List<List<string>> values to into string[][] array C#

Question

I have the a following two variables:

        List<List<string>> result
        string[][] resultarray

I want to take the values from result and read store them in resultarray like so: [["one", "two"], ["three"], ["four", "five", six"], ["seven"], ["eight"]] etc.

I have the following code:

        string[][] resultarray = new string[resultint][];
        int a = new int();
        int b = new int();
        foreach (List<string> list in result)
        {
            foreach (string s in list)
            {
                resultarray[b][a] = s;
                a++;
            }
            b++;
        }
        return resultarray;

However, when debugging, I get a "NullExceptionError: Object reference not set to an instance of an object" when trying to increment a or b. I've also tried declaring them as:

    int a = 0
    int b = 0

...This doesn't work either. Am I not declaring these correctly or does it have to do with the foreach loop?

second foreach is unnecessary, just use resultarray[b] = list.ToArray(); — sasjaq
– sasjaq, Commented Oct 24, 2013 at 13:01

Marc Gravell · Accepted Answer · 2013-10-24 12:59:43Z

10

Each sub-array starts as null - you need to create the inner arrays.

But a simpler approach is:

var resultarray = result.Select(x => x.ToArray()).ToArray();

which gives the stated outcome if we assume the input is something like:

var result = new List<List<string>> {
    new List<string> { "one", "two" },
    new List<string> { "three" },
    new List<string> { "four", "five", "six" },
    new List<string> { "seven" },
    new List<string> { "eight" },
};

answered Oct 24, 2013 at 12:59

Marc Gravell

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3 Comments

reallybadatmath Over a year ago

Thanks. I've been trying to wrap my head around LINQ but I still don't quite understand it. I'll do some more reading.

Marc Gravell Over a year ago

@reallybadatmath basically what it says is "create a projection, where each item becomes the result of ToArray() on that item; and then create an array from the results of that projection"

reallybadatmath Over a year ago

Thankyou again Marc. Your explanation makes sense -- a LOT simpler than trying to target the indexes with incrementing integers.

Maciej Stachowski · Accepted Answer · 2013-10-24 13:00:57Z

3

Before the inner loop:

resultarray[b] = new string[list.Count];

answered Oct 24, 2013 at 13:00

Maciej Stachowski

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1 Comment

Marc Gravell Over a year ago

If we were going to anything with the inner loop, @sasjaq has the truth of it

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