Sending data to Xively using Arduino mega + Gprs shield sim 900 - arduino

Question

I am using Arduino Mega2560 and GPRS Shield Sim900 and trying to send data to Xively using the code below.
What should I change in this code to send data to Xively with Sim900 GPRS Shield?
/*
* Xively WiFi Sensor Tutorial
*
* This sketch is designed to take sensors (from photocell) and
* upload the values to Xively at consistant intervals. At the
* same time it gets a setable value from Xively to adjust the
* brigthness of an LED. This sketch is reusable and can be
* adapted for use with many different sensors.
*
* Derived from Xively Ardino Sensor Client by Sam Mulube.
*
* By Calum Barnes 3-4-2013
* BSD 3-Clause License - [http://opensource.org/licenses/BSD-3-Clause]
* Copyright (c) 2013 Calum Barnes
*/
#include <SPI.h>
#include <WiFi.h>
#include <HttpClient.h>
#include <Xively.h>
char ssid[] = "SSID_HERE"; // your network SSID (name)
char pass[] = "PASS_HERE"; // your network password (use for WPA, or use as key for WEP)
int keyIndex = 0; // your network key Index number (needed only for WEP)
int status = WL_IDLE_STATUS;
// Your Xively key to let you upload data
char xivelyKey[] = "API_KEY_HERE";
//your xively feed ID
#define xivelyFeed FEED_ID_HERE
//datastreams
char sensorID[] = "LIGHT_SENSOR_CHANNEL";
char ledID[] = "LED_CHANNEL";
// Analog pin which we're monitoring (0 and 1 are used by the Ethernet shield)
#define sensorPin A2
//led connected pin
#define ledPin 9
// Define the strings for our datastream IDs
XivelyDatastream datastreams[] = {
XivelyDatastream(sensorID, strlen(sensorID), DATASTREAM_FLOAT),
XivelyDatastream(ledID, strlen(ledID), DATASTREAM_FLOAT),
};
// Finally, wrap the datastreams into a feed
XivelyFeed feed(xivelyFeed, datastreams, 2 /* number of datastreams */);
WiFiClient client;
XivelyClient xivelyclient(client);
void printWifiStatus() {
// print the SSID of the network you're attached to:
Serial.print("SSID: ");
Serial.println(WiFi.SSID());
// print your WiFi shield's IP address:
IPAddress ip = WiFi.localIP();
Serial.print("IP Address: ");
Serial.println(ip);
// print the received signal strength:
long rssi = WiFi.RSSI();
Serial.print("signal strength (RSSI):");
Serial.print(rssi);
Serial.println(" dBm \n");
}
void setup() {
// put your setup code here, to run once:
Serial.begin(9600);
//pin setup
pinMode(sensorPin, INPUT);
pinMode(ledPin, OUTPUT);
Serial.println("Starting single datastream upload to Xively...");
Serial.println();
// attempt to connect to Wifi network:
while (status != WL_CONNECTED) {
Serial.print("Attempting to connect to SSID: ");
Serial.println(ssid);
status = WiFi.begin(ssid, keyIndex, pass);
// wait 10 seconds for connection:
delay(10000);
}
Serial.println("Connected to wifi");
printWifiStatus();
}
void loop() {
//adjust LED level. set from Xively
int getReturn = xivelyclient.get(feed, xivelyKey); //get data from xively
if (getReturn > 0) {
Serial.println("LED Datastream");
Serial.println(feed[1]);
} else {
Serial.println("HTTP Error");
}
//write value to LED - change brightness
int level = feed[1].getFloat();
if (level < 0) {
level = 0;
} else if (level > 255) {
level = 255;
}
//actually write the value
digitalWrite(ledPin, level);
//read sensor values
int sensorValue = analogRead(sensorPin);
datastreams[0].setFloat(sensorValue);
//print the sensor valye
Serial.print("Read sensor value ");
Serial.println(datastreams[0].getFloat());
//send value to xively
Serial.println("Uploading it to Xively");
int ret = xivelyclient.put(feed, xivelyKey);
//return message
Serial.print("xivelyclient.put returned ");
Serial.println(ret);
Serial.println("");
//delay between calls
delay(15000);
}