Easy way to read integers from a file

Question

I am trying to read in a file, which is essentially a list of integers separated by a line. Obviously, file input can never be trusted, so I need to filter out non-integer numbers.

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I know that the as operator usually converts if it can, and then sets it to null, however, since int not null, it is not. I thought that maybe I could pounce on the Nullable<int> . I never delved into this, I thought that maybe I could do:

 var lines = File.ReadAllLines(""); var numbers = lines.Select(line => line as int?).Where(i => i != null); 

I know that I could potentially get around this by doing:

 var numbers = lines.Select(line => { int iReturn = 0; if (int.TryParse(line, out iReturn )) return iReturn; else return null; }).Where(i => i != null); 

I could also do this as an extension method.

I was just looking for a neat, concise way to make the cast in a statement, and also to understand why my code is invalid.

Answer 1

I always use this simple extension method:

 public static int? TryGetInt(this string item) { int i; bool success = int.TryParse(item, out i); return success ? (int?)i : (int?)null; } 

Then it’s easy:

  var numbers = lines.Select(line => line.TryGetInt()) .Where(i => i.HasValue) .Select(i => i.Value); 

You can also use int.TryParse without an extension, but this is undocumented, therefore, it may stop working in the future:

 int i = 0; var numbers = lines.Where(line => int.TryParse(line, out i)) .Select(line => i); 

Edit

"also understands why my code is invalid"

corresponding code:

 if (int.TryParse(line, out iReturn )) return iReturn; else return null; 

It will work if you replace

 else return null; 

with

 else return (int?)null; 

because you return int , but null implicitly converted to int .

Answer 2

There is no short way to do this, because here you do not need to make a throw (you cannot throw) - you need to convert from one type to another. The types of courses are int and string (so they aren’t quite “any” types), but, as in the general case, any conversion between unrelated types cannot be done “exactly the same”.

Answer 3

Nope. C # is intentionally cautious about changing strings in numbers.

You can make the code shorter (no more than null) using the foreach loop

 var numbers = new List<int>(); foreach(string line in lines) { int n; if (int.TryParse(line, out n)) numbers.Add(n); } 

Answer 4

You can create and extend a method.

 public static int? ToInt(this string s, int default){ ... } 

and use it in LINQ:

 var lines = File.ReadAllLines(path); var numbers = lines.Select(line => line.ToInt()) .Where(i => i != null); 

Answer 5

If I understand you correctly, and you just want to filter non-integer strings, maybe regex is an option?

 var lines = File.ReadAllLines(""); var numbers = lines.Where(i => Regex.IsMatch(i, "[0-9]+")); 

Answer 6

Here is the best I came up with:

Use this extension method:

 public static class Int32Extensions { public static int? ParseOrDefault(this string text) { int iReturn = 0; if (int.TryParse(text, out iReturn)) { return iReturn; } return null; } } 

Like this:

 var query = lines.Select(x => x.ParseOrDefault()).Where(x => x.HasValue);