Filter odd numbers without using filter function in Clojure

Question

how can I filter odd numbers and put them into a vector? (For educational purposes I know a better way to use the filter function)

My attempt is:

(map
  (fn [x] (if (odd? x) (into [] cat x)))
  (range 0 10))

Expected output:

;=> [1 3  5 7 9]

Second question: In (if) function how can I set the when if condition is false then do exactly nothing. When if I leave it empty It brings nil. I don't want that.

Thank you for your time.

Answer 1

Here is one way to do it:

(ns tst.demo.core
  (:use demo.core tupelo.core tupelo.test))

(defn filter-odd
  [vals]
  (reduce
    (fn [cum item]
      ; Decide how to modify `cum` given the current item
      (if (odd? item)
        (conj cum item) ; if odd, append to cum result
        cum ; if even, leave cum result unchanged
        ))
    []    ; init value for `cum`
    vals ; sequence to reduce over
  ))

(verify
  (is= (filter-odd (range 10))
    [1 3 5 7 9]))

You could also use loop/recur to mimic the functionality of the reduce function.

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