I am trying to read from a JSON response in Casablanca. The sent data looks like this:
{
"devices":[
{"id":"id1",
"type":"type1"},
{"id":"id2",
"type":"type2"}
]
}
Does anyone know how to do this? Casablanca tutorials only seem to care about creating such arrays and not about reading from them.
Let's assume you got your json as an http response:
web::json::value json;
web::http::http_request request;
//fill the request properly, then send it:
client
.request(request)
.then([&json](web::http::http_response response)
{
json = response.extract_json().get();
})
.wait();
Note that no error checking is done here, so let's assume everything works fine (--if not,see the Casablanca documentation and examples).
The returned json can then be read via the at(utility::string_t) function. In your case it is an array (you either know that or check it via is_array()):
auto array = json.at(U("devices")).as_array();
for(int i=0; i<array.size(); ++i)
{
auto id = array[i].at(U("id")).as_string();
auto type = array[i].at(U("type")).as_string();
}
With this you get the entries of the json response stored in string variables.
In reality, you further might want to check whether the response has the coresponding fields, e.g. via has_field(U("id")), and if so, check whether the entries are not null via is_null() -- otherwise, the as_string() function throws an exception.
The following is a recursive function I made for parsing JSON values in cpprestsdk, if you would like additional info or elaboration feel free to ask.
std::string DisplayJSONValue(web::json::value v)
{
std::stringstream ss;
try
{
if (!v.is_null())
{
if(v.is_object())
{
// Loop over each element in the object
for (auto iter = v.as_object().cbegin(); iter != v.as_object().cend(); ++iter)
{
// It is necessary to make sure that you get the value as const reference
// in order to avoid copying the whole JSON value recursively (too expensive for nested objects)
const utility::string_t &str = iter->first;
const web::json::value &value = iter->second;
if (value.is_object() || value.is_array())
{
ss << "Parent: " << str << std::endl;
ss << DisplayJSONValue(value);
ss << "End of Parent: " << str << std::endl;
}
else
{
ss << "str: " << str << ", Value: " << value.serialize() << std::endl;
}
}
}
else if(v.is_array())
{
// Loop over each element in the array
for (size_t index = 0; index < v.as_array().size(); ++index)
{
const web::json::value &value = v.as_array().at(index);
ss << "Array: " << index << std::endl;
ss << DisplayJSONValue(value);
}
}
else
{
ss << "Value: " << v.serialize() << std::endl;
}
}
}
catch (const std::exception& e)
{
std::cout << e.what() << std::endl;
ss << "Value: " << v.serialize() << std::endl;
}
return ss.str();
}
Related
File:
{
"somestring":{
"a":1,
"b":7,
"c":17,
"d":137,
"e":"Republic"
},
}
how can I read the somestring value by jsoncpp?
Use the getMemberNames() method.
Json::Value root;
root << jsonString;
Json::Value::Members propNames = root.getMemberNames();
std::string firstProp = propNames[0];
std::cout << firstProp << '\n'; // should print somestring
If you want to see all the properties, you can loop through it using an iterator:
for (auto it: propNames) {
cout << "Property: " << *it << " Value: " << root[*it].asString() << "\n";
}
This simple loop will only work for properties whose values are strings. If you want to handle nested objects, like in your example, you'll need to make it recursive, which I'm leaving as an exercise for the reader.
I have yahoo finance json file from which I want to isolate Date,Close and volume from the quote list and save it in the same order with a comma separtion in a single text file. This is my json script.
Json::Value root; // will contains the root value after parsing.
Json::Reader reader;
bool parsingSuccessful = reader.parse( YahooJson, root );
if(not parsingSuccessful)
{
// Report failures and their locations
// in the document.
std::cout<<"Failed to parse JSON"<<std::endl
<<reader.getFormatedErrorMessages()
<<std::endl;
return 1;
}else{
std::cout<<"\nSucess parsing json\n"<<std::endl;
std::cout << root<< std::endl;
std::cout <<"No of Days = "<< root["query"]["count"].asInt() << std::endl;
//below for loop returns an error
for (auto itr : root["query"]["result"]["quote"]) {
std::string val = itr.asString();
}
}
I was able to succed in fetching the json values and print root["query"]["count"].asInt() but when I go to the list values(quote) I dont know how to iterate through quote (query->result->quote) to get Date,close and volume values?
EDIT
Also tried this method
const Json::Value& quotes = root["query"]["results"]["quote"];
for (int i = 0; i < quotes.size(); i++){
std::cout << " Date: " << quotes[i]["Date"].asString();
std::cout << " Close: " << quotes[i]["Close"].asFloat();
std::cout << " Volume: " << quotes[i]["Volume"].asFloat();
std::cout << std::endl;
}
It works only when output was Date. For close and volume output it exits with a runtime error message and also this error
what() type is not convertible to string
You haven't specified which JSON library you are using, and I don't know the Yahoo finance data well enough to know the exact field names, but if you are using the JsonCpp library, which has documentation here, and you are asking about how to iterate over a JSON array, then one way to do it using iterators would look something like this
const Json::Value quote = root["query"]["results"]["quote"];
for (Json::ValueConstIterator itr = quote.begin(); itr != quote.end(); ++itr)
{
const Json::Value date = (*itr)["Date"];
const Json::Value close = (*itr)["Close"];
const Json::Value volume = (*itr)["Volume"];
std::cout << "Date: " << date.asString() << std::endl;
std::cout << "Close: " << close.asString() << std::endl;
std::cout << "Volume: " << volume.asString() << std::endl;
}
I am trying to print a map in an organized way. My map is defined like this:
map<std::string,std::vector<message *> > data;
where message is a struct like this:
struct message{
static unsigned int last_id;
unsigned int id;
std::string msg;
std::string timestamp;
message(const std::string& recvbuf_msg,const std::string& a_timestamp) :
msg(recvbuf_msg), timestamp(a_timestamp), id(++last_id)
{
}
};
I tried this way of printing it:
std::cout << (data[username]).at(0)->msg << std::endl;
But it gives a debug error when reaching that function, how can i solve it?
Error R6010 - abort() has been called suggests that either there is no entry for key username in the map, or the vector of messages for that user is empty. You need to make sure the containers are nonempty before accessing elements. It is a good idea to use iterators, here is an example of how to print the messages for all usernames:
for(auto mapIt = data.cbegin(); mapIt != data.cend(); ++mapIt)
{
std::cout << "printing data for " << mapIt->first << ":" << std::endl;
for(auto vectIter = mapIt->second.cbegin(); vectIter != mapIt->second.cend(); ++vectIter)
{
std::cout << (*vectIter)->msg << ", " << (*vectIter)->timestamp << ", "
<< (*vectIter)->id << std::endl;
}
}
The code uses auto, so if you are not using a C++11 compliant compiler, you will have to write the iterator types yourself.
I have a problem in using the std::map correctly. The class Example is a class with an ID, a label, a vector of keypoints and a descriptor matrix. The class Examples is a map for retrieving an example given its ID. The examples are read from files on disk, stored in the map, then used later.
Even if it is conceptually very simple, I am not able to fill the map properly.
I have the following class:
class Example
{
public:
std::string id;
std::string label;
std::vector<cv::KeyPoint> keypoints;
cv::Mat descriptors;
Example(std::string id_, std::string label_)
: id(id_), label(label_)
{
// ... nothing ...
}
string to_string() const
{
stringstream ss;
ss << "#" << id
<< " (" << label << ")"
<< " - #keypoints " << keypoints.size()
<< ", descr " << descriptors.rows << " x " << descriptors.cols;
return ss.str();
} // to_string
}; // class Example
ostream& operator <<(ostream & out, const Example &ex)
{
out << ex.to_string();
return out;
} // operator <<
And this one:
// OLD: class Examples : public std::map<std::string, Example*> {
class Examples {
// New line after Martini's comment
std::map<std::string, Example*> _map;
[...]
void fill() {
// create an example
Example *example = new Example(id, label);
// inputstream in
// Read all the keypoints
cv::KeyPoint p;
for(int i=0; ... ) {
in.read(reinterpret_cast<char *>(&p), sizeof(cv::KeyPoint));
example->keypoints.push_back(p); // push_back copies p
} // for
// ... misc code
cv::Mat descr(n_keypoints, d_size, cv_type, cv::Scalar(1));
// ... read Mat from inputstream in, then assign it to the example
example->descriptors = descr;
// SEE THE OUTPUT BELOW
std::cout << "INSERT THIS: " << (*example) << std::endl;
_map.insert(std::pair<string,Example*>(id, example));
std::cout << "READ THIS: " << *(get_example(id)) << std::endl;
// ... other code
} // fill
// Code modified after Martini's comment.
Example* get_example(const std::string &id) const {
std::map<std::string, Example*>::const_iterator it = _map.find(id);
if( it == _map.end()) {
// ... manage error
// ... then exit
} // if
return it->second;
} // get_example
} // class Examples
The output from the insert/get lines is:
INSERT THIS: #122225084 (label) - #keypoints 711, descr 711 x 128
READ THIS: #122225084 (label) - #keypoints 0, descr 0 x 0
In the insert I had a pointer to an example with 711 keypoints and a 711x128 descriptor matrix. If I read the example using its ID right after the insert, I get a pointer to an example with 0 keypoints and an empty matrix.
What am I doing wrong?
Looking into your code one possible explanation is that you already have element in the map with the same key. To diagnose that first of all print value of pointer before you add object and after that (something like this):
std::cout << "INSERT THIS: " << (void *)example << " " << (*example) << std::endl;
_map.insert(std::pair<string,Example*>(id, example));
std::cout << "READ THIS: " << (void *)get_example(id) << " " << *(get_example(id)) << std::endl;
Next or another way is to check result of insert:
if( !_map.insert(std::pair<string,Example*>(id, example)).second )
std::cout << "ERROR: example:" << id << " is already there";
If you want to override element unconditionally you can use oprator[]:
_map[ id ] = example;
If there are really duplicates you will get memory leak (you are getting it anyway) so I would strongly recommend to use smart pointer to store data in your map.
I'm trying to write a copy constructor for an object managing a STL map containing pointers, where the key is a string. However, when I attempt to insert new values in the map, the pointers are set to NULL:
// ...
for(std::map<std::string, data_base*, order>::const_iterator it = other.elements.begin();
it != other.elements.end(); ++it){
data_base *t = it->second->clone();
std::cout << "CLONE: " << std::hex << t << std::endl;
elements[it->first] = t;
std::cout << "INSERTED: " << std::hex << elements[it->first] << std::endl;
}
// ...
other is the object being copied and elements the map. The clone() method returns a pointer to a new object (via new).
Running the code above I get something like:
CLONE: 0xcfbbc0
INSERTED: 0
I'm not a very experienced programmer and this issue is probably simple to fix, but I didnt find any solution to it searching around.
Thanks a lot for your time.
I don't see any problem with this code, other than maybe
std::map<std::string, data_base*, order>::const_iterator it
Here order gives the key comparator to use to sort the pairs contained in the map (often implemented as a tree).
Maybe you're doing something wrong in it, making your [] operator don't find the right ke, making your last line logging a new pair with a null ptr.
First, try without that order, using the default key-comparator (std::less), then if it don't work, post your order definition and the map declaration. If it's not enough, just provide a simple complete program that reproduce the problem.
I just wrote a simple similar test, using the default key-comparator :
#include <map>
#include <string>
#include <iostream>
struct Data
{
int k;
Data* clone() { return new Data(); }
};
typedef std::map< std::string, Data* > DataMap;
DataMap data_map;
int main()
{
data_map[ "hello" ] = new Data();
data_map[ "world" ] = new Data();
DataMap other_map;
for( DataMap::const_iterator it = data_map.begin(); it != data_map.end(); ++it)
{
Data*t = it->second->clone();
std::cout << "CLONE: " << std::hex << t << std::endl;
other_map[it->first] = t;
std::cout << "INSERTED: " << std::hex << other_map[it->first] << std::endl;
}
std::cin.ignore();
return 0;
}
On VS2010SP1, this outputs :
CLONE: 00034DD0
INSERTED: 00034DD0
CLONE: 00035098
INSERTED: 00035098
So it should be the problem, or maybe you're doing something wrong before.
Try this out, to help debug the issue. I'd recommend double-checking that the order function is correct. You can remove it to use std::less<T>, which is known to work.
// ...
typedef std::map<std::string, data_base*, order> string_db_map;
for(string_db_map::const_iterator it = other.elements.begin();
it != other.elements.end();
++it)
{
data_base *t = it->second->clone();
std::cout << "CLONE: " << std::hex << t << std::endl;
std::pair<string_db_map::iterator, bool) result = elements.insert(
string_db_map::value_type( it->first, t));
if ( !result.second )
{
std::cout << "element['" << it->first << "'] was already present, and replaced." << std::endl;
}
std::coud << "INSERTED [iterator]: " << std::hex << (*result.first).second << std::endl;
std::cout << "INSERTED [indexed]: " << std::hex << elements[it->first] << std::endl;
}
// ...