C# Type of String Index

Question

I need to access a very large number in the index of the string which int and long can't handle. I had to use ulong but the problem is that the indexer can only handle the type int.

This is my code and I have marked the line where the error is located. Any ideas how to solve this?

       string s = Console.ReadLine();
        long n = Convert.ToInt64(Console.ReadLine());               
        var cont = s.Count(x => x == 'a');
        Console.WriteLine(cont);
        Console.ReadKey();

The main idea of the code is to identify how many 'a's there are in the string. What are some other ways I can do this?

EDIT:

i didn't know that is the string index Capicity cant exceed the int type. and i fixed my for loop by replacing it with this linq line

var cont = s.Count(x => x == 'a');

now since my string can't exceed certain amount. so how i can repeat my string to append its char for 1,000,000,000,000 times rather than using this code

for (int i = 0; i < 20; i++)
{
    s += s;
}

since this code is generating random char numbers in the string and if i raised the 20 may cause to overflow so i need to adjust it to repeat itself to make the string[index] = n // the long i declared above. so for example if my string input is "aba" and n is 10 so the string will be "abaabaabaa" // total chars 10

PS: I Edited the original code

Answer 1

I assume you got a programming assignment or online coding challenge, where the requirement was "Count all instances of the letter 'a' in this > 2 GB file". You solution is to read the file in memory at once, and loop over it with a variable type that allows values over 2GB.

This causes an XY problem. You cannot have an array that large in memory in the first place, so you're not going to reach the point where you need a uint, long or ulong to index into it.

Instead, use a StreamReader to read the file in chunks, as explained in for example Reading large file in chunks c#.

Answer 2

I highly recommend you rethink the way you are doing this, but a quick fix would be to use a foreach loop instead:

foreach(char c in s)
{
    if (c == 'a')
        cont++;         
}

Alternative using Linq:

cont = s.Count(c => c == 'a');

---

I'm not sure about what n is supposed to do. According to your code it limits the string length but your question never mentions why or to what end.

Answer 3

You can repeat your string using an infinite sequence. I haven't added any check for valid arguments, etc.

static void Main(string[] args)
{
    long count = countCharacters("aba", 'a', 10);
    Console.WriteLine("Count is {0}", count);

    Console.WriteLine("Press ENTER to exit...");
    Console.ReadLine();
}

private static long countCharacters(string baseString, char c, long limit)
{
    long result = 0;

    if (baseString.Length == 1)
    {
        result = baseString[0] == c ? limit : 0;
    }
    else
    {
        long n = 0;
        foreach (var ch in getInfiniteSequence(baseString))
        {
            if (n >= limit)
                break;

            if (ch == c)
            {
                result++;
            }
            n++;
        }
    }

    return result;
}

//This method iterates through a base string infinitely
private static IEnumerable<char> getInfiniteSequence(string baseString)
{
    int stringIndex = 0;
    while (true)
    {
        yield return baseString[stringIndex++ % baseString.Length];
    }
}

For the given inputs, the result is 7

Answer 4

i need to access a very large number in the index of the string which int, long can't handle

this statement is not true

c# string's max length is int.Max since string.Length is an integer and it is limited by that. You should be able to do

for (int i = 0; i <= n; i++)

Answer 5

The maximum length of a string cannot exceed the size of an int so there really is no point in using ulong or long to index into the string.

Simply put, you're trying to solve the wrong problem.

If we disregard the fact that the program is likely to cause an out of memory exception when building such a long string, you can simply fix your code by switching to an int instead of a ulong:

for (int i = 0; i <= n; i++)

Having said that you can also use LINQ to do this:

int cont = s.Take(n + 1).Count(c => c == 'a');

Now, in the first sentence of your question you state this:

I need to access a very large number in the index of the string which int and long can't handle.

This is wholly unnecessary because any legal index of a string will fit inside an int.

Answer 6

If you need to do this on some input that's longer than the maximum length of a string in .NET, you'll need to change your approach; use a Stream instead trying to read all input into a string.

char seeking = 'a';
ulong count = 0;

char[] buffer = new char[4096];
using (var reader = new StreamReader(inStream))
{
    int length;
    while ((length = reader.Read(buffer, 0, buffer.Length)) > 0)
    {
        count += (ulong)buffer.Count(c => c == seeking);
    }
}