CODEWITHSUNDEEP
rvector
Serpahimz
Serpahimz

Reputation: 175

R: How to find the maximal index for which an entry in a vector meets a certain condition

I have a vector X which can potentially contain a very large number of entries and I want to find the maximal index i such that X[i] <= a*i/m (for some constants a,m) and I'd really rather not loop over the vector. I thought about using Position but I can't figure out how to make a suitable function that would take into account the indexes of the vector.

I forgot to mention that the entries of the vector will be sorted so that might help.

Help would be appreciated

Upvotes: 0

Views: 46

Answers (2)

Carl Witthoft
Carl Witthoft

Reputation: 21532

I interpret your question (as of "right now" ) differently from ChinmayP, so maybe this:

foo <- which ( X/(1:(length(X)) < a/m)

rev(foo)[1]

That will give you the element of X with the largest index i (as in X[i] ) which meets your condition.

Upvotes: 2

CHP
CHP

Reputation: 17189

Following should give you what you want. 

max(which(X <= a/m * seq_along(X)))

Looping backwards might still be better idea though, as you only want maximum index value.

for ( i in length(X):1 ) { if (X[i] <= a*i/m) break } ;  i

When the loop breaks, i will contain your required max index.

Upvotes: 1

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