CODEWITHSUNDEEP
vectorclojureindexofindices
truffle
truffle

Reputation: 555

Alternative to .indexOf to get the indices of a vector in Clojure

This is a simple problem but nonetheless, I am stuck. So let's say I have an input vector with negatives, zeros and positive numbers.

[-1 -1 -2 -1 -4 0 -6 -1 -5 -2 -10 **4** -12 -4 -6 -1 -16 **3** -18 **2** -10 -8 -22 **12** -19
-10 -14 0 -28 **12** -30 -1 -18 -14 -22 **19** -36 -16 -22 **10** -40 **12** -42 -4 -12 -20 -46 **28** -41 -7]

And I want to return a vector of the indices of the previous vectors, where the value is a positive.

So the return value would be 

 (11 17 19 23 29 35 39 41 47)

11 is the index of where the 4 sits, 17 is the index of where the 3 is, etc

I am using .indexOF

and this is what it is returning: (11 17 19 23 23 35 39 23 47)

I figured that b/c at index 23, the positive value is 12, and at index 29, the value is also 12, so it is just returning the index where it first sees the positive value of "12" But how can I get around this?

I've read through the thread How do I find the index of an item in a vector? But I am still struggling since that one seems to talk about how to find the indices of "two" in a vector if you are looking for "two" specifically.

Upvotes: 1

Views: 636

Answers (2)

user4813927
user4813927

Reputation:

If you are searching the index numbers of the positive arguments of your vector you might also use map-indexed which gives you the arguments indexed from 0 to (dec (count your-vector)) in combination with (filter pos?) to filter out the positive entries of your vector. filter function goes through your vector and filters the numbers which fill the prerequisites of its predicate (in this case they should be positives), then you can easily ask for their index which you had just created using the map-indexed:

(defn indexof [a]
  (->> a
    (map-indexed vector)
    (filter #(pos? (second %)))
    (map first)))

(indexof [-1 -1 -2 -1 -4 0 -6 -1 -5 -2 -10 4 -12 -4 -6 -1 -16 3 -18 2 -10 -8 -22 12 -19
 -10 -14 0 -28 12 -30 -1 -18 -14 -22 19 -36 -16 -22 10 -40 12 -42 -4 -12 -20 -46 28 -41 -7])

=> (11 17 19 23 29 35 39 41 47)

If you search for zero's, just change the predicate of the filter to zero? to search for zero's:

(defn indexof-zero [a]
  (->> a
    (map-indexed vector)
    (filter #(zero? (second %)))
    (map first)))

(indexof-zero [-1 -1 -2 -1 -4 0 -6 -1 -5 -2 -10 4 -12 -4 -6 -1 -16 3 -18 2 -10 -8 -22 12 -19
-10 -14 0 -28 12 -30 -1 -18 -14 -22 19 -36 -16 -22 10 -40 12 -42 -4 -12 -20 -46 28 -41 -7])

=> (5 27)

Upvotes: 1

Lee
Lee

Reputation: 144136

You can use keep-indexed:

(keep-indexed (fn [idx v] (if (pos? v) idx)) input-vector)

Upvotes: 6

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