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Can somebody tell me why this code is not working? I am trying to find the 10001st prime number

The code gives me the answer 43739 which is wrong. I don't know which part of the code I messed up, help would be much appreciated.
{
int primenumbers = 4;
int thenumber = 2;
int finalnumber = 0;
while (primenumbers <= 10001)
{
for (int x = 2; x < 10; x++)
{
if (thenumber % x == 0)
{
x = 10;
}
if (x == 9)
{
finalnumber = thenumber;
primenumbers += 1;
break;
}
}
thenumber += 1;
}
Console.WriteLine(finalnumber);
}

Let's split the initial problem into smaller ones: first, check for being prime:
private static bool IsPrime(int value) {
if (value <= 1)
return false;
else if (value % 2 == 0)
return value == 2;
int n = (int) (Math.Sqrt(value) + 0.5);
for (int d = 3; d <= n; d += 2)
if (value % d == 0)
return false;
return true;
}
Then enumerate prime numbers (2, 3, 5, 7, 11...):
// Naive; sieve algorithm will be better off
private static IEnumerable<int> Primes() {
for (int i = 1; ; ++i)
if (IsPrime(i))
yield return i;
}
Finally, query with a help of Linq:
using System.Linq;
...
int result = Primes().Skip(10000).First();
Console.Write(result);
And you'll get
104743

Why Is This Code Running Longer Character Combinations Than Necessary?

I'm a math student with little to no experience programming, but I wrote this to act like a brute force algorithm. It seems to run fine except that it runs all the password combinations out to 3 characters for passwords as short as 2. Also I'm sure there's a way to refactor the for and if statements as well. Any help would be appreciated, thanks.
I've already been testing to see if some of the if statements aren't executing, and it looks like the statements with "console.writeln(Is this executing)" aren't executing but I'm not really sure.
public Form1()
{
InitializeComponent();
}
static char[] Match ={'0','1','2','3','4','5','6','7','8','9','a','b','c','d','e','f','g','h','i','j' ,'k','l','m','n','o','p',
'q','r','s','t','u','v','w','x','y','z','A','B','C','D','E','F','G','H','I','J','C','L','M','N','O','P',
'Q','R','S','T','U','V','X','Y','Z','!','?',' ','*','-','+'};
private string[] tempPass;
private void button1_Click(object sender, EventArgs e)
{
string tempPass1 = "lm";
string result = String.Empty;
int passLength = 1;
int maxLength = 17;
tempPass = new string[passLength];
for (int i = 0; i < Match.Length; i++)
{
if (tempPass1 != result)
{
tempPass[0] = Match[i].ToString();
result = String.Concat(tempPass);
if (passLength > 1)
{
for (int j = 0; j < Match.Length; j++)
{
if (tempPass1 != result)
{
tempPass[1] = Match[j].ToString();
result = String.Concat(tempPass);
if (passLength > 2)
{
for (int k = 0; k < Match.Length; k++)
{
if (tempPass1 != result)
{
tempPass[2] = Match[k].ToString();
result = String.Concat(tempPass);
if (tempPass[0] == "+" && tempPass[1] == "+" && tempPass[2] == "+" && tempPass1 != result)
{
Console.WriteLine("This will execute?");
passLength++;
tempPass = new string[passLength];
k = 0;
j = 0;
i = 0;
}
else if (result == tempPass1)
{
Console.WriteLine("Broken");
Console.WriteLine("This is big gay: " + result);
break;
}
}
}
}
if (tempPass[0] == "+" && tempPass[1] == "+" && tempPass1 != result)
{
Console.WriteLine("Did this execute?");
passLength++;
tempPass = new string[passLength];
j = 0;
i = 0;
}
else if (result == tempPass1)
{
Console.WriteLine("Broken");
Console.WriteLine("This is bigger gay: " + result);
break;
}
}
}
}
//tempPass[1] = "World!";
//Console.WriteLine(result);
if (tempPass[tempPass.Length - 1] == "+" && tempPass1 != result)
{
passLength++;
tempPass = new string[passLength];
Console.WriteLine(tempPass.Length + " " + result + " " + "Success");
Console.WriteLine(i);
i = 0; /**update
j = 0;
k = 0;
l = 0;
m = 0;*/
}
else if (result == tempPass1)
{
Console.WriteLine("Broken");
Console.WriteLine("This is biggest gay: " + result);
}
}
}
}

Play with this; modified from my answer here. It'll show you all the 2 and 3 length combinations. Clicking the button will start/stop the generation process. You need a button, label, and a timer:
public partial class Form1 : Form
{
private Revision rev;
public Form1()
{
InitializeComponent();
rev = new Revision("0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ!? *-+", "00");
label1.Text = rev.CurrentRevision;
}
private void button1_Click(object sender, EventArgs e)
{
timer1.Enabled = !timer1.Enabled;
}
private void timer1_Tick(object sender, EventArgs e)
{
rev.NextRevision();
if (rev.CurrentRevision.Length == 4)
{
timer1.Stop();
MessageBox.Show("Sequence Complete");
// make it start back at the beginning?
rev = new Revision("0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ!? *-+", "00");
label1.Text = rev.CurrentRevision;
}
else
{
label1.Text = rev.CurrentRevision;
}
}
}
public class Revision
{
private string chars;
private char[] values;
private System.Text.StringBuilder curRevision;
public Revision()
{
this.DefaultRevision();
}
public Revision(string validChars)
{
if (validChars.Length > 0)
{
chars = validChars;
values = validChars.ToCharArray();
curRevision = new System.Text.StringBuilder(values[0]);
}
else
{
this.DefaultRevision();
}
}
public Revision(string validChars, string startingRevision)
: this(validChars)
{
curRevision = new System.Text.StringBuilder(startingRevision.ToUpper());
int i = 0;
for (i = 0; i <= curRevision.Length - 1; i++)
{
if (Array.IndexOf(values, curRevision[i]) == -1)
{
curRevision = new System.Text.StringBuilder(values[0]);
break;
}
}
}
private void DefaultRevision()
{
chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
values = chars.ToCharArray();
curRevision = new System.Text.StringBuilder(values[0]);
}
public string ValidChars
{
get { return chars; }
}
public string CurrentRevision
{
get { return curRevision.ToString(); }
}
public string NextRevision(int numRevisions = 1)
{
bool forward = (numRevisions > 0);
numRevisions = Math.Abs(numRevisions);
int i = 0;
for (i = 1; i <= numRevisions; i++)
{
if (forward)
{
this.Increment();
}
else
{
this.Decrement();
}
}
return this.CurrentRevision;
}
private void Increment()
{
char curChar = curRevision[curRevision.Length - 1];
int index = Array.IndexOf(values, curChar);
if (index < (chars.Length - 1))
{
index = index + 1;
curRevision[curRevision.Length - 1] = values[index];
}
else
{
curRevision[curRevision.Length - 1] = values[0];
int i = 0;
int startPosition = curRevision.Length - 2;
for (i = startPosition; i >= 0; i += -1)
{
curChar = curRevision[i];
index = Array.IndexOf(values, curChar);
if (index < (values.Length - 1))
{
index = index + 1;
curRevision[i] = values[index];
return;
}
else
{
curRevision[i] = values[0];
}
}
curRevision.Insert(0, values[0]);
}
}
private void Decrement()
{
char curChar = curRevision[curRevision.Length - 1];
int index = Array.IndexOf(values, curChar);
if (index > 0)
{
index = index - 1;
curRevision[curRevision.Length - 1] = values[index];
}
else
{
curRevision[curRevision.Length - 1] = values[values.Length - 1];
int i = 0;
int startPosition = curRevision.Length - 2;
for (i = startPosition; i >= 0; i += -1)
{
curChar = curRevision[i];
index = Array.IndexOf(values, curChar);
if (index > 0)
{
index = index - 1;
curRevision[i] = values[index];
return;
}
else
{
curRevision[i] = values[values.Length - 1];
}
}
curRevision.Remove(0, 1);
if (curRevision.Length == 0)
{
curRevision.Insert(0, values[0]);
}
}
}
}

How to increment mixed Alphanumeric in C#?

I have requirements in a project to generate sequential rows and columns which are alphanumeric values.
The end user will pass the start value of row and column he would like to start from, and how many rows and columns he wants to generate.
For letters the max value is Z
For numbers the max values is 9
If the end user passed these parameters:
StartRow = 0A
StartColumn = A9Z
rowsCount = 2
columnsCount = 5
I would like to get this result:

You might want to reconsider your approach. Rather than maintaining an alphanumeric value and trying to increment it, maintain the value as a class containing Row and Column values, and then use ToString to convert it to the alphanumeric representation. Like this:
class RowCol
{
private int _row;
private int _col;
public int Row
{
get { return _row; }
set
{
// Row is of the form <digit><letter
// giving you 260 possible values.
if (value < 0 || value > 259)
throw new ArgumentOutOfRangeException();
_row = value;
}
}
public int Col
{
get { return _col; }
set
{
// Col is <letter><digit><letter>,
// giving you 6,760 possible values
if (value < 0 || value > 6759)
throw new ArgumentOutOfRangeException();
_col = value;
}
}
public string RowString
{
get
{
// convert Row value to string
int r, c;
r = Math.DivMod(_row, 26, out c);
r += '0';
c += 'A';
return string.Concat((char)r, (char)c);
}
set
{
// convert string to number.
// String is of the form <letter><digit>
if (string.IsNullOrEmpty(value) || value.Length != 2
|| !Char.IsDigit(value[0] || !Char.IsUpper(value[1]))
throw new ArgumentException();
_row = 26*(value[0]-'0') + (value[1]-'A');
}
}
public string ColString
{
get
{
int left, middle, right remainder;
left = Math.DivRem(_col, 260, out remainder);
middle = Math.DivRem(remainder, 26, out right);
left += 'A';
middle += '0';
right += 'A';
return string.Concat((char)left, (char)middle, (char)right);
}
set
{
// Do standard checking here to make sure it's in the right form.
if (string.IsNullOrEmpty(value) || value.Length != 3
|| !Char.IsUpper(value[0] || !Char.IsDigit(value[1]) || !Char.IsUpper(value[2]))
throw new ArgumentException();
_col = 260*(value[0] - 'A');
_col += 26*(value[1] - '0');
_col += value[2] - 'A';
}
}
public override string ToString()
{
return RowString + '-' + ColString;
}
public RowCol(int row, int col)
{
Row = _row;
Col = _col;
}
public RowCol(string row, string col)
{
RowString = row;
RowString = col;
}
}
(Code not yet tested, but that's the general idea.)
That's a bit more code than you have, it hides the complexity in the RowCol class rather than forcing you to deal with it in your main program logic. The point here is that you just want to increment the row or column; you don't want to have to think about how that's done. It makes your main program logic easier to understand. For example:
string startRow = "0A";
string startCol = "B0A";
RowCol rc = new RowCol("0A", "B0A");
for (int r = 0; r < rowsCount; r++)
{
rc.ColString = "B0A";
for (int c = 0; c < columnsCount; c++)
{
Console.WriteLine(rc);
rc.Row = rc.Row + 1;
}
rc.Col = rc.Col + 1;
}
By casting this as a simple conversion problem and encapsulating it in a class, I've made the code more robust and flexible, and easier to test, understand, and use.

I have come up with very simple solution to implement that and I would like to share this Console application :
class Program
{
static void Main(string[] args)
{
var row = "0A";
var column = "A9Z";
var rowsCount = 2;
var columnsCount = 5;
var rowCharArray =row.ToArray().Reverse().ToArray();
var columnCharArray = column.ToArray().Reverse().ToArray();
for (int i = 0; i < rowsCount; i++)
{
for (int j = 0; j < columnsCount; j++)
{
columnCharArray = incrementChar(columnCharArray);
var currentColumn = string.Join("", columnCharArray.Reverse().ToArray());
var currentRow= string.Join("", rowCharArray.Reverse().ToArray());
Console.WriteLine(currentRow + "-" + currentColumn);
}
columnCharArray = column.ToArray().Reverse().ToArray();
rowCharArray= incrementChar(rowCharArray);
Console.WriteLine("-------------------------------");
}
Console.ReadLine();
}
static char[] incrementChar(char[] charArray,int currentIndex=0)
{
char temp = charArray[currentIndex];
if (charArray.Length -1 == currentIndex && (temp == '9' || temp == 'Z'))
throw new Exception();
temp++;
if(Regex.IsMatch(temp.ToString(),"[A-Z]"))
{
charArray[currentIndex] = temp;
}
else
{
if (Regex.IsMatch(temp.ToString(), "[0-9]"))
{
charArray[currentIndex] = temp;
}
else
{
currentIndex++;
incrementChar(charArray, currentIndex);
}
}
if (currentIndex != 0)
charArray = resetChar(charArray, currentIndex);
return charArray;
}
static char[] resetChar(char[] charArray,int currentIndex)
{
for (int i = 0; i < currentIndex; i++)
{
if (charArray[i] == 'Z')
charArray[i] = 'A';
else if (charArray[i] == '9')
charArray[i] = '0';
}
return charArray;
}
}

How to show in a label first 100 even numbers using While and Do While ? c#

Actually i have a button to display the first 100 even numbers using For
int a = 100;
int res = 0;
int i;
string npares = "";
for (i = 0; i <= a; i++)
{
if (i % 2 == 0)
{
res = res + i;
if (i < a)
npares += i + ",";
else
npares += i;
}
}
LBLstatus.MaximumSize = new Size(200, 0);
LBLstatus.Text = npares;
But i need to make the same with more two buttons using While and Do While , how i can make this ?
EDIT >>>>>>
Using while i got this way :
int a = 100;
int i = 0;
string npares = "";
int res = 0;
while (i <= a)
{
i++;
if
(i % 2 == 0)
{
res = res + i;
if (i < a)
npares += i + ",";
else
npares += i;
}
LBLstatus.Text = npares;

(This answer shows the relationship between constructs, but does not otherwise attempt to provide a solution.)
The construct
for (init;cond;post)
{
body;
}
can be generally rewritten as / considered equivalent to
init;
while (cond) {
body;
post;
}
On the other hand a do-while has no analogous simple for form, because it delays evaluation of cond until after the body has been executed once, but it can be written as
for (init;;post) {
body;
if (!cond) break;
}

Using Take
List<int> ints;
List<int> positiveInts = ints.Where(i => i % 2 == 0).Take(100).ToList();
Using Aggregate
List<int> ints;
string positiveInts = ints.Where(i => i % 2 == 0).Take(100).Select(i => i.ToString()).Aggregate((a,b) => b += String.IsNullOrEmpty(b) ? a : "," + a);

How to add or subtract very large numbers without bigint in C#?

So let me start by saying that I'm a newbie with little to moderate knowledge about C#.
Coming to the topic: I need to make a program that is able to add/subtract very large integers. Initially, used BigInt only to find out it's not allowed. There should be a logical workaround for this? I have an idea which is using "elementary school method" where you add each digit starting from right to left.
I made a string which I split into char array and added each digit from right to left(GetUpperBound-i). But it doesn't seem to work.
My Code:
string s, s2;
char[] c_arr, c_arr2;
int i, erg;
s = "1234";
s2 = "5678";
c_arr = s.ToCharArray();
c_arr2 = s2.ToCharArray();
for (i = 0; i <= c_arr.GetUpperBound(0); i++)
{
erg = c_arr[c_arr.GetUpperBound(0)-i]+c_arr2[c_arr2.GetUpperBound(0)-i];
Console.Write(erg);
}
Console.ReadKey();

There are a few things wrong with your code for the 'elementary school method'. You don't account for carry, you're adding up ascii values rather than actual values between 0-9, and you're outputting the results in the wrong order.
The code below, whilst not very elegant, does produce the correct results:
var s1 = "12345";
var s2 = "5678";
var carry = false;
var result = String.Empty;
if(s1.Length != s2.Length)
{
var diff = Math.Abs(s1.Length - s2.Length);
if(s1.Length < s2.Length)
{
s1 = String.Join("", Enumerable.Repeat("0", diff)) + s1;
}
else
{
s2 = String.Join("", Enumerable.Repeat("0", diff)) + s2;
}
}
for(int i = s1.Length-1;i >= 0; i--)
{
var augend = Convert.ToInt32(s1.Substring(i,1));
var addend = Convert.ToInt32(s2.Substring(i,1));
var sum = augend + addend;
sum += (carry ? 1 : 0);
carry = false;
if(sum > 9)
{
carry = true;
sum -= 10;
}
result = sum.ToString() + result;
}
if(carry)
{
result = "1" + result;
}
Console.WriteLine(result);

The following program can be used to add two large numbers, I have used string builder to store the result. You can add numbers containing digits upto '2,147,483,647'.
Using System;
using System.Text;
using System.Linq;
public class Test
{
public static void Main()
{
string term1="15245142151235123512352362362352351236";
string term2="1522135123612646436143613461344";
StringBuilder sum=new StringBuilder();
int n1=term1.Length;
int n2=term2.Length;
int carry=0;
int n=(n1>n2)?n1:n2;
if(n1>n2)
term2=term2.PadLeft(n1,'0');
else
term1=term1.PadLeft(n2,'0');
for(int i=n-1;i>=0;i--)
{
int value=(carry+term1[i]-48+term2[i]-48)%10;
sum.Append(value);
carry=(carry+term1[i]-48+term2[i]-48)/10;
}
char[] c=sum.ToString().ToCharArray();
Array.Reverse(c);
Console.WriteLine(c);
}
}

string Add(string s1, string s2)
{
bool carry = false;
string result = string.Empty;
if(s1[0] != '-' && s2[0] != '-')
{
if (s1.Length < s2.Length)
s1 = s1.PadLeft(s2.Length, '0');
if(s2.Length < s1.Length)
s2 = s2.PadLeft(s1.Length, '0');
for(int i = s1.Length-1; i >= 0; i--)
{
var augend = Convert.ToInt64(s1.Substring(i,1));
var addend = Convert.ToInt64(s2.Substring(i,1));
var sum = augend + addend;
sum += (carry ? 1 : 0);
carry = false;
if(sum > 9)
{
carry = true;
sum -= 10;
}
result = sum.ToString() + result;
}
if(carry)
{
result = "1" + result;
}
}
else if(s1[0] == '-' || s2[0] == '-')
{
long sum = 0;
if(s2[0] == '-')
{
//Removing negative sign
char[] MyChar = {'-'};
string NewString = s2.TrimStart(MyChar);
s2 = NewString;
if(s2.Length < s1.Length)
s2 = s2.PadLeft(s1.Length, '0');
for (int i = s1.Length - 1; i >= 0; i--)
{
var augend = Convert.ToInt64(s1.Substring(i,1));
var addend = Convert.ToInt64(s2.Substring(i,1));
if(augend >= addend)
{
sum = augend - addend;
}
else
{
int temp = i - 1;
long numberNext = Convert.ToInt64(s1.Substring(temp,1));
//if number before is 0
while(numberNext == 0)
{
temp--;
numberNext = Convert.ToInt64(s1.Substring(temp,1));
}
//taking one from the neighbor number
int a = int.Parse(s1[temp].ToString());
a--;
StringBuilder tempString = new StringBuilder(s1);
string aString = a.ToString();
tempString[temp] = Convert.ToChar(aString);
s1 = tempString.ToString();
while(temp < i)
{
temp++;
StringBuilder copyS1 = new StringBuilder(s1);
string nine = "9";
tempString[temp] = Convert.ToChar(nine);
s1 = tempString.ToString();
}
augend += 10;
sum = augend - addend;
}
result = sum.ToString() + result;
}
//Removing the zero infront of the answer
char[] zeroChar = {'0'};
string tempResult = result.TrimStart(zeroChar);
result = tempResult;
}
}
return result;
}
string Multiply(string s1, string s2)
{
string result = string.Empty;
//For multipication
bool Negative = false;
if(s1[0] == '-' && s2[0] == '-')
Negative = false;
else if(s1[0] == '-' || s2[0] == '-')
Negative = true;
char[] minusChar = {'-'};
string NewString;
NewString = s2.TrimStart(minusChar);
s2 = NewString;
NewString = s1.TrimStart(minusChar);
s1 = NewString;
List<string> resultList = new List<string>();
for(int i = s2.Length - 1; i >= 0; i--)
{
string multiplycation = string.Empty;
for (int j = s1.Length - 1; j >= 0; j--)
{
var augend = Convert.ToInt64(s1.Substring(j,1));
var addend = Convert.ToInt64(s2.Substring(i,1));
long multiply = augend * addend;
// print(multiply);
multiplycation = multiply.ToString() + multiplycation;
}
//Adding zero at the end of the multiplication
for (int k = s2.Length - 1 - i; k > 0; k--)
{
multiplycation += "0";
}
resultList.Add(multiplycation);
}
for (int i = 1; i < resultList.Count; i++)
{
resultList[0] = Add(resultList[0],resultList[i]);
}
//Finally assigning if negative negative sign in front of the number
if(Negative)
result = resultList[0].Insert(0,"-");
else
result = resultList[0];
return result;
}
string Divide(string dividend, string divisor)
{
string result = string.Empty;
int remainder = 0;
int intNumberstoGet = divisor.Length;
int currentInt = 0;
int dividing = int.Parse(dividend.Substring(currentInt,intNumberstoGet));
int intDivisor = int.Parse(divisor);
while(currentInt < dividend.Length)
{
if(dividing == 0)
{
currentInt++;
result += "0";
}
else
{
while(dividing < intDivisor)
{
intNumberstoGet++;
dividing = int.Parse(dividend.Substring(currentInt,intNumberstoGet));
}
if (dividing > 0)
{
remainder = dividing % intDivisor;
result += ((dividing - remainder) / intDivisor).ToString();
intNumberstoGet = 1;
if(currentInt < dividend.Length - 2)
currentInt += 2;
else
currentInt++;
if(currentInt != dividend.Length)
{
dividing = int.Parse(dividend.Substring(currentInt,intNumberstoGet));
remainder *= 10;
dividing += remainder;
}
}
}
}
return result;
}

Here you go. Another example. It's 10 to 30 times faster than the accepted answer.
static string AddNumStr(string v1, string v2)
{
var v1Len = v1.Length;
var v2Len = v2.Length;
var count = Math.Max(v1Len, v2Len);
var answ = new char[count + 1];
while (count >= 0) answ[count--] = (char)((v1Len > 0 ? v1[--v1Len] & 0xF:0) + (v2Len>0 ? v2[--v2Len]&0xF : 0));
for (var i = answ.Length - 1; i >= 0; i--)
{
if (answ[i] > 9)
{
answ[i - 1]++;
answ[i] -= (char)10;
}
answ[i] = (char)(answ[i] | 48);
}
return new string(answ).TrimStart('0');
}

Below SO question has some interesting approaches. Though the answer is in Java, but you will surely get to know what needs to be done.
How to handle very large numbers in Java without using java.math.BigInteger

public static int[] addTwoNumbers(string s1, string s2)
{
char[] num1 = s1.ToCharArray();
char[] num2 = s2.ToCharArray();
int sum = 0;
int carry = 0;
int size = (s1.Length > s2.Length) ? s1.Length + 1 : s2.Length + 1;
int[] result = new int[size];
int index = size - 1;
int num1index = num1.Length - 1;
int num2index = num2.Length - 1;
while (true)
{
if (num1index >= 0 && num2index >= 0)
{
sum = (num1[num1index]-'0') + (num2[num2index]-'0') + carry;
}
else if(num1index< 0 && num2index >= 0)
{
sum = (num2[num2index]-'0') + carry;
}
else if (num1index >= 0 && num2index < 0)
{
sum = (num1[num1index]-'0') + carry;
}
else { break; }
carry = sum /10;
result[index] = sum % 10;
index--;
num1index--;
num2index--;
}
if(carry>0)
{
result[index] = carry;
}
return result;
}