c# expressions and loop beauty

Question

This is probably pretty subjective, but how do people generally lay out their loop control in C# when the control variable is updated in the loop? The pedant in me doesn't like the separate declaration and repetition involved. eg.

string line = reader.ReadLine();
while (line != null)
{
    //do something with line

    line = reader.ReadLine();
}

The C coder in me wants to change this to

while (string line = reader.ReadLine() != null)
{
    //do something with line
}

but C#'s expressions don't seem to work that way :(

score 7 · Answer 1 · edited Nov 24 '11 at 14:11

7

Options:

1) Declare the variable the loop, but assign it in the condition:

string line;
while ((line = reader.ReadLine()) != null)
{
}

2) Use a for loop instead:

for (string line = reader.ReadLine(); line != null; line = reader.ReadLine())
{
}

3) Create an extension method to turn a reader into an IEnumerable<String> and then use:

foreach (string line in reader.ReadLines())
{
}

Personally I like the last one where possible - otherwise I'd use the first form.

edited Nov 24 '11 at 14:11

C.Evenhuis

25,996
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answered Nov 24 '11 at 14:07

Jon Skeet

1,421,763
867
9,128
9,194

You could make that last one a `var`, no? I mean, if your colleagues cannot figure out that the result of `ReadLines` is a `string`, the solution is not explicit typing but immediate sacking :-) – Jörg W Mittag Nov 24 '11 at 14:10
1

@JörgWMittag: I could, but I'm not sure there's any benefit in doing so. – Jon Skeet Nov 24 '11 at 14:11

score 4 · Accepted Answer · answered Nov 24 '11 at 14:06

4

You can't declare a variable inside an expression.

You can write

string line;
while (line = reader.ReadLine() != null)

To make it clearer, I prefer to write

string line;
while (null != (line = reader.ReadLine()))

However, the best alternative is

foreach(string line in File.ReadLines(path) {

}

This will perform equivalently.
If you're reading some other stream, you can create an extension method that uses the previous syntax to enable foreach loops.

answered Nov 24 '11 at 14:06

SLaks

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And don't forget to declare the variable before. – Vladimir Perevalov Nov 24 '11 at 14:08
1

@VladimirPerevalov: I didn't. – SLaks Nov 24 '11 at 14:10

score 2 · Answer 3 · answered Nov 24 '11 at 14:07

2

Personally, I prefer:

string line;
while (line = reader.ReadLine() != null)
{
  ...
}

There's always the for construct as an alternative though:

for (string line = reader.ReadLine(); line != null; line = reader.ReadLine())
{
  ...
}

answered Nov 24 '11 at 14:07

Rich O'Kelly

41,274
9
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114

score 1 · Answer 4 · answered Nov 24 '11 at 14:08

1

I usually write something like:

string line;
while((line = reader.ReadLine()) != null)
{
}

answered Nov 24 '11 at 14:08

Stmated

477
5
17

score 0 · Answer 5 · answered Nov 24 '11 at 14:08

0

I agree that repeating the reader.ReadLine() expression isn't good. One way is to use while(true) and break:

while(true)
{
  string line = reader.ReadLine();
  if(line == null)
    break;

  //do something with line
}

answered Nov 24 '11 at 14:08

Anders Abel

67,989
17
150
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Daren Thomas · Answer 6 · 2011-11-24T16:02:08.113

How about:

public static IEnumerable<R> ToSequence<T, R>(this T self, 
    Func<T, R> yielder, Func<R, bool> condition)
{
    R result = yielder(self, R);
    while (condition(result))
    {
        yield return result;
    }
}

and then use it something like this:

foreach (var line in reader.ToSequence(
    (r) => r.ReadLine(), 
    (line) => line != null))
{
    // do some stuff to line
}

or would you prefer:

foreach (var line in reader.NotNull(r => r.ReadLine()))
{
    // do some stuff to line...
}

that could be defined as:

public static IEnumerable<R> NotNull<T, R>(this T self, Func<T, R> yielder)
{
    return self.ToSequence(yielder, (r) => r != null);
}

Not sure that really improves the beauty ;) – GazTheDestroyer Nov 24 '11 at 14:28 — GazTheDestroyer, Nov 24 '11 at 14:28

c# expressions and loop beauty

6 Answers6