Remove Duplicate character from string in Mongodb

Question

I want to remove duplicate characters from strings in MongoDB. Example: Input string: xxxyzzxcdv Output string: xyzcdv

Answer 1

Query

• reduce on range(count string)
• keep 2 values {"previous": [], "string": ""} (initial value of reduce)
• get the cur-char {"$substrCP": ["$mystring", "$$this", 1]} this is the current index on the string, and i take the next char
• if it is in previous kep "string" as it is, else concat to add the new character

heelo

reduce on (0 1 2 3 4) `{"$range": [0, {"$strLenCP": "$mystring"}]}`
we start from  `{"previous": [], "string": ""}`

- get 1 character start from index 0  
 `{"$substrCP": ["$mystring", "$$this", 1]}}`  = "h"
- if this character is on previous don't add it  
 `{"$in": ["$$cur_char", "$$value.previous"]}`
- else add it on previous and on the string the 2 concats in code

Repeat for `index($$this)`= 1
- get 1 character start from index 1  
 `{"$substrCP": ["$mystring", "$$this", 1]}}` = "e"
.....

PlayMongo

aggregate(
[{"$set": 
   {"mystring": 
     {"$getField": 
       {"field": "string",
        "input": 
         {"$reduce": 
           {"input": {"$range": [0, {"$strLenCP": "$mystring"}]},
            "initialValue": {"previous": [], "string": ""},
            "in": 
             {"$let": 
               {"vars": {"cur_char": {"$substrCP": ["$mystring", "$$this", 1]}},
                "in": 
                {"$cond": 
                  [{"$in": ["$$cur_char", "$$value.previous"]},
                   "$$value",
                   {"previous": 
                     {"$concatArrays": ["$$value.previous", ["$$cur_char"]]},
                   "string": 
                   {"$concat": ["$$value.string", "$$cur_char"]}}]}}}}}}}}}])

---

Edit

The second query removed only the duplicates we choose.

Query

• removes only the characters in the array, here only ["x"]
• i removed the $getField because its only for MongoDB 5 +

aggregate(
[{"$set": 
    {"mystring": 
      {"$reduce": 
        {"input": {"$range": [0, {"$strLenCP": "$mystring"}]},
          "initialValue": {"previous": [], "string": ""},
          "in": 
          {"$let": 
            {"vars": {"cur_char": {"$substrCP": ["$mystring", "$$this", 1]}},
              "in": 
              {"$cond": 
                [{"$and": 
                    [{"$in": ["$$cur_char", "$$value.previous"]},
                      {"$in": ["$$cur_char", ["x"]]}]},
                  "$$value",
                  {"previous": 
                    {"$concatArrays": ["$$value.previous", ["$$cur_char"]]},
                    "string": 
                    {"$concat": ["$$value.string", "$$cur_char"]}}]}}}}}}},
  {"$set": {"mystring": "$mystring.string"}}])

---

Edit

If you need to use this aggregation for update, you can use it as pipeline update like.

update({},
[{"$set": ......])

See your driver to find how to do update with pipeline, in Java its like above, alternative run it as database command