Question

I am trying to solve the following problem but cannot find an elegant solution. Any ideas?
Thanks.

Input – a variable length string of numbers, e.g.,
string str = “5557476374202110373551116201”;

Task – Check (from left to right) that each number (ignoring the repetitions) does not appear in the following 2 indexes. Using eg. above, First number = 5. Ignoring reps we see that last index of 5 in the group is 2. So we check next 2 indexes, i.e. 3 and 4 should not have 5. If it does we count it as error. Goal is to count such errors in the string.

In the above string errors are at indexes, 3,10 and 16.

Answer 1

in addition to the other excellent solutions you can use a simple regexp:

foreach (Match m in Regexp.Matches(str, @"(\d)(?!\1)(?=\d\1)"))
    Console.WriteLine("Error: " + m.Index);

returns 3,10,16. this would match adjacent errors using lookahead with a backreference. handles repetitions. .net should support that. if not, you can use a non-backreference version:

(?<=0[^0])0|(?<=1[^1])1|(?<=2[^2])2|(?<=3[^3])3|(?<=4[^4])4|(?<=5[^5])5|(?<=6[^6])6|(?<=7[^7])7|(?<=8[^8])8|(?<=9[^9])9