Parsing HTTP get response to extract values

Question

Basically I created a web page to control 4 channel relay, then I made an HTTP get request from that page and got the following response:

Linked
T+CIPSEND=4,50
> GET /ard/sensor.html HTTP/1.1
Host: localho
SEND OK
+IPD,4,351:HTTP/1.1 200 OK
Date: 2018年4月07日 01:22:58 GMT
Server: Apache/2.4.9 (Win32) PHP/5.5.12
Last-Modified: 2018年4月07日 01:17:13 GMT
ETag: "72-56937eec1c277"
Accept-Ranges: bytes
Content-Length: 114
Content-Type: text/html
<p> Relay1 = 1# </p> </br>
<p> Relay2 = 0# </p> </br>
<p> Relay3 = 0# </p> </br>
<p> Relay4 = 1# </p> </br>
OK
OK
Unlink

Now I want to store Relay1 status (which is 1 here) in a variable, also all other Relays in order to control them high or low Note : # is supposed to be the separator and here is my code:

 #include <SoftwareSerial.h>
const int LED_PIN = 13;
String ssid = "ABC";
String password = "a14102016a";
const byte rxPin = 6;
const byte txPin = 7;
SoftwareSerial ESP8266 (rxPin, txPin);
unsigned long lastTimeMillis = 0;
void setup() {
 Serial.begin(9600);
 ESP8266.begin(9600);
 reset();
 connectWifi();
 delay(2000);
}
//reset the esp8266 module
void reset() {
 ESP8266.println("AT+RST");
 ESP8266.println("AT+CIOBAUD=9600");
 delay(1000);
 if (ESP8266.find("OK") ) Serial.println("Module Reset");
}
//connect to your wifi network
void connectWifi() {
 String cmd = "AT+CWJAP=\"" + ssid + "\",\"" + password + "\"";
 ESP8266.println(cmd);
 delay(4000);
 if (ESP8266.find("OK")) {
 Serial.println("Connected!");
 }
 else {
 connectWifi();
 Serial.println("connect to wifi");
 }
}
void printResponse() {
 while (ESP8266.available()) {
 Serial.println(ESP8266.readStringUntil('\n'));
 }
}
void loop() {
 if (millis() - lastTimeMillis > 30000) {
 lastTimeMillis = millis();
 ESP8266.println("AT+CIPMUX=1");
 delay(1000);
 printResponse();
 ESP8266.println("AT+CIPSTART=4,\"TCP\",\"192.168.1.161\",80");
 delay(1000);
 printResponse();
 String cmd = "GET /ard/sensor.html HTTP/1.1\r\nHost: localhost";
 ESP8266.println("AT+CIPSEND=4," + String(cmd.length() + 4));
 delay(1000);
 ESP8266.println(cmd);
 delay(1000);
 ESP8266.println(""); 
 }
 if (ESP8266.available()) {
 Serial.write(ESP8266.read());
Byte R[ ] = ESP8266.read () ;
Serial write (R[ ]);
// if (readString.indexOf("Relay1=1">0)) 
{
 Serial.write("found");
}
 }
}

Any suggestions ?

• Juraj: why do you send it as html like text? send 1001

Answer 1

First of all, I would avoid using the String class, because of the memory fragmentation issues that come with it. The most obvious solution is probably to store the result in a character array, then parse the array, with strstr() or sscanf(), to extract the values you want. However, this requires storing the whole response in RAM, which is expensive. A better option is to parse the string as it comes, character by character, using a kind of finite state machine.

Here is how I would do it. First, I assume you know what you want to do with these values. Since I don't, I will instead use this dummy function to just print the values out to the serial port:

static void parse_relay_callback(uint8_t index, uint8_t value)
{
 Serial.print("parsed: relay ");
 Serial.print(index);
 Serial.print(": ");
 Serial.println(value);
}

You will have to replace this with whatever is appropriate for your application.

Then here is the parser:

static void parse_relay(char c)
{
 static const char templ[] = "RelayI = V#"; // I: index, V: value
 static uint8_t pos, index, value;
 char expected = templ[pos++];
 if (expected == 'I' && c >= '0' && c <= '4')
 index = c - '0';
 else if (expected == 'V' && (c == '0' || c == '1'))
 value = c - '0';
 else if (expected == '#' && c == '#')
 parse_relay_callback(index, value);
 else if (c != expected)
 pos = 0;
}

You just feed it one character at a time, with something like:

while (ESP8266.available())
 parse_relay(ESP8266.read());

Feeding the parser with your response example gives the following on the serial port:

parsed: relay 1: 1
parsed: relay 2: 0
parsed: relay 3: 0
parsed: relay 4: 1