How can I copy and compare a string?

Question

I have the following code (I realise String isn't very efficient, but this sketch doesn't have to be)

debugOutput gets called every loop, but I only want to spam the serial bus (to debug) if the status changes.

However, it runs once, despite changes to the pins being made.

String prevState = "";
void debugOutput() {
  String s = "";
  //s = s + digitalRead(6) ? "1" : "0";
  //s = s + digitalRead(5) ? "1" : "0";
  s = s + digitalRead(4) ? "1" : "0";
  s = s + digitalRead(3) ? "1" : "0";
  s = s + digitalRead(2) ? "1" : "0";
  if(!prevState.equals(s)) {
    Serial.println("State Change: "+s);
    //strcpy(s, prevState);
    prevState = s;
  }
}

My suspicion is it was because of the pointers being the same, but I declare a new string every time this function runs. Just in case, I tried to swap this to strcpy, which isn't valid since I'm using String instead of character arrays.

What can I do, I'm really confused.

This is the Serial output, btw

State Change: 1 So it's clear to me that my string appending isn't even working as expected.

Answer 1

There is this thing called "operator precedence" that defines the hierarchy of operators.

The ?: will be calculated after the +. That's why

s = s + digitalRead(2) ? "1" : "0";

actually means

s = (s + digitalRead(2)) ? "1" : "0";

But you want

s = s + (digitalRead(2) ? "1" : "0");

Answer 2

I'm not sure why string appending using + was failing, but changing it from

s = s + digitalRead(2) ? "1" : "0";

to

s += digitalRead(2) ? "1" : "0";

fixed my issue.