Replace repeated char with something else in a string

Question

I need help to figure out a logic:

So, let's say I have a string, and whenever there is a Char repeated inside that string , I need to replace it with a ( Char + sequence of number ).

For example:

Original String: "abcdefgabfabc"

Expected output: "abcdefga2b2f2a3b3c2"

'a' occurs 3 times, so the first 'a' remains as 'a', but the second 'a' becomes 'a2', and the third 'a' becomes 'a3' and the same goes to other chars like b, b2, b3...

Answer 1

1. Create a Dictionary of letters and the # of occurrences of each letter
2. Create a StringBuilder to store the output
3. Loop through the input string letter by letter
4. Output the letter to the new string
5. If a letter is not in the dictionary, add it as the key with a '1' as the value
6. If a letter is already in the dictionary, increase the value by 1 and append the value to the output string

Answer 2

Try this:

var foundChars = new SortedDictionary<char, int>();
var stringBuilder = new StringBuilder();

foreach (var c in originalString)
{
    var count = 0;

    if (!foundChars.TryGetValue(c, out count)
    {
        foundChars.Add(c, 1);
    }
    else
    {
        count += 1;
        foundChars[c] = count;
    }

    stringBuilder.Append(c);
    if (count > 0) stringBuilder.Append(count);
}

Note that while less pretty this will be more performant than LINQ based solutions and be retrocompatible with .NET 2.0.

Answer 3

var str = "abcdefgabfabc";
var chars = str.Select((c, index) =>
        {
            int count = str.Substring(0, index).Count(x => c == x);
            if (count > 0) return c.ToString() + (count+1);
            else return c.ToString();
        }).SelectMany(c => c).ToArray();
var result = new string(chars); // abcdefga2b2f2a3b3c2

Answer 4

I would use LINQ to iterate over each character, and then keep a counter of each one you encounter along the way.

For example..

var count = new Dictionary<string, int>();
var string = "abcdefabcdef";
var result = "";

string.Select().Each(c => {
    if (count.ContainsKey(c)) 
        count.Add(c, 1);
    else
        count[c]++;
    result += count[c] > 1? c + count[c] : c;
});

Answer 5

Some of the other answers suffers from being O(n^2) ( Selman22 , Stumblor ) or O(n*log n) ( Chrono1981 ) even though a O(n) solution is simple. The right solution is really what D Stanley and I hinted to. Here it goes:

var input = "abcdefgabfabc";
var counts = new Dictionary<char, int>();

var sb = new StringBuilder();

foreach (var c in input)
{
    int count;
    counts.TryGetValue(c, out count);  // If "counts" doesn't have the key, then count will be 0
    counts[c] = ++count;

    sb.Append(c);

    if (count > 1)
        sb.Append(count);
}

var result = sb.ToString();