How to search and replace in a Clojure script data structure?

Question

I would like to have a search and replace on the values only inside data structures:

(def str [1 2 3 
              {:a 1 
               :b 2 
               1  3}])

and

(subst  str  1  2)

to return

[2 2 3 {:a 2, :b 2, 1 3}]

Another example:

(def str2  {[1 2 3] x, {a 1 b 2} y} )

and

(subst  str2  1  2)

to return

{[1 2 3] x, {a 1 b 2} y}

Since the 1's are keys in a map they are not replaced

The restriction that only values and not map keys should be replaced means you'll likely be rolling your own. — Alex
– Alex, Commented Sep 18, 2014 at 15:09
In {[1 2 3] x, {a 1 b 2} y}, the both 1s are values inside keys. Are you saying that values only are naked values or naked values inside values? — Thumbnail
– Thumbnail, Commented Sep 19, 2014 at 7:23
Function in my answer seems to work just like you've described. I wonder if it works for you and if not, what's the problem with it? — Mark Karpov
– Mark Karpov, Commented Sep 23, 2014 at 16:17
@Zubair, try it for yourself, that's the way it works (second solution). — Mark Karpov
– Mark Karpov, Commented Sep 24, 2014 at 1:29

Mark Karpov · Accepted Answer · 2014-09-18 15:15:52Z

3

One option is using of postwalk-replace:

user> (def foo [1 2 3 
              {:a 1 
               :b 2 
               1  3}])
;; => #'user/foo
user> (postwalk-replace {1 2} foo)
;; => [2 2 3 {2 3, :b 2, :a 2}]

Although, this method has a downside: it replaces all elements in a structure, not only values. This may be not what you want.

Maybe this will do the trick...

(defn my-replace [smap s]
  (letfn [(trns [s]
            (map (fn [x]
                   (if (coll? x)
                       (my-replace smap x)
                       (or (smap x) x)))
                 s))]
    (if (map? s)
      (zipmap (keys s) (trns (vals s)))
      (trns s))))

Works with lists, vectors and maps:

user> (my-replace {1 2} foo)
;; => (2 2 3 {:a 2, :b 2, 1 3})

...Seems to work on arbitrary nested structures too:

user> (my-replace {1 2} [1 2 3 {:a [1 1 1] :b [3 2 1] 1 1}])
;; => (2 2 3 {:a (2 2 2), :b (3 2 2) 1 2})

edited Sep 18, 2014 at 15:15

answered Sep 18, 2014 at 14:58

Mark Karpov

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3 Comments

A. Webb Over a year ago

OP only wants to replace values. There is also a 1 key that should not be replaced.

Mark Karpov Over a year ago

@A.Webb, Good point! I'll see if I can come up with something more relevant.

amalloy Over a year ago

"Only values" is not super well defined. What about an input like {[1 2 3] x, {a 1 b 2} y}? Here the first 1 is not a key, but it is a value inside of a key. And the second 1 is the value in a map, but that map is the key of another map. Are these being replaced? In your edited solution, no. But either way it's pretty weird; I recommend that OP think about whether there's an important reason for map keys to be left untouched.

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