Accessing JSON data from Open Weater Maps API

[CieloInvernale]

I'm trying to interface with OpenWeaterMaps API (http://openweathermap.org/). All API options and parameters are fully explained, anyway I've got troubles when I came to the Lazarus part (sorry, I'm a newbye with Lazarus...).

The application is simple: retrieve JSON data from the appropriate URL then parse each part as a simple string.

My program 'till now tries to retrieve the "main" data in JSON structure ("Clouds" in the following example JSON code - notice that there are 2 different "main" nodes in the response, I'm trying to access the first one nested within "weather" or, in other words, "weather->main:Clouds").

JSON API's response sample:

Code: Text  [Select][+][-]

1. {"coord":
2. {"lon":145.77,"lat":-16.92},
3. "weather":[{"id":803,"main":"Clouds","description":"broken clouds","icon":"04n"}],
4. "base":"cmc stations",
5. "main":{"temp":293.25,"pressure":1019,"humidity":83,"temp_min":289.82,"temp_max":295.37},
6. "wind":{"speed":5.1,"deg":150},
7. "clouds":{"all":75},
8. "rain":{"3h":3},
9. "dt":1435658272,
10. "sys":{"type":1,"id":8166,"message":0.0166,"country":"AU","sunrise":1435610796,"sunset":1435650870},
11. "id":2172797,
12. "name":"Cairns",
13. "cod":200}
14.  

My code so far (MYCITYID and MYAPPID are placeholders for the real keys, obviously not to be published here...):

Code: Pascal  [Select][+][-]

1. unit Unit1;
2.  
3. {$mode objfpc}{$H+}
4.  
5. interface
6.  
7. uses
8.   Classes, SysUtils, FileUtil, Forms, Controls, Graphics, Dialogs, StdCtrls, fphttpclient,
9.   fpjson, jsonparser;
10.  
11. type
12.  
13.   { TForm1 }
14.  
15.   TForm1 = class(TForm)
16.     Memo1: TMemo;
17.     procedure FormCreate(Sender: TObject);
18.   private
19.     { private declarations }
20.   public
21.     { public declarations }
22.   end;
23.  
24. var
25.   Form1: TForm1;
26.   jData : TJSONData;
27.   jObject : TJSONObject;
28.   wheater_main : String;
29.  
30. implementation
31.  
32. {$R *.lfm}
33.  
34. { TForm1 }
35.  
36. procedure TForm1.FormCreate(Sender: TObject);
37. begin
38.   jData := GetJSON(TFPHTTPClient.SimpleGet('http://api.openweathermap.org/data/2.5/weather?id=MYCITYID&appid=MYAPPID'));
39.  
40.   wheater_main := jData.FindPath('main').AsString;
41.  
42.   Memo1.Lines.Clear;
43.  
44.   Memo1.Lines.Add(wheater_main);
45.  
46. end;
47.  
48. end.
49.  

Code compiles correctly and an executable is produced, anyway when I launch it it returns the following error:

Cannot convert data from object value.

Any idea about what's wrong with my code??

Thanks in advance for your time and attention.

[molly]

The path main is not a complete end path, rather a path to a more complex (sub)structure in json format.

Either access the full path directly by supplying that full path (e.g. "main.pressure") or retrieve path "main" into a separate JSONData structure (jData2 := jData.FindPath('main')) and then search for the path e.g. "pressure").

edit: if your aim is really to get a string representation of the main json data structure then you can use jData.FindPath('main').AsJSon;

[bytebites]

Code: Pascal  [Select][+][-]

1. jData.FindPath('weather').Items[0].FindPath('main').AsString

[molly]

Ah, good point bytebites.

 I overlooked that there were multiple "main"'s 

In that case:

Code: Pascal  [Select][+][-]

1. Writeln('weather[0].main = ' , jData.FindPath('weather[0].main').AsString);

[magleft]

I tried downloading data from openweathermap. The raw data is in the form shown below:

Code: Pascal  [Select][+][-]

1. {"cod":"200","message":0,"cnt":40,"list":[{"dt":1695988800,"main":{"temp":300.37,"feels_like":300.12,"temp_min":300.37,"temp_max":301.04,"pressure":1018,"sea_level":1018,"grnd_level":1015,"humidity":39,"temp_kf":-0.67},"weather":[{"id":500,"main":"Rain","description":"light rain","icon":"10d"}],"clouds":{"all":31},"wind":{"speed":3.12,"deg":231,"gust":2.49},"visibility":10000,"pop":0.22,"rain":{"3h":0.17},"sys":{"pod":"d"},"dt_txt":"2023-09-29 12:00:00"},{"dt":1695999600,"main":{"temp":297.16,"feels_like":297.13,"temp_min":295.72,"temp_max":297.16,"pressure":1018,"sea_level":1018,"grnd_level":1016,"humidity":58,"temp_kf":1.44},"weather":[{"id":500,"main":"Rain","description":"light rain","icon":"10d"}],"clouds":{"all":41},"wind":{"speed":2.08,"deg":357,"gust":4.8},"visibility":10000,"pop":0.71,"rain":{"3h":1.47},"sys":{"pod":"d"},"dt_txt":"2023-09-29 15:00:00"},{"dt":1696010400,"main":{"temp":291.93,"feels_like":292,"temp_min":291.93,"temp_max":291.93,"pressure":1019,"sea_level":1019,"grnd_level":1017,"humidity":82,"temp_kf":0},"weather":[{"id":501,"main":"Rain","description":"moderate rain","icon":"10n"}],"clouds":{"all":54},"wind":{"speed":2.85,"deg":15,"gust":4.62},"visibility":10000,"pop":0.9,"rain":{"3h":3.45},"sys":{"pod":"n"},"dt_txt":"2023-09-29 18:00:00"},{"dt":1696021200,"main":{"temp":292.09,"feels_like":292.02,"temp_min":292.09,"temp_max":292.09,"pressure":1019,"sea_level":1019,"grnd_level":1017,"humidity":76,"temp_kf":0},"weather":[{"id":500,"main":"Rain","description":"light rain","icon":"10n"}],"clouds":{"all":38},"wind":{"speed":1.82,"deg":57,"gust":2.37},"visibility":10000,"pop":0.6,"rain":{"3h":0.16},"sys":{"pod":"n"},"dt_txt":"2023-09-29 21:00:00"},{"dt":1696032000,"main":{"temp":292.82,"feels_like":292.72,"temp_min":292.82,"temp_max":292.82,"pressure":1018,"sea_level":1018,"grnd_level":1016,"humidity":72,"temp_kf":0},"weather":[{"id":803,"main":"Clouds","description":"broken clouds","icon":"04n"}],"clouds":{"all":68},"wind":{"speed":1.55,"deg":21,"gust":1.54},"visibility":10000,"pop":0.34,"sys":{"pod":"n"},"dt_txt":"2023-09-30 00:00:00"},{"dt":1696042800,"main":{"temp":292.45,"feels_like":292.32,"temp_min":292.45,"temp_max":292.45,"pressure":1017,"sea_level":1017,"grnd_level":1015,"humidity":72,"temp_kf":0},"weather":[{"id":500,"main":"Rain","description":"light rain","icon":"10n"}],"clouds":{"all":100},"wind":{"speed":1.55,"deg":35,"gust":1.52},"visibility":10000,"pop":0.37,"rain":{"3h":0.11},"sys":{"pod":"n"},"dt_txt":"2023-09-30 03:00:00"},
2.  

I tried the following code without success:

Code: Pascal  [Select][+][-]

1.   Http := TFPHTTPClient.Create(nil);
2.    try
3.      Http.IOTimeout := 1000;
4.      Http.AllowRedirect := True;
5.      jData := GetJSON(Http.Get(urltext));
6.      Memo1.Lines.Add(jData.FindPath('list.items[0].main.items.temp').AsString);
7.     Json.Free;
8.    finally
9.      Http.Free;
10.    end;
11.  
12.  

Can anyone help?

[paweld]

In your case, the path should look like this:

Code: Pascal  [Select][+][-]

1. Memo1.Lines.Add(jData.FindPath('list[0].main.temp').AsString);

clearer example:

Code: Pascal  [Select][+][-]

1. uses
2.   fpjson;
3.  
4. procedure TForm1.Button1Click(Sender: TObject);
5. const
6.   json = '{"cod":"200","message":0,"cnt":40,"list":[{"dt":1695988800,"main":{"temp":300.37,"feels_like":300.12,"temp_min":300.37,"temp_max":301.04,"pressure":1018,"sea_level":1018,"grnd_level":1015,"humidity":39,"temp_kf":-0.67},"weather":[{"id":500,"main":"Rain","description":"light rain","icon":"10d"}],"clouds":{"all":31},"wind":{"speed":3.12,"deg":231,"gust":2.49},"visibility":10000,"pop":0.22,"rain":{"3h":0.17},"sys":{"pod":"d"},"dt_txt":"2023-09-29 12:00:00"},{"dt":1695999600,"main":{"temp":297.16,"feels_like":297.13,"temp_min":295.72,"temp_max":297.16,"pressure":1018,"sea_level":1018,"grnd_level":1016,"humidity":58,"temp_kf":1.44},"weather":[{"id":500,"main":"Rain","description":"light rain","icon":"10d"}],"clouds":{"all":41},"wind":{"speed":2.08,"deg":357,"gust":4.8},"visibility":10000,"pop":0.71,"rain":{"3h":1.47},"sys":{"pod":"d"},"dt_txt":"2023-09-29 15:00:00"},{"dt":1696010400,"main":{"temp":291.93,"feels_like":292,"temp_min":291.93,"temp_max":291.93,"pressure":1019,"sea_level":1019,"grnd_level":1017,"humidity":82,"temp_kf":0},"weather":[{"id":501,"main":"Rain","description":"moderate rain","icon":"10n"}],"clouds":{"all":54},"wind":{"speed":2.85,"deg":15,"gust":4.62},"visibility":10000,"pop":0.9,"rain":{"3h":3.45},"sys":{"pod":"n"},"dt_txt":"2023-09-29 18:00:00"},{"dt":1696021200,"main":{"temp":292.09,"feels_like":292.02,"temp_min":292.09,"temp_max":292.09,"pressure":1019,"sea_level":1019,"grnd_level":1017,"humidity":76,"temp_kf":0},"weather":[{"id":500,"main":"Rain","description":"light rain","icon":"10n"}],"clouds":{"all":38},"wind":{"speed":1.82,"deg":57,"gust":2.37},"visibility":10000,"pop":0.6,"rain":{"3h":0.16},"sys":{"pod":"n"},"dt_txt":"2023-09-29 21:00:00"},{"dt":1696032000,"main":{"temp":292.82,"feels_like":292.72,"temp_min":292.82,"temp_max":292.82,"pressure":1018,"sea_level":1018,"grnd_level":1016,"humidity":72,"temp_kf":0},"weather":[{"id":803,"main":"Clouds","description":"broken clouds","icon":"04n"}],"clouds":{"all":68},"wind":{"speed":1.55,"deg":21,"gust":1.54},"visibility":10000,"pop":0.34,"sys":{"pod":"n"},"dt_txt":"2023-09-30 00:00:00"},{"dt":1696042800,"main":{"temp":292.45,"feels_like":292.32,"temp_min":292.45,"temp_max":292.45,"pressure":1017,"sea_level":1017,"grnd_level":1015,"humidity":72,"temp_kf":0},"weather":[{"id":500,"main":"Rain","description":"light rain","icon":"10n"}],"clouds":{"all":100},"wind":{"speed":1.55,"deg":35,"gust":1.52},"visibility":10000,"pop":0.37,"rain":{"3h":0.11},"sys":{"pod":"n"},"dt_txt":"2023-09-30 03:00:00"}]}';
7. var
8.   jd: TJSONData;
9.   ja: TJSONArray;
10.   jo: TJSONObject;
11.   i: Integer;
12. begin
13.   jd := GetJSON(json);
14.   if jd.FindPath('list') <> nil then
15.   begin
16.     ja := TJSONArray(jd.FindPath('list'));
17.     if ja.Count > 0 then
18.     begin
19.       jo := TJSONObject(ja.Items[0]);
20.       if jo.FindPath('main.temp') <> nil then
21.         Memo1.Lines.Add(Format('%5d     %f', [i, jo.FindPath('main.temp').AsFloat]));
22.     end;
23.     ////or all items
24.     //for i := 0 to ja.Count - 1 do
25.     //begin
26.     //  jo := TJSONObject(ja.Items[i]);
27.     //  if jo.FindPath('main.temp') <> nil then
28.     //    Memo1.Lines.Add(Format('%5d     %f', [i, jo.FindPath('main.temp').AsFloat]));
29.     //end;
30.   end;
31.   jd.Free;
32. end;
33.  

[TRon]

Also note that you get a list of 40! entries. It is a forecast based upon this. I have no idea if the data in the list is sorted in a particular order (I assume it is returned with a sort on dt values).

[magleft]

Indeed, it concerns 40 registrations. I modified the example paweld sent a bit, but it doesn't work. What am I doing wrong;

Code: Pascal  [Select][+][-]

1. procedure TFwheather.Button2Click(Sender: TObject);
2. var
3.   Http: TFPHTTPClient;
4.   info, Content: string;
5.   Json: TJSONData;
6.   JsonObject: TJSONObject;
7.   wheater_main : String;
8.   jdata:TJSONData;
9.   enum: TJSONEnum;
10.   jd: TJSONData;
11.   ja: TJSONArray;
12.   jo: TJSONObject;
13.   i: Integer;
14.   urltext :String;
15. begin
16.   urltext:='api.openweathermap.org/data/2.5/forecast?lat=39.1520282768&lon=20,9768486023&APPID=45a17f4586f55c714ae04d9c36e8a834' ;//{API key}
17.   try
18.   Http.IOTimeout := 1000;
19.   Http.AllowRedirect := True;
20.   jd := GetJSON(Http.Get(urltext));
21.   if jd.FindPath('list') <> nil then
22.   begin
23.     ja := TJSONArray(jd.FindPath('list'));
24.     if ja.Count > 0 then
25.  {   begin
26.       jo := TJSONObject(ja.Items[0]);
27.       if jo.FindPath('main.temp') <> nil then
28.         Memo1.Lines.Add(Format('%5d     %f', [i, jo.FindPath('main.temp').AsFloat]));
29.     end;
30. }    ////or all items
31.     for i := 0 to ja.Count - 1 do
32.     begin
33.       jo := TJSONObject(ja.Items[i]);
34.       if jo.FindPath('main.temp') <> nil then
35.         Memo1.Lines.Add(Format('%5d     %f', [i, jo.FindPath('main.temp').AsFloat]));
36.     end;
37.   end;
38.  
39.   finally
40.   jd.Free;
41.   end;
42. end;
43.  
44.  

[rvk]

Indeed, it concerns 40 registrations. I modified the example paweld sent a bit, but it doesn't work. What am I doing wrong;

Well. The first thing you are doing wrong is not telling us what the problem is 
Error message????? Crash????
What does happen?

Second... looking at your code... I see a http declaration in the procedure but I don't see you creating the object itself.
So you are missing a

Code: Pascal  [Select][+][-]

1. http := TFPHTTPClient.Create(nil);
2. try
3.   //...
4. finally
5.   http.free;
6. end;
7.  

Also... urltext needs to contain a protocol (so https:// in front of it).

And don't forget to include opensslsockets in your uses.

[paweld]

First of all, don't share your API key - they can block your access.

Code: Pascal  [Select][+][-]

1. uses
2.   fpjson, fphttpclient, opensslsockets;
3.  
4. procedure TForm1.Button1Click(Sender: TObject);
5. const
6.   url = 'https://api.openweathermap.org/data/2.5/forecast?lat=39.1520282768&lon=20,9768486023&APPID={yourAPPID}';
7. var
8.   jd: TJSONData;
9.   ja: TJSONArray;
10.   jo: TJSONObject;
11.   i: Integer;
12.   hc: TFPHttpClient;
13.   json: String;
14. begin
15.   hc := TFPHttpClient.Create(nil);
16.   hc.AllowRedirect := True;
17.   try
18.     json := hc.Get(url);
19.     if json <> '' then
20.     begin
21.       jd := GetJSON(json);
22.       if jd.FindPath('list') <> nil then
23.       begin
24.         ja := TJSONArray(jd.FindPath('list'));
25.         for i := 0 to ja.Count - 1 do
26.         begin
27.           jo := TJSONObject(ja.Items[i]);
28.           if jo.FindPath('main.temp') <> nil then
29.             Memo1.Lines.Add(Format('%5d     %f', [i, jo.FindPath('main.temp').AsFloat]));
30.         end;
31.       end;
32.       jd.Free;
33.     end
34.     else
35.       ShowMessage('Empty response');
36.   finally
37.     hc.Free;
38.   end;
39. end;    

[magleft]

I would like to apologize as the code was not working due to my mistake. Due to an error in the url I didn't have the prefix ' https://'. Now all works.
Thank you all for your help.