将Int List转换为Integer

Question

This is a bit of an odd case I must admit, but I'm trying to find a succinct and elegant way of converting a List<int> into an actual int . For example, if my list contained entries 1, 2, and 3, then the final integer would be 123. The only other way I thought of doing it was converting the array to a string and then parsing the string.

Any hints?

Answer 1

Iterate the list, as if adding a sum, but multiply the running total by 10 at each stage.

int total = 0;
foreach (int entry in list)
{
    total = 10 * total + entry;
}

Answer 2

List<int> l = new List<int>();
// add numbers
int result = int.Parse(string.Join(",", l).Replace(",", ""))

You'd have to take care if the list is long enough so that the resulting number would exceed the limits for an int though.

Answer 3

I think you suggestion is pretty good, something like this works:

var list = new List<int>() { 1, 2, 3, 4 };
var listAsString = String.Join("", list.ConvertAll(x => x.ToString()).ToArray());
Console.WriteLine(listAsString);
int result = Int32.Parse(listAsString);
Console.WriteLine(result);

Answer 4

You could do it by adding all the numbers in a loop. Not sure it's faster than string parsing though:

List<int> list = new List<int> {1, 2, 3};
int sum = 0;
int pow = list.Count - 1;
for (int i = 0; i <= pow; i++)
{
    sum += list[i] * (int)Math.Pow(10,(pow-i));
}

Also if you have a long list, you might want to use .Net 4's BigInteger class instead of an int.

Answer 5

Well just for fun (I have no idea how effecient this is, probably not very), you can also do this easily with Linq...

first convert the list of ints to a list of strings, then use the aggregate function to concatenate them, then at the end use in32.TryParse to make sure the resulting value is in the int range.

string val = ints.Select(i=> i.ToString()).Aggregate((s1, s2) => s1 + s2);

Answer 6

Say, you have an enumerable like

var digits = [1, 2, 3 ...];

then you could do:

// simplest and correct way; essentially the accepted answer but using LINQ
var number = digits.Aggregate((a, b) => a * 10 + b);

// string manipulation; works for sets like [10, 20, 30]
var number = int.Parse(string.Join("", digits));

// using `Select` instead of `Aggregate` but verbose; not very useful
var number = (int)digits.Select((d, i) => d * Math.Pow(10, digits.Count() - i - 1)).Sum();