如何在clojure中获取矩阵中的元素

Question

I have a vector of vectors [[plate,'p1',0,1],[plate,'p2',0,2],[plate,'p3',1,1]] containing x,y positions of detected plates.

How do I retrieve the x position of plate p3 ?

It seems to be a simple task but I'm more familiar with python, so I'm not sure how to do this in clojure.

Answer 1

You don't really provide much context on what you're trying to do but it feels like you want to filter the vector of tuples to those that have the symbol p3' in the second position and then return just the third and fourth elements of such a match?

If so, the following would work:

dev=> (def plate :plate)
#'dev/plate
dev=> (def data [[plate,'p1',0,1],[plate,'p2',0,2],[plate,'p3',1,1]])
#'dev/data
dev=> (let [[[_ _ x y]] (filter (comp #{'p3'} second) data)]
 #_=>   [x y])
[1 1]

This doesn't feel very idiomatic, so perhaps you could explain more of the context?

Note: 'p3' is a symbol whose name is p3' so I wonder if you mean "p3" for a string?

The vector of vector format doesn't seem very conducive to the sort of access you want to perform - perhaps changing it to a hash map, whose keys are the plate IDs (if that's what p1 , p2 , and p3 are?) would be better to work with?

Answer 2

i would go with something like this:

(def data [[:plate "p1" 0 1] [:plate "p2" 0 2] [:plate "p3" 1 1]])

(some (fn [[_ v x y]] (when (= v "p3") [x y])) data)
;;=> [1 1]

(some (fn [[_ v x y]] (when (= v "p123") [x y])) data)
;;=> nil

Answer 3

(def p '[[plate,"p1",0,1],[plate,"p2",0,2],[plate,"p3",1,1]])
;; be aware, 'p1' you can use in Python, but because in Clojure `'` at beginning
;; of a symbol is parsed as `quote`, you can't use `''` instead of `""` in contrast to Python.

;; create a nested map out of the vec of vecs
(defn vecs-to-map [vecs]
  (reduce (fn [m [_ id x y]] (assoc m id {:x x :y y}))
          {}
          vecs))
(def m (vecs-to-map p))
;;=> {"p1" {:x 0, :y 1}, "p2" {:x 0, :y 2}, "p3" {:x 1, :y 1}}

;; you can access such a nested list via `get-in` and giving the nested map
;; and the keys it should apply on it.
(get-in m ["p3" :x])
;;=> 1

Since the irregularity that one key is a string and the other a keyword is not so nice, I would make out of them all keywords:

(defn vecs-to-map [vecs]
  (reduce (fn [m [_ id x y]] (assoc m (keyword id) {:x x :y y}))
          {}
          vecs))

(def m (vecs-to-map p))

;; access it by:
(get-in m [:p3 :x])
;;=> 1

Additional Thoughts

We ignored the first element of the vec plate . Let's say there exist also another vectors like

(def b '[[box "b1" 0 1] [box "b2" 0 2] [box "b3" 1 1]])

And if we want a nested map which contains :plate and :box in the outer level as keys, we have to change the vecs-to-map function.

(defn vecs-to-map [vecs]
  (reduce (fn [m [k id x y]] (assoc m (keyword k)
                                    (assoc (get m (keyword k) {})
                                           (keyword id) {:x x :y y})))
          {}
          vecs))

Then we can generate the map containing everything by:

(def m (vecs-to-map (concat p b)))

;; or alternatively in two steps:
;; (def m (vecs-to-map p))
;; (def m (merge m (vecs-to-map b)))


m
;; => {:plate {:p1 {:x 0, :y 1}, :p2 {:x 0, :y 2}, :p3 {:x 1, :y 1}}, :box {:b1 {:x 0, :y 1}, :b2 {:x 0, :y 2}, :b3 {:x 1, :y 1}}}

And we access the content by:

;; access through:
(get-in m [:plate :p3 :x])
;; => 1

(get-in m [:box :b2 :y])
;; => 2

Answer 4

Here is how I would do it, based on my favorite template project . Please also note that in Clojure strings always use double-quotes like "p1" . Single quotes are totally different!

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

(defn vec-has-label
  "Returns true iff a vector has the indicated label"
  [vec lbl]
  (= lbl (nth vec 1)))

(defn vec->x
  "Given a vector, return the x value"
  [vec]
  (nth vec 2))

(defn find-label
  [matrix label]
  (let [tgt-vecs (filterv #(vec-has-label % label) matrix) ; find all matching vectors
        x-vals   (mapv vec->x tgt-vecs)] ; extract the x values
    x-vals))

The unit tests show the code in action

(dotest
  (isnt (vec-has-label '[plate "p1" 0 1] "p0"))
  (is   (vec-has-label '[plate "p1" 0 1] "p1"))

  (is= 9 (vec->x '[plate "p1" 9 1]))

  (let [matrix (quote
                 [[plate "p1" 0 1]
                  [plate "p2" 0 2]
                  [plate "p3" 1 1]])]
    (is= (find-label matrix "p3")
      [1])
  ))

The unit test show the 2 ways of "quoting" a data structure that contains one or more symbols. This would be unneeded if the redundant plate symbol weren't present.

See also this list of documentation sources .