How to treat seq as return value in Nim
I am facing a problem with Nim sequences and returning them from a function.
json_p.nim(42, 33) template/generic instantiation from here
json_p.nim(28, 22) Error: no generic parameters allowed for seq
Line 28 is where I define mine key_list
proc get_json_keys(json_data: JsonNode) : seq =
var key_list: seq[string] = @[] # 28
var key: string
for record in json_data:
for key, value in record:
if (not key_list.contains(key)):
key_list.add(key)
return key_list
I'll just name it from the main one.
proc main() : void =
var file = get_url()
var json_data = file.parseFile()
[...]
var key_list = get_json_keys(json_data)
for key in key_list:
echo key
The code works fine inside the main function.
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Problems:
* seq is a generic dynamic array and you can only add a key and the whole search will be linear as it is similar to a C language array.
* All return values of functions have a default "result" with a variable name. You must use it to return your values.
* Using ".contains" will cause nim to search the entire array to check. The best option is to use a fast search container.
I assume you need:
* function to process json duplicate keys and return a unique list with fast key lookup.
Implementation:
import json,tables
proc get_json_keys(json : JsonNode):OrderedTable[string,string]=
#initialize the result object
result = initOrderedTable[string,string]()
#more info,see https://nim-lang.org/docs/json.html
for key,value in json.pairs():
#debugging...
#echo key & "-" & value.getStr()
if not result.contains(key):
result[key]=value.getStr()
var json_data = parseJson("""{"key1" :"value1","key2" :"value2" }""")
var key_list = get_json_keys(json_data)
for key in key_list.pairs() :
echo key
Output:
(Field 0: "key1", field1: "value1")
(Field 0: "key2", field1: "value2")
If search speed is not an issue, you can also do this:
Implementation using seq:
proc get_json_keys(json : JsonNode):seq[string]=
result = newSeq[string](0)
for key,value in json.pairs():
if not result.contains(key):
result.add(key)
var json_data = parseJson("""{"key1" :"value1","key2" :"value2","key1":"value3" }""")
var key_list = get_json_keys(json_data)
echo key_list
Output:
@ ["key1", "key2"]
obs: edited my answer because seq is not immutable if declared with 'var'. It is only immutable if "let" is declared.
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