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.

• Delta_G: Why are you using a String for this? Just save the pin states and compare them and if any have changed send a message. Take out the intermediate step of creating a String from that data and compare the data directly.
• KIIV: Why using a String? Anyway making a copy si as simple as prevState = s; And check for inequality is simply prevState != s.
• Ryan Leach: This was intended to be quick and dirty debug code that wouldn't be in the final thing, I was attempting to do what I thought was easy and quick for me at the time, but when you need to start debugging your debugging code, you start to get frustrated.
• Ryan Leach: I've added an edit as I realized I missed including what my serial output was.
• KIIV: Yeah, operator priority is kinda need to know (or you have to use paretheses). Even += operator would let you avoid stuff like adding result of digital with s and then use it as bool value for ?: :D

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");

• Ryan Leach: To make this a more useful answer, you should really mention how the types are ignored / coerced. I've awarded you the answer regardless though. I was aware of operator precedence, but not how it interacted with the types under C.

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.

• Ryan Leach: I won't be accepting this as the answer, even though that it is AN answer, in order to award the answer points to someone that explains what was wrong.
• Juraj: first of all you shouldn't use String for this. you can store numbers, compare numbers, print numbers. digitalRead returns a number