Select integers from string[], and return int[]

Question

I have an array string[] with values that are mostly convertible to integers.

var values = new [] {"", "1", "2", "a", "3"};

I need to convert values to an array of integers, discarding any items that aren't convertible. So I end up with

var numbers = new [] {1, 2, 3};

What would be the most efficient (quickest and clean code) way to do this?

Answer 1

You could try the following

public static IEnumerable<int> Convert(this IEnumerable<string> enumerable) {
  Func<string,int?> convertFunc = x => {
    int value ;
    bool ret = Int32.TryParse(x, out value);
    return ret ? (int?)value : null;
  };
  return enumerable
    .Select(convertFunc)
    .Where(x => x.HasValue)
    .Select(x => x.Value);
}

This can easily then be converted to an array.

var numbers = values.Convert().ToArray();

Answer 2

var numbers = values.Select(
    s => {
        int n;
        if (!int.TryParse((s ?? string.Empty), out n)) 
        {
            return (int?)null;
        }
        return (int?)n;
    }
)
.Where(n => n != null)
.Select(n => n.Value)
.ToArray();

Answer 3

With some straightforward LINQ:

var numbers = values.Where(x => { int num = 0; return Int32.TryParse(x, out num); })
                    .Select(num => Int32.Parse(num));

Notably this converts every string twice. Doing this imperatively you'll lose some clarity but gain some speed (as an IEnumerable extension):

public static IEnumerable<int> TryCastToInt<T>(this IEnumerable<T> values)
  int num = 0;
  foreach (object item in values) {
    if (Int32.TryParse(item.ToString(), num)) {
      yield return num;
    }
  }
}

Answer 4

EDIT: Updated to not use try/catch, since StackOverflow users have pointed out that it's slow.

Try this.

var values = new[] { "", "1", "2", "a", "3" };
List<int> numeric_list = new List();
int num_try = 0;
foreach (string string_value in values)
{
    if (Int32.TryParse(string_value, out num_try) {
        numeric_list.Add(num_try);
    }

    /* BAD PRACTICE (as noted by other StackOverflow users)
    try
    {
        numeric_list.Add(Convert.ToInt32(string_value));
    }
    catch (Exception)
    {
        // Do nothing, since we want to skip.
    }
    */
}

return numeric_list.ToArray();

Answer 5

int n;
var values = new[] { "", "1", "2", "a", "3" };
var intsonly = values.Where (v=> Int32.TryParse(v, out n)).Select(x => Int32.Parse(x));

Answer 6

This can be done in a single linq statement without having to do the parse twice:

var numbers = values
    .Select(c => { int i; return int.TryParse(c, out i) ? i : (int?)null; })
    .Where(c => c.HasValue)
    .Select(c => c.Value)
    .ToArray();

Answer 7

I personally use an extension method that's a little different from what others have posted so far. It specifically uses a custom WeakConverter<TSource, TResult> delegate to avoid the issue in Ron's answer of calling ToString() on every object, while maintaining genericity unlike Jared's answer (though I will concede that sometimes trying to make everything generic is overdoing it -- but in this case, the added effort is really not much for what I consider a significant benefit in terms of reusability).

public delegate bool WeakConverter<TSource, TResult>(TSource source, out TResult result);

public static IEnumerable<TResult> TryConvertAll<TSource, TResult>(this IEnumerable<TSource> source, WeakConverter<TSource, TResult> converter)
{
    foreach (TSource original in source)
    {
        TResult converted;
        if (converter(original, out converted))
        {
            yield return converted;
        }
    }
}

With this in place, you can convert a string[] to an int[] quite simply and robustly (no double-parsing necessary):

string[] strings = new[] { "1", "2", "abc", "3", "", "123" };

int[] ints = strings.TryConvertAll<string, int>(int.TryParse).ToArray();

foreach (int x in ints)
{
    Console.WriteLine(x);
}

Output:

1
2
3
123

Answer 8

Here is my take on this. It uses a separate method but cleanly expresses the conditional parse without using nullable values:

private static IEnumerable<int> ParseInt32s(IEnumerable<string> value)
{
    foreach(var value in values)
    {
        int n;

        if(Int32.TryParse(value, out n))
        {
            yield return n;
        }
    }
}

Usage:

string[] values;

var parsedValues = ParseInt32s(values).ToArray();

Answer 9

var numbers = values
    .Where(x => !String.IsNullOrEmpty(x))
    .Where(x => x.All(Char.IsDigit))
    .Select(x => Convert.ToInt32(x))
    .ToArray();