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C# Array - identify the input if it is repeating or not

I want to create an algorithm that would identify if the user input is repeating or not. If it is repeating it will prompt the user a message if its not repeating it will continue the process.
public static class Program
{
public static void Main()
{
Console.WriteLine("input array of numbers: );
int[] array = new int[4];
for(int i=0; i<3; i++)
{
array[i] = int.Parse(Console.ReadLine());
if(array[i] == array[0])
{
Console.WriteLine("repeating inputs")
}
}
Console.WriteLine("Highest number is:" + array.MaxLenght);
Console.ReadLine();
}
}
Explanation: The user will be prompted by message "inter array of numbers:" then the user will now input the numbers. If the user inputs the same number or if the number was already inputted, the user will be prompted by a message something like "repeating inputs! Input another number". After the user input another number unique to the previously interred the program will continue and print out the largest number base on the inputs.

i'm not sure if I understood you correctly but this is what i can extrapolated from your post :
you want to get input from the user and check if it's repeating or not and then print the highest number (based on your Console.WriteLine("Highest number is:" + array.MaxLenght); )
this is how i'd approach it
Console.WriteLine("input array of numbers: ");
List<int> uniqueInts = new List<int>();
int[] array = new int[4];
for (int i = 0; i < 3; i++)
{
array[i] = int.Parse(Console.ReadLine());
if (!uniqueInts.Contains(array[i]))
uniqueInts.Add(array[i]);
}
//getting the max number
//assuming the first number is the max
int max = uniqueInts[0];
for (int i = 1; i < uniqueInts.Count; i++)
{
if (max < uniqueInts[i])
max = uniqueInts[i];
}
Console.WriteLine("The highest number is : " + max);

There are a lot of assumptions that I'm making with this answer. I'm assuming you're struggling to get the value of the item prior to the current iteration considering you have if(array[i] == array[0]).
If that's the case, then simply change array[0] to array[i-1].
Wait! Before you do that, you need to add a check to make sure you aren't on the first iteration. If you don't, you'll get an exception thrown on the first iteration, because you'll be trying to grab array[-1], which isn't valid.
for(int i=0; i<3; i++)
{
array[i] = int.Parse(Console.ReadLine());
if(i > 0)
{
if (array[i] == array[i-1])
Console.WriteLine("repeating inputs")
}
}
Make these few changes, and I think you'll get what you're after.

How find average of numbers(by input) and also count numbers

I mean how to count and sum input numbers until receive "end".
thanks !
And also how to find out input is number or letter in c#?
class Program
{
static void Main(string[] args)
{
int n = 0;
int sum = 0;
string inp;
do
{
Console.Write("Numbers ");
inp = Console.ReadLine();
int num= Convert.ToInt16(inp);
sum = sum + num;
n++;
} while (too == "end");
int average = sum / n;
Console.WriteLine(" " + average);
Console.ReadLine();
}
}

I would suggest you use a normal while loop and also add validation to check to integer input.
For the while loop you want to loop until the input is not equal to "end":
while(inp != "end")
For the validation, you can use int.TryParse method:
int num = 0;
if (int.TryParse(inp, out num)) { }
Here is a modified example of your code:
int n = 0;
int sum = 0;
string inp = null;
while(inp != "end")
{
Console.Write("Numbers ");
inp = Console.ReadLine();
int num = 0;
if (int.TryParse(inp, out num))
{
sum = sum + num;
n++;
}
}
int average = sum / n;
Console.WriteLine(" " + average);
Console.ReadLine();

// A list to hold all of the numbers entered
List<int> numbers = new List<int>();
// Will hold the inputted string
string input;
// This needs to be outside the loop so it's written once
Console.Write("Numbers: " + Environment.NewLine);
// Keep going until we say otherwise
while (true)
{
// Get the input
input = Console.ReadLine();
// Will hold the outcome of parsing the input
int number = -1;
// Check to see if input was a valid number
bool success = int.TryParse(input, out number);
// If it was a valid number then remember it
// If ANY invalid or textual input is detected then stop
if (success)
numbers.Add(number);
else
break;
}
// Write the count and average
Console.WriteLine("Count:" + numbers.Count);
Console.WriteLine("Average:" + numbers.Average());
Console.ReadLine();
Input:
Numbers:
1
2
3
4
5
Output:
Count: 5
Average: 3
The only thing here a little different to what you specified is ANY invalid or textual entry causes it to finish, not just typing the word "end", although that obviously works too.

Sorting array numbers in c#

I have an assingment and I'm a bit lost. In an array of 10 (or less) numbers which the user enters (I have this part done), I need to find the second smallest number. My friend sent me this code, but I'm having a hard time understanding it and writing it in c#:
Solved it!!! :
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
namespace ConsoleApplication1
{
class Program
{
static void Main(string[] args)
{
int vnesena;
int? min1 = null;
int? min2 = null;
for(int i=1; i<11; i=i+1)
{
Console.WriteLine("Vpiši " + i +"." + " število: ");
vnesena = Convert.ToInt32(Console.ReadLine());
if (vnesena == 0)
{
break;
}
if (min1 == null || vnesena < min1)
{
min2 = min1;
min1 = vnesena;
}
else if (vnesena != min1 && (min2==null || vnesena<min2))
{
min2 = vnesena;
}
}
if (min1 == null || min2 == null)
{
Console.WriteLine("Opozorilo o napaki");
}
else
{
Console.WriteLine("Izhod: " + min2);
}
Console.ReadKey();
}
}
}

That code is too complicated, so try something like this.
int[] numbers = new int[10];
for (int i = 0; i < 10; i++)
{
numbers[i] = int.Parse(Console.ReadLine());
}
Array.Sort(numbers);
Console.WriteLine("Second smallest number: " + numbers[1]);
If the code isn't too obvious, let me explain:
Declare an array of 10 integers
Loop 10 ten times and each time, ask for user input & place input as an integer to the array
Sort the array so each number is in the number order (smallest first, biggest last).
The first integer is smallest (input at index 0, so numbers[0]) and the second smallest is obviously numbers[1].
Of course, for this piece of code to work, you have to use this code in console program.
As you didn't mention if you are allowed to use built in sorting functions etc, I assume that Array.Sort() is valid.
EDIT: You updated your topic so I'll change my code to match criterias.
int[] numbers = new int[10];
bool tooShortInput = false;
for (int i = 0; i < 10; i++)
{
int input = int.Parse(Console.ReadLine());
if (input != 0)
{
numbers[i] = input;
}
else
{
if (i == 2)
{
Console.WriteLine("You only entered two numbers!");
tooShortInput = true;
break;
}
else
{
for (int j = 0; j < 10; j++)
{
if (numbers[j] == 0)
{
numbers[j] = 2147483647;
}
}
break;
}
}
}
// Sort the array
int temp = 0;
for (int write = 0; write < numbers.Length; write++) {
for (int sort = 0; sort < numbers.Length - 1; sort++) {
if (numbers[sort] > numbers[sort + 1]) {
temp = numbers[sort + 1];
numbers[sort + 1] = numbers[sort];
numbers[sort] = temp;
}
}
}
if (!tooShortInput)
{
Console.WriteLine("Second smallest number: " + numbers[1]);
}
If you don't understand the updated code, let me know, I will explain.
NOTE: This is fastly coded and tested with android phone so obviously this code isn't 5 star quality, not even close, but it qualifies :-).
Regards, TuukkaX.

To paraphrase the code given:
Set 2 variables to nothing. (This is so that there can be checks done later. int? could be used if you want to use null for one idea here.
Start loop through values.
Get next value.
If the minimum isn't set or the new value is lower than the minimum, replace the second lowest with the former lowest and lowest with the new value that was entered.
Otherwise, check if the new value isn't the same as the minimum and if the minimum isn't set or the entered value is lower than the second lowest then replace the second lowest with this new value.
Once the loop is done, if either minimum value isn't filled in then output there isn't such a value otherwise output the second lowest value.
Imagine if you had to do this manually. You'd likely keep track of the lowest value and second lowest value as you went through the array and the program is merely automating this process. What is the problem?
This is a rough translation of what your friend gave you that isn't that hard to translate to my mind.
int enteredValue;
int? smallest = null, secondSmallest = null;
for (int i = 0; i < 10; i = i + 1)
{
Console.WriteLine("Vpiši " + i+1 + " število: ");
enteredValue = Convert.ToInt32(Console.ReadLine());
if (smallest==null || enteredValue<smallest) {
secondSmallest=smallest;
smallest = enteredValue;
} else if (enteredValue!=smallest && enteredValue<secondSmallest) {
secondSmallest= enteredValue;
}
}

Why use a loop and not take advantage of the Array.Sort method?
int[] numbers = new int[4] { 4, 2, 6, 8 };
Array.Sort(numbers);
int secondSmallestNumber = numbers[1];

IndexOutOfRange exception when using int variable as array selector

I'm trying to create a very basic login system in C# using arrays for username and password comparing.
I'm using a for() loop to compare the username and password the user provides with the usernames and passwords that are in my arrays. Here's my loop code:
string user = null, usrpassword = null;
string[] usernames = {"admin", "guest"};
string[] userpasswords = {"adminpw", "guestpw"};
Console.Write("Username: "); //Username
user = Console.ReadLine();
Console.Write("Password: "); //Password
usrpassword = Console.ReadLine();
Console.WriteLine("Processing...");
for (int i = 0; i <= usernames.Length; i++)
{
if (user == usernames[i] && usrpassword == userpasswords[i])
{
loginloop = false;
Console.WriteLine("Login Successful.");
}
else if (i > usernames.Length)
{
//incorrect username
Console.WriteLine("Incorrect username or password!");
}
} //for-loop-end
I don't get any syntax errors when I build, but when it reaches the for loop it crashes and gives me a IndexOutOfRange exception.

Array indexes start at 0 and go up to Length - 1, so you only want to continue the loop while the iterator is less than Length.
Change the <= to a <:
for (int i = 0; i < usernames.Length; i++)
{
...
}

You just have an "off by one" style error in your for loops conditions;
for (int i = 0; i <= usernames.Length; i++)
should instead be
for (int i = 0; i < usernames.Length; i++)
The array is "zero indexed" where as the Length property is the length counting from one to n. The final index is actually the value one less than Length or 0 in the case of an empty array.

Ask user for starting and stopping point within the array?

In C# how do i ask user for starting and stopping point within the array?
Below is my code so far:
class Program
{
static void Main(string[] args)
{
double[] num = { 10, 20, 30, 40, 50 };
double n = num.Length;
Console.Write("Elements of, arrary are:" + Environment.NewLine);
for (int i = 0; i < n; i++)
{
Console.WriteLine(num[i]);
}
double sum = 0;
for (int i = 0; i < n; i++)
{
sum = sum + num[i];
}
Console.WriteLine("The sum of elements:" + sum);
Console.ReadKey();
}
}

You'll take the sum of the elements between starting and stopping point, as I guess. Take two inputs from the user and assign them to starting and ending points to the for-loop. Such as:
int startingPoint = Convert.ToInt32(Console.ReadLine());
int endingPoint = Convert.ToInt32(Console.ReadLine());
for(int i = startingPoint; i <= endingPoint; i++)
{
//take sum etc.
}
Don't forget to inform the user about the element values in the array and what input value they are entering at that moment.
Another important thing here is to control the inputs. They should be numeric and between 0-n, starting point should be smaller than ending point.
For numeric control you can write like follows:
if (int.TryParse(n, out startingPoint))
{
// operate here
}
else
{
Console.WriteLine("That's why I don't trust you, enter a numeric value please.");
}
startingPoint should be between 0-n and cannot be n. To control it:
if (startingPoint >= 0 && startingPoint < n)
{
// operate here
}
else
{
Console.WriteLine("Please enter a number between 0 and " + n + ".");
}
After taking startingPoint successfully, you should control if endingPoint. It should be between startingPoint-n. After controlling for being numeric you can write as follows:
if (endingPoint >= startingPoint && endingPoint < n)
{
// operate here
}
else
{
Console.WriteLine("Please enter a number between " + startingPoint + " and " + n + ".");
}
I don't know what can I explain more for this question. Please let me know for further problems.

If you want to prompt the user for the start and end indexes:
Console.WriteLine("Please enter start index");
string startIndexAsString = Console.ReadLine();
int startIndex = int.Parse(startIndexAsString);
Console.WriteLine("Please enter end index");
string endIndexAsString = Console.ReadLine();
int endIndex = int.Parse(endIndexAsString);
var sum = num.Skip(startIndex).Take(endIndex - startIndex + 1).Sum();