Arduino turn char into a string - arduino

Question

I am trying to turn a char into a string so I can extract the values I am interested in, however it just appears empty.
The variable I am interested in is content.
I am performing a get and it returns a JSON object. And want to extract the sunrise and sunset values.
#include <SPI.h>
#include <Ethernet2.h>
#include <String.h>
#include <stdlib.h>
EthernetClient client;
const char* server = "api.sunrise-sunset.org"; // server's address
const char* resource = "/json?lat=53.440&lng=0.200&date=today"; // http resource
const unsigned long BAUD_RATE = 9600; // serial connection speed
const unsigned long HTTP_TIMEOUT = 10000; // max respone time from server
const size_t MAX_CONTENT_SIZE = 512; // max size of the HTTP response
// ARDUINO entry point #1: runs once when you press reset or power the board
void setup() {
initSerial();
initEthernet();
}
// ARDUINO entry point #2: runs over and over again forever
void loop() {
if (connect(server)) {
if (sendRequest(server, resource) && skipResponseHeaders()) {
char response[MAX_CONTENT_SIZE];
String str(response);
Serial.println(str);
char* field;
char* sunset;
char* sunrise;
field = strtok(response,"{,");
while (field != NULL)
{
field = strtok (NULL, ",}");
if(field != NULL)
{
if(strstr(field, "sunrise") != NULL)
{
int length = strlen(field);
sunrise = new char[length + 1];
strncpy(sunrise, field, length + 1); // +1 to copy a terminating null as well
}
if(strstr(field, "sunset") != NULL)
{
int length = strlen(field);
sunset = new char[length + 1];
strncpy(sunset, field, length + 1); // +1 to copy a terminating null as well
}
}
}
//Serial.println("SUNRISE DATA: %s\n\n", sunrise);
//Serial.println("SUNSET DATA: %s\n\n", sunset);
free(sunrise); // do not forget to free the memory if not needed anymore
free(sunset); // do not forget to free the memory if not needed anymore
}
disconnect();
}
wait();
}
// Initialize Serial port
void initSerial() {
Serial.begin(BAUD_RATE);
while (!Serial) {
; // wait for serial port to initialize
}
Serial.println("Serial ready");
}
// Initialize Ethernet library
void initEthernet() {
byte mac[] = {0xDE, 0xAD, 0xBE, 0xEF, 0xFE, 0xED};
byte ip[] = { 192,168,0,202 };
if (!Ethernet.begin(mac)) {
Serial.println("Failed to configure Ethernet");
return;
}
Serial.println("Ethernet ready");
delay(1000);
}
// Open connection to the HTTP server
bool connect(const char* hostName) {
Serial.print("Connect to ");
Serial.println(hostName);
bool ok = client.connect(hostName, 80);
Serial.println(ok ? "Connected" : "Connection Failed!");
return ok;
}
// Send the HTTP GET request to the server
bool sendRequest(const char* host, const char* resource) {
Serial.print("GET ");
Serial.println(resource);
client.print("GET ");
client.print(resource);
client.println(" HTTP/1.1");
client.print("Host: ");
client.println(server);
client.println("Connection: close");
client.println();
return true;
}
// Skip HTTP headers so that we are at the beginning of the response's body
bool skipResponseHeaders() {
// HTTP headers end with an empty line
char endOfHeaders[] = "\r\n\r\n";
client.setTimeout(HTTP_TIMEOUT);
bool ok = client.find(endOfHeaders);
if (!ok) {
Serial.println("No response or invalid response!");
}
return ok;
}
void disconnect() {
Serial.println("Disconnect");
client.stop();
}
// Pause for a 1 minute
void wait() {
Serial.println("Wait 60 seconds");
delay(60000);
}

Answer 1

I think there is a misunderstanding from your side. Certainly you want to process the response of the server and according to your code, this is char response[MAX_CONTENT_SIZE] where the response is stored.
Now this already is a string, more or less. An array of characters, chars. Definiton from here.
Strings are actually one-dimensional array of characters terminated by a null character '\0'. Thus a null-terminated string contains the characters that comprise the string followed by a null.
You can extract the relevant parts from it straight away.
Your response should look like something like this, according to sunrise-sunset.org/api. Note that I just copied the data into an array for testing purposes.
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#define MAX_CONTENT_SIZE 512
char response[MAX_CONTENT_SIZE] = \
"{\
\"results\":\
{\
\"sunrise\":\"7:27:02 AM\",\
\"sunset\":\"5:05:55 PM\",\
\"solar_noon\":\"12:16:28 PM\",\
\"day_length\":\"9:38:53\",\
\"civil_twilight_begin\":\"6:58:14 AM\",\
\"civil_twilight_end\":\"5:34:43 PM\",\
\"nautical_twilight_begin\":\"6:25:47 AM\",\
\"nautical_twilight_end\":\"6:07:10 PM\",\
\"astronomical_twilight_begin\":\"5:54:14 AM\",\
\"astronomical_twilight_end\":\"6:38:43 PM\"\
},\
\"status\":\"OK\"\
}";
You can easily process it using strtok function from string.h. Using a {, delimiter first will separate "result":{ from "sunrise .... Then you can use a }, delimiter.
With strstr you can check for "sunrise" and "sunset" field, and if you find them you can copy their value into a new string with strncpy.
int main()
{
char* field;
char* sunset;
char* sunrise;
field = strtok(response,"{,");
while (field != NULL)
{
field = strtok (NULL, ",}");
if(field != NULL)
{
if(strstr(field, "sunrise") != NULL)
{
int length = strlen(field);
sunrise = malloc(length * sizeof(char) + 1); // +1 for terminating null character '\0'
strncpy(sunrise, field, length + 1); // +1 to copy a terminating null as well
}
if(strstr(field, "sunset") != NULL)
{
int length = strlen(field);
sunset = malloc(length * sizeof(char) + 1); // +1 for terminating null character '\0'
strncpy(sunset, field, length + 1); // +1 to copy a terminating null as well
}
}
}
printf("SUNRISE DATA: %s\n\n", sunrise);
printf("SUNSET DATA: %s\n\n", sunset);
free(sunrise); // do not forget to free the memory if not needed anymore
free(sunset); // do not forget to free the memory if not needed anymore
return 0;
}
The output of this program is:
SUNRISE DATA: "sunrise":"7:27:02 AM"
SUNSET DATA: "sunset":"5:05:55 PM"
You can further process these strings with strtok again if you like. This is just an example code, you can use it to implement your solution.