Parse an integer from a string with trailing garbage.

Question

I need to parse a decimal integer that appears at the start of a string.

There may be trailing garbage following the decimal number. This needs to be ignored (even if it contains other numbers.)

e.g.

"1" => 1
" 42 " => 42
" 3 -.X.-" => 3
" 2 3 4 5" => 2

Is there a built-in method in the .NET framework to do this?

int.TryParse() is not suitable. It allows trailing spaces but not other trailing characters.

It would be quite easy to implement this but I would prefer to use the standard method if it exists.

Answer 1

string s = " 3 -.X.-".Trim();
string collectedNumber = string.empty;
int i;

for (x = 0; x < s.length; x++) 
{

  if (int.TryParse(s[x], out i))
     collectedNumber += s[x];
  else
     break;     // not a number - that's it - get out.

} 

if (int.TryParse(collectedNumber, out i))
    Console.WriteLine(i); 
else
    Console.WriteLine("no number found");

Answer 2

foreach (var m in Regex.Matches(" 3 - .x. 4", @"\d+"))
{
    Console.WriteLine(m);
}

Updated per comments

Not sure why you don't like regular expressions, so I'll just post what I think is the shortest solution.

To get first int:

Match match = Regex.Match(" 3 - .x. - 4", @"\d+");
if (match.Success)
    Console.WriteLine(int.Parse(match.Value));

Answer 3

There's no standard .NET method for doing this - although I wouldn't be surprised to find that VB had something in the Microsoft.VisualBasic assembly (which is shipped with .NET, so it's not an issue to use it even from C#).

Will the result always be non-negative (which would make things easier)?

To be honest, regular expressions are the easiest option here, but...

public static string RemoveCruftFromNumber(string text)
{
    int end = 0;

    // First move past leading spaces
    while (end < text.Length && text[end] == ' ')
    {
        end++;
    }

    // Now move past digits
    while (end < text.Length && char.IsDigit(text[end]))
    {
        end++;
    }

    return text.Substring(0, end);
}

Then you just need to call int.TryParse on the result of RemoveCruftFromNumber (don't forget that the integer may be too big to store in an int).

Answer 4

I'm not sure why you would avoid Regex in this situation.

Here's a little hackery that you can adjust to your needs.

" 3 -.X.-".ToCharArray().FindInteger().ToList().ForEach(Console.WriteLine);

public static class CharArrayExtensions
{
    public static IEnumerable<char> FindInteger(this IEnumerable<char> array)
    {
        foreach (var c in array)
        {
            if(char.IsNumber(c))
                yield return c;
        }
    }
}

EDIT: That's true about the incorrect result (and the maintenance dev :) ).

Here's a revision:

    public static int FindFirstInteger(this IEnumerable<char> array)
    {
        bool foundInteger = false;
        var ints = new List<char>();

        foreach (var c in array)
        {
            if(char.IsNumber(c))
            {
                foundInteger = true;
                ints.Add(c);
            }
            else
            {
                if(foundInteger)
                {
                    break;
                }
            }
        }

        string s = string.Empty;
        ints.ForEach(i => s += i.ToString());
        return int.Parse(s);
    }

Answer 5

    private string GetInt(string s)
    {
        int i = 0;

        s = s.Trim();
        while (i<s.Length && char.IsDigit(s[i])) i++;

        return s.Substring(0, i);
    }

Answer 6

Might as well add mine too.

        string temp = " 3 .x£";
        string numbersOnly = String.Empty;
        int tempInt;
        for (int i = 0; i < temp.Length; i++)
        {
            if (Int32.TryParse(Convert.ToString(temp[i]), out tempInt))
            {
                numbersOnly += temp[i];
            }
        }

        Int32.TryParse(numbersOnly, out tempInt);
        MessageBox.Show(tempInt.ToString());

The message box is just for testing purposes, just delete it once you verify the method is working.

Answer 7

You can use Linq to do this, no Regular Expressions needed:

public static int GetLeadingInt(string input)
{
   return Int32.Parse(new string(input.Trim().TakeWhile(c => char.IsDigit(c) || c == '.').ToArray()));
}

This works for all your provided examples:

string[] tests = new string[] {
   "1",
   " 42 ",
   " 3 -.X.-",
   " 2 3 4 5"
};

foreach (string test in tests)
{
   Console.WriteLine("Result: " + GetLeadingInt(test));
}

Answer 8

This is how I would have done it in Java:

int parseLeadingInt(String input)
{
    NumberFormat fmt = NumberFormat.getIntegerInstance();
    fmt.setGroupingUsed(false);
    return fmt.parse(input, new ParsePosition(0)).intValue();
}

I was hoping something similar would be possible in .NET.

This is the regex-based solution I am currently using:

int? parseLeadingInt(string input)
{
    int result = 0;
    Match match = Regex.Match(input, "^[ \t]*\\d+");
    if (match.Success && int.TryParse(match.Value, out result))
    {
        return result;
    }
    return null;
}