How to convert Int To String in C# without using library functions?

Question

What is the most effective code to convert int to string without using a native convert function.

 public static void Main(string[] args)
    {
        string y = Convert(990);
        Console.WriteLine(y);
        Console.ReadLine(); 
    }


    public static string Convert(int x)
    {

         char[] Str = new char[20];
         int i = 0;
         while (x != 0)
         {
             Str[i++] = x % 10 + '0';
             x = x / 10;
         }

         return Str.ToString();// How do I handle this without the native function?
    }

The above doesnt seem to work. Please advise.

This is a school assignment, and therefore not for us to do. — It'sNotALie.
– It'sNotALie., Commented Jul 4, 2013 at 21:21
While an obvious classroom or interview question, at least you posted code. Plus it gave me an excuse to write 'clever' code. Fun occasionally. — Mark Peters
– Mark Peters, Commented Jul 4, 2013 at 22:11

Mark Peters · Accepted Answer · 2013-07-04 22:04:53Z

1

Here's a solution without a String constructor or a .ToString() call. This one handles negative numbers. It's a bit too 'clever' for my taste, but it's an academic exercise anyway...

void Main()
{
    Console.WriteLine(Convert(-5432));
}

String Convert(int i)
{
    return String.Join("",Digits(i).Reverse());
}

IEnumerable<char> Digits(int i)
{
    bool neg = false;
    if(i==0) {yield return '0'; yield break;}
    if(i<0) { i = -i; neg = true;}
    while (i!=0)
    {
        char digit = (char)(i % 10 + '0');
        i = i / 10;
        yield return digit;
    }
    if(neg) yield return '-';

    yield break;
}

edited Jul 4, 2013 at 22:04

answered Jul 4, 2013 at 21:54

Mark Peters

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Comments

Douglas · Accepted Answer · 2013-07-04 21:42:54Z

1

It might be easier to construct your result using a StringBuilder rather than a char array:

public static string Convert(int x)
{
     StringBuilder sb = new StringBuilder();

     while (x != 0)
     {
         sb.Insert(0, (char)(x % 10 + '0'));
         x = x / 10;
     }

     return sb.ToString();
}

The Insert(0, x) allows you to insert the more significant digits at the front.

edited Jul 4, 2013 at 21:42

answered Jul 4, 2013 at 21:36

Douglas

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2 Comments

David Heffernan Over a year ago

What do you think x % 10 + '0' is?

Douglas Over a year ago

@DavidHeffernan: You're right; I was assuming that that part of the OP's code was correct. Now fixed.

David Heffernan · Accepted Answer · 2013-07-05 05:13:47Z

You are adding the least significant digits to the start of the string. You'll need to reverse the string, or add the digits in reverse order, or insert them rather than append them.

And you'll need to do some casting. For example, here's one way to do it:

Str[i++] = (char)(x % 10 + '0');

Finally, you cannot use ToString() like that. You want this:

return new string(Str, 0, i);

Although StringBuilder might be more suitable.

And note that your code won't work properly for negative input values, or for zero.

So, here's a version that handles all of that:

public static string Convert(int x)
{
    if (x == 0)
    {
        return "0";
    }
    StringBuilder sb = new StringBuilder();
    string prefix = "";
    if (x < 0)
    {
        prefix = "-";
        x = -x;
    }
    while (x != 0)
    {
        sb.Insert(0, (char)(x % 10 + '0'));
        x = x / 10;
    }
    return prefix + sb.ToString();
}

This is pretty ugly though!

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