Why aren't my conditional checks interchangeable?

Okay, so I'm going to start with showing the working version of my while and if-check loop:

  int letterIndex = 0;
  int rows = sizeof letterArr / sizeof letterArr[0];

  while(letterIndex < rows) {
    if(letter == letterArr[letterIndex][0]) {
      break;
    } else {
      letterIndex++;
    }
  }

...and then I obviously do some stuff when the loop's condition is no longer true. Cool.

But at first I was trying to break the loop with this condition:

  int letterIndex = 0;
  int rows = sizeof letterArr / sizeof letterArr[0];

  while( (letter != letterArr[letterIndex][0]) && (letterIndex < rows) ) {
    letterIndex++;
  }

...and I'd just end up in a never ending loop. The second condition letterIndex < rows never changed its evaluation to false.

And then when I tried this next configuraiton:

  int letterIndex = 0;
  int rows = sizeof letterArr / sizeof letterArr[0];

  while(letter != letterArr[letterIndex][0]) {
    if(letterIndex < rows) {
      break;
    } else {
      letterIndex++;
    }
  }

...I was super confused with what was going on. For some reason I switched the condition statements so letterIndex < rows broke the while loop but that just makes this more confusing for me. In this context shouldn't letterIndex < rows and letter == letterToArr[letterIndex][0] be interchangeable?

I printed out a ton of debugging messaging around the variable's values and from the outputted statements the other two versions on this code should be working like I'd assume: when the letterIndex is more than the rows value then the statement evaluates to false.

I'm not sure if this is super helpful but I'm working on an Arduino Uno board and I'm more of a web developer. I've dabbled in C++ but I've never done anything professional with it so no doubt this is due to my lack of understanding/knowledge for either the Arduino architecture or C++. Possibly both.

This code

while( (letter != letterArr[letterIndex][0]) && (letterIndex < rows) ) {

has undefined behaviour because you check if letterIndex is less than rows after you use letterIndex to access the letterArr array. Because the check is afterwards you will end up making an out of bound access on your array and that leads to undefined behaviour. That said I'm a bit surprised it led to an infinite loop.

The correct code is

while( (letterIndex < rows) && (letter != letterArr[letterIndex][0]) ) {

Operator && is always evaluated left to right. And if the left hand side evaluates to false, the right hand side is not evaluated at all. This avoids the out of bounds array access that your version had. Similar rules apply to operator ||. This is known as short circuit evaluation.

You are not allowed to access letterArr[letterIndex] when letterIndex is out of range. In the working version you don't because you first check the condition letterIndex < rows. In both versions that don't work you access letterArr[letterIndex] without checking first if letterIndex is a valid index resulting in Undefined Behavior.