Unable to store stream data in variable in Arduino programming - arduino

Question

I'm unable to store the serial.port value in a variable. I want to send a message from Android telnet app, on and off. If on comes I want to print fan on, if off comes I want to print off. I'm able to print on and off while I'm statically fixing value. I'm unable to store the stream in a variable.
String stringOne;
void setup() {
digitalWrite(13, LOW);
// Open serial communications and wait for port to open:
Serial.begin(9600);
while (!Serial) {
; // wait for serial port to connect. Needed for native USB port only
}
// send an intro:
Serial.println("\n\nString substring():");
Serial.println();
pinMode(13,OUTPUT);
}
void loop() {
digitalWrite(13,LOW);
// Set up a String:
stringOne ="+IPD 0,14 :ON";
int length = stringOne.length();
Serial.println(stringOne.length());
Serial.println(stringOne);
if (Serial.available() > 0) {
// substring(index) looks for the substring from the index position to the end:
if (stringOne.substring(length-2,length) == "ON") {
Serial.println("FAN ON");
digitalWrite(13,HIGH);
// delay(2000);
}
if (stringOne.substring(length-3,length) == "OFF") {
Serial.println("FAN OFF");
digitalWrite(13,LOW);
// delay(2000);
}
}
// you can also look for a substring in the middle of a string:
// do nothing while true:
while (true);
}

Answer 1

Well, while editing your question, I couldnt help noticing that infinite loop at the end of your code.
// do nothing while true:
while(true)
In this case, even if your code was all right, you cant expect to get next data.
void loop --> Remember it is itself a infinite loop
update 1:
your logic to use the serial port is wrong;
Remember, serial port only recieves a single character at a time.
if you send "hello" from pc, at the other end, arduino will recieve h, e, l, l, o
The trick is to collect all letters into a array. and then use our logic in it.
char commandbuffer[20]; //an array to hold our characters
int i=0;
if (Serial.available() > 0) {
while( Serial.available() >0) { //read until all data we send arrives
char c = Serial.read();
commandbuffer[i]= c; //we are actually storing it one by one
i++;
}
}
commandbuffer[i]='\n';
for(int j = 0; j<i; j++){
Serial.print(commandbuffer[j]);// and show it one by one too
}
now when you send "hello", it will print hello back. I hope this give you some idea. Happy coding.