A1: No, you need not use a proxy-file for this API
If one tries the API call, using a published Quandl syntax of:
<pragma>://<URL.ip>/<relative.URL>[?<par.i>=<val.i>[&<par.j>=<val.j>[&...]]]
the server side will push you the content of:
Date,Value
2013-12-31,4.0
So, your code may use Quandl API with like this:
void OnStart()
{
string cookie = NULL,
headers;
char post[],
result[];
int res;
/* TODO: *
* Must allow MT4 to access the server URL, *
* you should add URL "https://www.quandl.com/api/v3/datasets/ADB/LAB_UNEMP_JPN.csv" *
* in the list of allowed URLs *
* ( MT4 -> Tools -> Options -> [Tab]: "Expert Advisors" ): */
string aDataSOURCE_URL = "https://www.quandl.com/api/v3/datasets/ADB/LAB_UNEMP_JPN.csv";
string aDataSOURCE_API = "rows = 1&api_key=<My_API_Key>";
//-- Create the body of the POST request for API specifications and API-authorization
ArrayResize( post,
StringToCharArray( aDataSOURCE_API, // string text |--> [in] String to copy.
post, // uchar &array[] <--| [out] Array of uchar type.
0, // int start = 0 |--> [in] Position from which copying starts. Default - 0.
WHOLE_ARRAY, // int count = -1 |--> [in] Number of array elements to copy. Defines length of a resulting string. Default value is -1, which means copying up to the array end, or till terminating '\0'. Terminating zero will also be copied to the recipient array, in this case the size of a dynamic array can be increased if necessary to the size of the string. If the size of the dynamic array exceeds the length of the string, the size of the array will not be reduced.
CP_UTF8 // uint cp = CP_ACP |--> [in] The value of the code page. For the most-used code pages provide appropriate constants.
)
- 1
);
//-- Reset the last error code
ResetLastError();
//-- Loading a html page from Quandl
int timeout = 5000; //-- Timeout below 1000 (1 sec.) is not enough for slow Internet connection
res = WebRequest( "POST", // const string method |--> [in] HTTP method.
aDataSOURCE_URL, // const string URL |--> [in] URL.
cookie, // const string cookie |--> [in] Cookie value.
NULL, // const string referrer |--> [in] Value of the Referer header of the HTTP request.
timeout, // int timeout |--> [in] Timeout in milliseconds.
post, // const char &data |--> [in] Data array of the HTTP message body
ArraySize( post ), // int data_size |--> [in] Size of the data[] array.
result, // char &result <--| [out] An array containing server response data.
headers // string &result_headers <--| [out] Server response headers.
);
//-- Check errors
if ( res == -1 )
{ Print( "WebRequest Error. Error code = ", GetLastError() ); //-- Perhaps the URL is not listed, display a message about the necessity to add the address
MessageBox( "Add the address '" + aDataSOURCE_URL + "' in the list of allowed URLs on tab 'Expert Advisors'", "Error", MB_ICONINFORMATION );
}
else //-- Load was successfull
{
PrintFormat( "The data has been successfully loaded, size = %d bytes.", ArraySize( result ) );
//-- parse the content ---------------------------------------
/*
"Date,Value
2013-12-31,4.0"
*/
//-- consume the content -------------------------------------
//...
}
}
There are 4 principal items to take care of:
0: an MT4 permission to use a given URL
1: an API URL setup - <pragma>://<URL.ip>/<relative.URL>
2: an API const char &data[] assy. [?<par.i>=<val.i>[&<par.j>=<val.j>[&...]]]
3: an API int data_size length calculation
Addendum: This is more a list of reasons, why avoiding use of the New-MQL4.56789 WebRequest() function variants:
Whereas MQL4 documentation promises a simple use of WebRequest() function variants, (cit.:) "1. Sending simple requests of type "key=value" using the header Content-Type: application/x-www-form-urlencoded.", the reality is far from a promised simple use-case:
0: DONE: an MT4 administrative step ( weakness: cannot enforce MT4 to communicate { http | https } protocol(s) over other than their default port(s) ~ { :80 | :443 }
1: URL consists of two ( three, if using a :port specifier, which does not work in MT4 (ref. 0: right above ) ) parts. <URL.ip_address> is the first one and can be expressed in a canonical IPv4 form ( 10.38.221.136 ) or in a DNS-translateable form ( MT4_APAC_PRIMARY.broker.com ). The second part, the <relative.URL>, specifies the HttpServer itself, where to locate a file ( it is a HttpServer--relative file location ). Published WebRequest permit to use the both parts joined together, ref. aDataSOURCE_URL.
3: WebServer API, if constructed so, may permit to add some additional parameters, that can be specified and presented to the WebServer. The presentation depends whether the { HTTP GET | HTTP POST } protocol-option is selected in on a caller side.
4: each call to MT4 WebRequest() also requires the caller to specify a length of a data content parameter ( ref. the use of ArraySize( post ), // int data_size )
