        if (int.TryParse(items[i], out int value))
          result[i] = value;
        else {
          Console.WriteLine($"Syntax error in {i + 1} term");
          failed = true;

          break;
        }  
      }

      if (!failed)
        return result; 
    }
  }

Then you can use this routine like this:

  // We don't want any 2D matrices here, just 2 vectors to sum
  int[] A = ReadVector("Please, enter vector A");
  int[] B = ReadVector("Please, enter vector B");  

  if (A.Length != B.Length) 
    Console.WriteLine("Vectors A and B have diffrent size");
  else {
    // A pinch of Linq to compute C = A + B 
    int[] C = A.Zip(B, (a, b) => a + b).ToArray();

    Console.WriteLine($"[{string.Join(", ", A)}] + ");
    Console.WriteLine($"[{string.Join(", ", B)}] = ");
    Console.WriteLine($"[{string.Join(", ", C)}]");  
  }  

Edit: Sure you can sum vectors with a help of good old for loop instead of Linq:

   int[] C = new int[A.Length];

   for (int i = 0; i < A.Length; ++i)
     C[i] = A[i] + B[i]; 

Array which stores information from user input

Apart from a few minor issues, you were nearly there

var friends = new string[2];

for (var i = 0; i < friends.Length; i++)
{
   Console.Write($"Enter friend ({i + 1}) : ");
   friends[i] = Console.ReadLine();
}

Console.WriteLine();
Console.WriteLine("These are your friends");

foreach (var friend in friends)
   Console.WriteLine(friend);

Console.WriteLine("Game Over!");

Output

Enter friend (1) : bob
Enter friend (2) : john

These are your friends 
bob
john
Game Over!

It's worth noting, List<T> are good a match for these types of situations, you don't have to worry about indexes, and can just expand and add to the list with List.Add(something)

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