JSON: Parsing Input
July 12, 2011
JavaScript Object Notation, JSON, is a system for passing structured data between computers, similar to XML but far simpler. JSON provides Unicode strings with a limited number of escapes, decimal numbers, the booleans true and false, the null value, arrays, and key/value dictionaries with string keys.
Your task is to write a parser that converts JSON data to an object in your favorite computer language. When you are finished, you are welcome to read or run a suggested solution, or to post your own solution or discuss the exercise in the comments below.
Programming Praxis 
Here is my implementation in python. It may contain some bugs i only made some tests:
import exceptions
class json_parser:
def __init__(self, string):
self.json_data=self.__remove_blanks(string)
self.pointer=0
def __remove_blanks(self,string):
checker=0
new_list=[]
inside_string=False
for i in list(string):
if inside_string or i != ‘ ‘ :
new_list.append(i)
if i == ‘”‘:
inside_string = not inside_string
return “”.join(n for n in new_list)
def __parse_obj(self):
new_dic={}
self.pointer+=1
while self.json_data[self.pointer] != ‘}’:
if self.json_data[self.pointer] == ‘”‘:
key=self.__parse_string()
else:
raise Exception #The only possible type of value for a key is String
if self.json_data[self.pointer] == ‘:’:
self.pointer+=1
else:
raise Exception #invalid object
value=self.__parse_value()
if value == -1:
return -1
new_dic[key]=value
if self.json_data[self.pointer] == ‘,’:
self.pointer+=1
self.pointer+=1
return new_dic
def __parse_array(self):
new_array=[]
self.pointer+=1
while self.json_data[self.pointer] != ‘]’:
value=self.__parse_value()
if value == -1:
return -1
else:
new_array.append(value)
if self.json_data[self.pointer] == ‘,’:
self.pointer+=1
self.pointer+=1
return new_array
def __parse_string(self):
self.pointer+=1
start=self.pointer
while self.json_data[self.pointer]!='”‘:
self.pointer+=1;
if self.pointer==len(self.json_data):
raise Exception #the string isn’t closed
self.pointer+=1
return self.json_data[start:self.pointer-1]
def __parse_other(self):
if self.json_data[self.pointer:self.pointer+4]==’true’:
self.pointer+=4
return True
if self.json_data[self.pointer:self.pointer+4]==’null’:
self.pointer+=4
return None
if self.json_data[self.pointer:self.pointer+5]==’false’:
self.pointer+=5
return False
start=self.pointer
while (self.json_data[self.pointer].isdigit()) or (self.json_data[self.pointer] in ([‘-‘,’.’,’e’,’E’])):
self.pointer+=1
if ‘.’ in self.json_data[start:self.pointer]:
return float(self.json_data[start:self.pointer])
else:
return int(self.json_data[start:self.pointer])
def __parse_value(self):
try:
if self.json_data[self.pointer]=='{‘:
new_value = self.__parse_obj()
elif self.json_data[self.pointer]=='[‘:
new_value = self.__parse_array()
elif self.json_data[self.pointer]=='”‘:
new_value = self.__parse_string()
else:
new_value = self.__parse_other()
except Exception:
print ‘Error:: Invalid Data Format, unknown character at position’, self.pointer
return -1
return new_value
def parse(self):
if self.json_data[self.pointer]=='{‘ or self.json_data[self.pointer]=='[‘:
final_object = self.__parse_value()
else:
print ‘Error:: Invalid inicial Data Format’
final_object=None
return final_object
[…] one of their problems (an old one, from 2009) . So today i’m posting here my solution for the problem of this week. Basically they ask us to write a JSON parser in our favorite computer language, so i chose […]