String += Help

Projects Programming
1

Been trying to figure out what this line does in the code below. I've done a log of reverse engineering and found that if I set the (val) to (1) I get the high response or whatever I put in that first "" same thing goes if I put (0) in the place of Val I get Low or whatever I change the word to... But just don't understand how its working because I can't do s += (val)?"high":"Low":"LowLow";

s += (val)?"high":"Low";

so really just want to know what the ? does. I know this is probably really basic but looked on the leaning center under strings and can't figure it out.

thanks let me know if you need more info.

int val; // create the Integer val
if (req.indexOf("/gpio/0") != -1) //if the data sent to the webserver "request" matches /gpio/0"
{
val = 0; // Set val to 0
Serial.println(val); // print the val to serial
}
else if (req.indexOf("/gpio/1") != -1) // if the data sent to the webserver "request" matches /gpio/1"
{
val = 1; // Set val to 0
Serial.println(val); // print the val to serial
}
else { // for everything else print invalid request
Serial.println("invalid request");
client.stop();
return;
}

// Prepare the response
String s = "HTTP/1.1 200 OK\r\nContent-Type: text/html\r\n\r\n\r\n\r\nGPIO is now "; // http response
s += (val)?"high":"Low";
s += "\n";

// Send the response to the client
client.print(s);
delay(1);
Serial.println("Client disonnected");

2

Ternary operator

3 
s += (val)?"high":"Low";

If val is true append "high" to the String s else append "Low" to it

The C ternary operator in all its glory.

4

The ternary operator is shorthand for an if-else block. In your case:

   if (val != 0) {
      s += "high";      // s = s + "high"
   } else {
      s += "Low";       // s = s + "Low"
   }

That is, if the logical test on val is logic True (i.e., non-zero), "high" is added to s. If val is 0, then "Low" is added to s. Evidently, s is a String variable.

5

It is indeed a ternary operator:

<condition> ? <value if true> : <value if false>;

A simple example would be:

int valueA = 12;
int valueB = 7;
Serial.println((valueA > 10) ? "Greater than 10" : "Less than 10");
Serial.println((valueB > 10) ? "Greater than 10" : "Less than 10");

This will print:

Greater than 10
Less than 10

P.S: Please use code tags, using the </> button or by using [code][/code] around your code.

6

Syntax details...
Notice that a single question mark separates the boolean test from the values.
Notice that a SINGLE colon is used to separate the two (and only TWO) values.

7

It adds either one to the string. Then keeps adding stuff to the string before sending it to the browser to show one of two messages.

8

Wow first off thank you all that responded. I didn't have alerts on this post and didn't think anyone was responding.

I'm a network engineer and I have super limited experience with any programming so all the help is much appreciated.

Fully understand how it works now really motivated to keep learning now that I know there are many people to help when needed.

Thanks again.