How to locate first location in a vector where all the rest of values are the same

Question

I have a vector, X, looking like the following:

X = c(30, 10, 10, 15, 25, 1, 10, 55, 10, 1, 10, 5, 5, 5, 5, 10, 5, 5, 5, 5, 5, 5, 5, 5) 
                                                                ^
                                                                17

I want to make a function where if I input the above vector and a number, n, it shows where the consecutive n starts. In X above, the desired number is 17 if my n is 5 since starting the 17th location, the rest of the vector is all 5.

So, if I want a function, f, such that f(X,5) gives me 17. Can anyone help me on achieve this in the fastest manner?

There is a 10 in the 16th location. I want all the rest to be n. In this case, 5. — jay2020, Jan 20 '16 at 21:25
What happens if the number five is not at the end? And what have you tried to solve this yourself? — Rich Scriven, Jan 20 '16 at 21:27
Start at the end of the array, work backwards until the input `n` is not at the current array location. Done. — Draco18s no longer trusts SE, Jan 20 '16 at 21:27
So, you want to know where the first location of the last value (as it seems these are to be when you define consecutive)? Using the word consecutive doesn't really work. Because that would grab 12 and 17, as Richard mentioned. — Badger, Jan 20 '16 at 21:29
Well I flip the vector and locate where the diff of X is first non zero. And flip back to locate. But I need everything very fast and I am not sure if this is the best way — jay2020, Jan 20 '16 at 21:34
Or Draco18s suggestion works as well by locating first non zero. — jay2020, Jan 20 '16 at 21:35
You need this to be fast... are you varying `X` or varying the `5` when repeating this? Or both? — Frank, Jan 20 '16 at 21:36
Does `length(X) - (which.max(rev(X) != 5) - 2)` works for you? — David Arenburg, Jan 20 '16 at 21:49
Yes, it will always be 5, at least for this particular data set I am looking at.. — jay2020, Jan 20 '16 at 21:57

score 2 · Accepted Answer · answered Jan 20 '16 at 21:57

If we assume that the last element is always 5 (according to OP), we could just reverse the vector, find the first non five, subtract it from the length of the vector and add 2 in order to switch direction back again

length(X) - which.max(rev(X) != 5) + 2
# [1] 17

How to locate first location in a vector where all the rest of values are the same

1 Answers1